COMPARATIVE POPULATION CYTOGENETICS, SPECIATION, AND EVOLUTION OF THE IGUANID LIZARD GENUS SCELOPORUS

Transcription

1 COMPARATIVE POPULATION CYTOGENETICS, SPECIATION, AND EVOLUTION OF THE IGUANID LIZARD GENUS SCELOPORUS A thesis presented by William Purington Hall, III to The Department of Biology In partial fulfilment of the requirements for the degree of Doctor of Philosophy in the subject of Biology Harvard University Cambridge, Massachusetts May, 1973

2 2 CONTENTS Preface 10 pages Core Manuscript: Comparative Population 192 pages Cytogenetics, Speciation and Evolution of the Crevice Using Species of Sceloporus (Sauria, Iguanidae) Manuscript: Hybridization of Karyotypically Differentiated Populations In the Sceloporus grammicus complex (Iguanldae). [with R.K. Selander] Evolution 27 (In press). [Not included here see published paper - EvolBiolPapers/Thesis/content/HallandSelander1973/ HallandSelanderText.htm Manuscript: Three Probable Cases of Parthenogenesis In Lizards (Agamidae, Chamaeleontidae, Gekkonidae), Experientia 26: [Not included here see published paper - Hall1970/Hall1970.htm Miscellaneous Material 24 pages

3 3 PREFACE The Problem to be Treated The two major and one minor manuscripts which are the body of this thesis all deal with the cytogenetics of speciation in lizards. Evolution has two major aspects: change through time and the origin of new species from old through the development of reproductive isolation. Of the two aspects, the mechanisms of speciation are the less well understood. With regard to speciation, the greatest debates have concerned the relative roles that allopatric separation and intrinsic changes in the species' genetic systems have played in the development of reproductive isolation (see Mayr, 1963, 1970; Dobzhansky, 1970; Stebbins, 1950; Goldschmidt, 1955; Lewis, 1973; White, in press, etc., for discussions of these issues). Unfortunately, speciation is a process which is not readily susceptible to study in the laboratory, and probably the most effective way to understand the roles of the genetic system in speciation is through the comparative study of speciation in suitable natural radiations which show important differences between their branches in aspects of their genetic system. Unfortunately, few such studies have been made; and, in fact, few radiations are well enough known systematically or cytogenetically to be suitable for such studies. However, as will be outlined below, I had the good fortune to be introduced to the sceloporine radiation of the lizard family Iguanidae, which provides an exceptionally ideal system within which intrinsic aspects of speciation may be studied by the comparative approach. This informally designated subfamily (Savage, 1958; Etheridge, 1964; Presch,

4 4 1969) contains nine genera and is undoubtedly the systematically best known radiation of comparable size in all of the reptilia (Smith, 1936, 1939, etc.; Savage, 1958; Norris, 1958; Axtell, 1958; Etheridge, 1964; Presch, 1969; Ballinger and Tinkle, 1972). As can be determined from the cited monographs and systematic treatments, the radiation of the sceloporines is contemporaneous with the evolution of the North American deserts and is therefore comparatively recent. And, although this is, of course, difficult to prove, this radiation seems remarkably intact in that there seem to be few gaps to be accounted for by the extinctions of major lineages. Also, as will be documented in the two major manuscripts of this thesis, different branches of the sceloporine radiation show striking differences in their adaptive and evolutionary plasticities, as measured by their comparative abilities to proliferate species. These differences are closely correlated with equally striking differences in the genetic systems of the different branches. On the other hand, the sceloporine radiation as a whole, or even just that of Sceloporus, is far too vast to be encompassed in one thesis; so here I present only the analysis of one radiation within Sceloporus, that of the crevice-using species, which still allows a fruitful comparison to be made between a cytogenetically conservative line speciating allopatrically (the torquatus group) and a karyotypically diverse line which has speciated extensively without any obvious allopatric isolation (the grammicus complex). The manuscripts included in this thesis are presented in the reverse order to which they were written, because only in the last written has the nature of the speciation been well enough understood to be

5 5 adequately described as a theory to be tested. The theory is then given first in the thesis and followed by the studies required to test it. The third manuscript is trivial by comparison, but I include it since it provides examples of instantaneous speciation in lizards through parthenogenesis and polyploidy. I then conclude the thesis with a short summary of work I have in progress (along with relevant illustrations and tables which I have completed) which has materially contributed to the principal manuscripts. Origins of the Study Before I present the body of the thesis, I wish to review the history of my involvement with speciation in the sceloporines, because this provides the most appropriate way of acknowledging the many people who have contributed so much to the inception and progress of the study. The central problem was first outlined while I was an undergraduate at San Diego State College (now California State University, San Diego). As a fugitive from a disastrous attempt to major in physics at Occidental College and UCLA, Richard Etheridge introduced me to the studies of comparative biology, evolution, herpetology, Sceloporus, and Don Hunsaker, more or less in that order. Hunsaker then introduced me to the deserts and much more fully to the local species of Sceloporus; and it was Hunsaker that convinced me that far more interesting work could be done with these common, attractive, and easy-to-study lizards than could be done with intertidal fish living in the cold, wet Japanese Current, Hunsaker was also the first to ask what special feature(s) of their biology had enabled the Sceloporus to become the dominant and by

6 6 far most speciose lizard genus of North America. However, his greatest contribution to the study was when he suggested that I try the new tissue culture procedures for chromosome preparation on Sceloporus as my cytogenetics course project. The project was a total failure, at least in terms of providing original results, but I was able to compile enough evidence on iguanid karyology (Cavazos, 1951; Hunsaker, unpub. data; Matthey, 1931; Painter, 1921; Schroeder, 1962; and Zeff, 1962); which, although much of it was wrong in detail, accurately indicated a concentration of karyotypic variability in Sceloporus as compared to the remaining iguanids. Thanks to the background on the Iguanidae provided by Etheridge and Hunsaker, this immediately suggested the possibility that some form of chromosomal speciation might account for the great evolutionary proliferation of Sceloporus (Hall, 1963, 1965). On graduation from SDSC in 1964, because of my poor record as a physics major, I was unable to obtain support from them to continue in their graduate program, so I spent the next 15 months working as a field biologist in the Nevada desert at the UCLA Rock Valley Field Station under F. B. Turner, While there, on the Nevada Test Site, I worked with Joseph R. Lannom, Jr., an experienced collector, and joined him on several extensive herpetological collecting trips into the center of the Sceloporus radiation along the west coast of Mexico, which provided the beginnings of my knowledge of the field biologies of many of the species. Joe also showed me where and how to night collect the various sceloporines that sleep buried in the sand. Additionally, while I worked on the Test Site, I was able to maintain in large indoor

7 7 pens many of the species Lannom and I had collected, as well as several additional species kindly provided by various correspondents. I saw much behavior in these pens that I would not have under other conditions. Finally, while at the Test Site, although I was still using inferior squash techniques, I was able to obtain enough additional information on the karyotypic variability of Sceloporus to further document the karyotypic diversity of Sceloporus relative to the other iguanids, and to verify the validity of the problem. As a result of my report of these initial findings (Hall, 1965), Ralph W. Axtell invited me to work under him at Southern Illinois University, Edwardsville, and to pioneer their then non-existent graduate program. Since I still had no other options for graduate work because of my undergraduate record, I was glad to take the chance. Axtell taught me much of what I know about biogeography, and with his extensive knowledge of the sceloporine lizards and Mexico, he provided an invaluable sounding board with whom I could argue my developing ideas regarding the evolution of Sceloporus. Also, during the summer of 1966, I was able to accompany him on an extensive collecting trip through the southwestern United States and northwestern Mexico, during which I was able to karyotype almost 150 lizards and successfully developed the karyotyping procedures used for the remainder of the study. Also, while a student at SIU.E, I was able to study cytogenetics and evolution under Hampton Carson at Washington University (St. Louis), which vastly improved my comprehension of comparative cytogenetics. From the time the nature of the problem became clear, I had been searching for recent cases of karyotypic differentiation or polymorphism

8 8 in Sceloporus, since analyses of these would provide the clearest tests of the chromosomal speciation idea. Due to an erroneous report of a 2n = 30 for Sceloporus undulatus by Painter (1921), I had expected to find this variation in the undulatus group, and the 1966 field work was largely planned to test this possibility. It confirmed that there was no variation in chromosome number in the undulatus. However, a Christmas, 1966, field trip by Axtell to southern Texas provided some Sceloporus grammicus specimens from Kingsville that proved to be karyotypically quite different from some individuals that I had obtained from El Salto, Durango, while working on the Nevada Test Site. As can be seen in the body of the thesis, the variability of grammicus proved to exceed my wildest expectations. However, this was not confirmed until the summer of 1968, after I had been accepted into the doctoral program at Harvard. Again, thanks to my undergraduate record, most schools would still not consider supporting my further graduate work. Harvard, as encouraged by Ernest E, Williams and George C. Gorman, was the major exception. My work here has benefited greatly from my contacts with Williams and Ernst Mayr, and from my contacts with the many students working on the other major radiation of the Iguanidae, Anolis. While a student at Harvard, I was able to spend full summers in Mexico during the years of 1968, 1970 and 1971 and to work on the Sceloporus study, and I made other long trips during the summer of 1969 and the spring of The many people who helped my work during the Harvard years are acknowledged in the appropriate manuscripts. A final acknowledgment must be made to Hobart M. Smith, whose

9 9 monographs on Sceloporus provide the foundation without which the present study would have been impossible. Although, as he knows, I will eventually make considerable revisions in species he has defined and in his conceptions of the evolution of the genus, he has provided a constant source of encouragement and moral support for the continuation of the work, a support which began while I was academically isolated on the Nevada Test Site. This has helped to keep me at the study under some rather trying circumstances when I might have otherwise decided to give it up as a bad deal.

10 10 References Axtell, R. W A monographic revision of the iguanid genus Holbrookia. Ph.D. Thesis, University of Texas, Austin, 222 pp. Ballinger, R. E. and D. W. Tinkle Systematics and evolution of the genus Uta (Sauria: Iguanidae). Misc. Publ. Mus. Zool., Univ. Michigan no. 145: Cavazos, L. F Spermatogenesis of the horned lizard Phrynosoma cornutum. Amer. Natur. 85: Dobzhansky, T Genetics of the Evolutionary Process. New York, Columbia Univ. Press. 505 pp. Etheridge, R The skeletal morphology and systematic relationships of sceloporine lizards. Copeia 1964: Goldschmidt, R. B Theoretical Genetics. Berkeley, Univ. California Press. Hall, W. P Cytogenetic studies in the family Iguanidae, Unpub. MS, San Diego State College. Hall, W. P Preliminary chromosome studies of some Nevada Test Site lizards. Paper delivered to the annual meeting of the American Society of Ichthyologists and Herpetologists, Lawrence, Kansas. Lewis, H The origin of diploid neospecies in Clarkia, Amer. Natur. 107: Matthey, R Chromosomes de reptiles. Sauriens, Ophidiens,Cheloniens, L'évolution de formule chromosomiale chez les Sauriens. Rev. Suisse Zool. 38: Mayr, E Animal Species and Evolution. Cambridge, Harvard Univ.

11 11 Press. 797 pp. Mayr, Populations, Species, and Evolution, Cambridge, Harvard Univ. Press. 453 pp. Norris, K. S The evolution and systematics of the iguanid genus Uma and its relation to the evolution of other North American desert reptiles. Bull. Amer. Mus. Nat. Hist. 114: Painter, T. S, Studies in reptilian spermatogenesis. I. The spermatogenesis of lizards. J. Expt. Zool. 34: Presch, W Evolutionary osteology and relationships of the horned lizard genus Phrynosoma (family Iguanidae). Copeia 1969: Savage, J. M The iguanid lizard genera Urosaurus and Uta, with remarks on related groups. Zoologica 43: Schroeder, G Chromosome studies in the genus Sceloporus. Unpub. MS, San Diego State College. Smith, H. M [1938]. The lizards of the torquatus group of the genus Sceloporus Wiegmann, Univ. Kansas Sci. Bull. 24: Smith, H. M. 1939, The Mexican and Central American lizards of the genus Sceloporus. Zool. Ser. Field Mus. Nat. Hist. 26: Stebbins, G. L., Jr Variation and Evolution in Plants. NewYork, Columbia Univ. Press 643 pp. White, M.J.D. (in press). Animal Cytology and Evolution, 3rd Ed. [Cambridge Univ. Press. 961 pp. (1973)] Zeff, E. W A technique for delineation of chromosomal constitution of reptilian leucocytes grown in culture. Unpub. Ms, Univ. California, Los Angeles.

12 COMPARATIVE POPULATION CYTOGENETICS, SPECIATION, AND EVOLUTION OF THE CREVICE USING SPECIES OF SCELOPORUS (SAURIA, IGUANIDAE) by William P. Hall, III Museum of Comparative Zoology, Harvard University, Cambridge, Mass

13 iii CONTENTS Introduction 1 Scope 1 Sceloporus as a resource for the study of evolution 2 Karyotypic variation in Sceloporus 4 Chromosomal Speciation Theory 7 Basics 7 genetic revolutions 9 chromosomal differentiation 9 hybridization 12 The geographic component 21 Cascading revolutionary speciation 27 cascade initiation 28 chain termination 31 evidence 36 Karyotypes as a phyletic tool 47 Methods and Materials 49 Cytological procedure 49 Material examined 52 Results 54 The "standard" or S karyotype 54 The "enlarged micro" or Em karyotype 57 Karyotypic diversity of Sceloporus grammicus 58 Introductory comments 58

14 iv S or standard grammicus 60 P1 or polymorphic-1 grammicus 60 F6 or fission-6 grammicus 63 F5 or fission-5 grammicus 64 F5+6 or fissions 5+6 grammicus 64 FM or multiple fissions grammicus 66 Contact zones, hybridization, and reproductive isolation in grammicus 70 general 70 triploidy and possible incipient parthenogenesis 75 Karyotype Evolution 78 Karyotype evolution in the grammicus complex 78 caveat 78 possible phyletic sequences 80 the probable phyletic sequence 81 Clarkii group chromosomes and origin of Em karyotypes 86 Sex chromosome evolution 89 Evolution of the Species Groups 95 Karyotypic phylogeny 95 Alternative phylogenies 98 the Smith phylogeny 98 problems with the Smith phylogeny 100 the Larsen phylogeny 102 problems with the Larsen phylogeny 103 Reconciliation and phyletic synthesis 106

15 v the biology of crevice using 106 reproductive biology 113 biogeography 114 synthesis 118 Patterns of Speciation 122 Introductory comments 122 Conservative geographic speciation 125 Revolutionary speciation 128 Cascading revolutionary speciation 130 Geography of revolutionary speciation 133 Summary and Conclusion 138 Acknowledgements 143 Bibliography 145 Appendix: Specimens Examined 162

16 vi TABLES 1. Summary of Karyotyped Lizard Genera and Species, by Family Geographical and Ecological Distribution of Standard grammicus Geographical and Ecological Distribution of F6 grammicus Geographical and Ecological Distribution of P1 grammicus Geographical and Ecological Distribution of F5 grammicus Geographical and Ecological Distribution of F5+6 grammicus Geographical and Ecological Distribution of FMl grammicus Geographical and Ecological Distribution of FM2 grammicus 172

17 vii FIGURES 1. Distribution of karyotypic variation in the family Iguanidae The standard karyotype Standard karyotypes of the torquatus species group The standard and Em karyotypes of three species groups Major karyotypes of the grammicus complex Comparison of standard grammicus and Agama caucasica 178 7a. Distribution of grammicus karyotypes in Mexico 179 7b. Distribution of grammicus karyotypes in the Valley of Mexico Karyotypic relationships of the clarkii species group Male diakinesis arrays Microchromosomal arrays from male diakinesis Reconstruction of the karyotypic evolution of the crevice-using Sceloporus and the clarkii group Relationships of the species groups of Sceloporus as proposed by Smith (1939) Relationships and groupings of Sceloporus proposed by Larsen Geographic distributions of the 2n=34 orcutti group species Geographic distributions of the 2n=40 clarkii group species Geographic distributions of the Em karyotype species Geographic distributions of the standard species of the grammicus group Geographic distributions of the torquatus group species Early radiation of the large-sized, large-scaled Sceloporus 192

18 1 INTRODUCTION Scope In this series of reports I will attempt to trace both the evolutionarily important functions of and the evolutionary changes in different aspects of the cytogenetic systems of the various species of the iguanid lizard genus Sceloporus. Concomitantly, it will be necessary to review, and in some cases revise, the accepted species groupings and ideas regarding phylogenetic relationships within Sceloporus (Smith, 1939; Smith and Taylor, 1950). An analysis of karyotypic variation and its probable significance in speciation is central to my phylogenetic treatment, but other sets of characters will be considered when needed. In the present work I examine the radiation of the live-bearing Sceloporus which have specialized for the use of crevices in one form or another for escape and sleeping cover. This radiation includes Smith's (1939) species groups grammicus and torquatus plus megalepidurus and pictus of the megalepidurus group and asper of the formosus group. Also, to help interpret some of the karyotypic differences observed among these crevice-using Sceloporus, I include data for S. clarkii and melanorhinus, a natural group which I believe shares a close common ancestry with the crevice-users.

19 2 Sceloporus as a Resource for the Study of Evolution Sceloporus is the dominant lizard genus of continental North America, and one or more of its species can be found in virtually every non-fossorial lizard habitat from western Panama to the northern limit of lizard distribution. According to present taxonomy (Smith and Taylor, 1950; Smith and Bumzahem, 1953; Webb and Hensley, 1959; Langbartel, 1959; Cole, 1963; Lynch and Smith, 1965; Webb, 1967; Smith and Lynch, 1967; Stuart, 1970, 1971; Dixon et al., 1972) Sceloporus contains 64 species, and my planned revisions will raise this to above 70, which makes Sceloporus one of the more speciose lizard genera in the world. In the Iguanidae, only the tropical Anolis radiation of some 200 species and possibly the comparatively poorly known Liolaemus radiation of southern South America have more species. The magnitude of the radiation of Sceloporus is impressive even without other consider-ations, but osteology (Etheridge, 1964; Presch, 1969) and biogeography (Savage, 1960, 1966) indicate that it may be one of the more recently differentiated genera within the Iguanidae. Not only has the proliferation of Sceloporus been remarkably extensive, but it appears to have been even more remarkably rapid. Important evolutionary questions implicit in this list of superlatives are: Why and how has Sceloporus evolved so many more species than have been produced by other genera of related and older origins? Might understanding these questions provide a more general insight into problems of species formation in other groups? Although we cannot subject such questions to controlled experiments in the laboratory, subdivisions within Sceloporus and the existence of eight

20 3 other closely related genera in the sceloporine branch of the family (Savage, 1958; Etheridge, 1964; Presch, 1969) provide a series of natural experiments which may be studied by the comparative approach to determine possible relationships between various biological characteristics of the subdivisions and their present evolutionary successes.

21 4 Karyotypic Variation in Sceloporus Since the pioneering investigations of T. S. Painter (1921) and Robert Matthey (1931, 1933, 1949), lizards have been comparatively well studied karyotypically; and among them the family Iguanidae is presently best known. This is mainly due to recent work by Gorman, Cole, and Pennock, plus extensive but still unpublished work from the MCZ cytogenetics lab by T. P. Webster, R. B. Stamm, and myself. Gorman (in press) reviews most of the karyotypic work on lizards published through 1971, Table 1 abstracts data from his article plus additional unpublished information from the MCZ cytogenetics lab to summarize what is known about chromosomal variation in lizards. Of 54 iguanid genera the 29 karyotyped represent all major radiations in the family. Figure 1, based on a tentative phylogeny of the Iguanidae by Etheridge and Estes (unpub.; see also, Etheridge, 1964), summarizes the known karyotypic variability of each genus in the family. In the sceloporine branch, half or more of the species in each of its nine genera have been karyotyped. And within Sceloporus itself, more than 2000 individuals from 56 of the 64 recognized species have been karyotyped, and these karyotyped species represent all 15 of Smith's (1939) species groups. Some interesting conclusions can be drawn from these data. In general, iguanids are karyotypically conservative. Of the 29 karyotyped genera, 27 have species with either a pattern believed by Gorman (in press; Gorman et al., 1967; Gorman et al., 1969; Bury et al., 1969) and myself to be primitive in lizards; i.e., a 2n=36, 12 metacentric macrochromosome, 24 microchromosome karyotype, or a 2n=34

22 5 pattern which differs from the primitive karyotype only by the absence of a pair of micro chromosomes. Only four genera show striking deviations from the general karyotpic conservatism of the family, with these deviations mainly due to fixation of Robertsonian rearrangements (i.e., centric fusions and/or centric fissions--hsu and Mead, 1969) between species. These four strikingly variable genera include the three largest in the family: Sceloporus, with 64+ species and 2n's ranging from 22 to 46; Anolis, with 200+ species and 2n's ranging from 26 to 48; and Liolaemus, with 50+ species and 2nls ranging from 32 to 40 in a small sample of species (unpub. data from R. D. Sage). The fourth notably variable genus is Polychrus, with five species and 2n's ranging from 20 to 30 without obvious Robertsonian relationships between the present karyotypes (Gorman, in press; Peccinini, 1969; Becak et al., 1972). Excepting the three genera with 50+ species, each of which shows remarkable karyotypic diversity, no other iguanid genus contains more than about 15 species (excluding the insular radiation of the Caribbean genus Leiocephalus with 16 species and the approximately eight Tropidurus species in the Galapagos); and, excepting Polychrus and Plica (the latter with two species and a 2n=40 in the one karyotyped), none of the other 24 genera karyotyped show any significant deviation from the 2n=36 or 2n=34 patterns. The concentration of interspecific karyotypic variation in the remarkably speciose genera such as Sceloporus is notable and immediately suggests that there may be some functional relationship between the establishment of these interspecific differences and the proliferation of species in these genera. Further-

23 6 more, Sceloporus and the related sceloporine genera show more than enough different patterns of speciation and karyotype variation to provide an ideal system of natural experiments which may be compared for studying this possible relationship. Even an analysis of the crevice-using radiation within Sceloporus, which will be described in the present report, will suggest some answers. However, before I turn to this analysis it will be appropriate to review, and in part to develop theory regarding the roles interspecific chromosomal differences may have played in species formation.

24 7 CHROMOSOMAL SPECIATION THEORY Basics Wallace (1959), White (1969, in press), Nadler (1969), Mayr (1969a, 1970) and others have noted that certain classes of chromosomal rearrangements are frequently found fixed between closely related species but are only rarely found as polymorphisms within populations of these species. Such mutations usually have no obvious adaptive functions as polymorphisms, and in many cases are suspected to cause meiotic assortment difficulties in heterozygotes that would reduce their fertility. Other kinds of rearrangements, such as the paracentric inversions of Drosophila (Dobzhansky, 1970) and the reciprocal translocations of Oenothera (Stebbins, 1950) have clear adaptive functions as polymorphisms within the genetic systems of the species they characterize. Robertsonian rearrangements clearly belong to the former class of mutations. For example, about one of every four Drosophila species differs by a Robertsonian rearrangement; but, in spite of the extensive cytological sampling of Drosophila, Wallace (1959) could find only one instance of Robertsonian heterozygosity from nature; and this occurs only in the zone of hybridization between D. americana americana and D. a. texana (Stone and Patterson, 1947; Hsu, 1952; Carson and Blight, 1952). Sceloporus would appear to provide another example of this situation, and many other similar examples from cytologically well known groups are cited by White (1969, 1973). On theoretical grounds we might expect meiotic malassortment from Robertsonian trivalents to produce some percentage of aneuploid gametes (Ohno, 1965), which would then reduce the effective fertility

25 8 of the heterozygotes if these gametes competed for fertilization. Among vertebrates such meiotic malassortment and consequent reduced reproductive fitness in heterozygotes has been demonstrated in humans (Hamerton, 1971), mice (Tettenborn and Gropp, 1970; Gropp et al., 1972), and cattle (Gustavsson, 1971a, 1971b). Certainly cases are also known where Robertsonian rearrangements survive as apparently stable polymorphisms (e.g.. Ford et al., 1957; Meylan, 1964, 1965; Matthey, 1966; Jotterand, 1972; etc.; and see below), but these are not frequent, and to my knowledge, no reasonable mechanism whereby the Robertsonian polymorphism as such would have an important adaptive function in the genetic system of a species has been documented. On the other hand, the frequent fixation between species of mutations which theoretically might reduce heterozygote fertility has led many cytogeneticists and systematists to propose models of chromosomal speciation based on the assumption that such chromosomal mutations do in fact reduce heterozygote fertility during the speciation process (Callan and Spurway, 1951; Spurway, 1953; Spurway and Callan, 1960; Lewis, 1953, 1966; Lewis and Raven, 1958; Wallace, 1959; Matthey, 1964, 1965; John and Lewis, 1965, 1966; White et al., 1967; White, 1968, 1969, 1973; Key, 1968; Mayr, 1969a, 1970; Todd, 1970; Arnason, 1972; etc.). However, rather than attempt to review each specific model here, I will summarized what I think are some of the more important aspects in the development of a more-or-less synthetic theory of revolutionary (frequently, but not always chromosomal) speciation. With an admitted (over-?) simplification, two extremes of species formation may be recognized: conservative or "classical"

26 9 allopatric speciation, and a revolutionary mode of speciation. The conservative type involves the geographic separation of comparatively large populations followed by their probably slow genetic divergence in response to the average differences in selection pressures operating on the respective isolates, and is a process which no rational biologist now disputes. Such speciation probably needs geological times to reach completion. The revolutionary mode of speciation invokes the founder principle and genetic revolution (Mayr, 1942, 1954, 1963, 1970). Genetic revolutions. A genetic revolution involves large shifts in the frequency of alleles at polymorphic loci (or even their fixation) in local populations as a consequence of their restriction to such effectively small sizes (say N < 20) that large sampling errors may occur in the reproduction of gene frequencies from one generation to the next (Wright, 1940, 1951, 1970). This effectively small population size is possible only if gene exchange with other populations is minimal or absent. Given the necessary isolation, the chance effects plus local environmental selection may occasionally produce in only a few generations a new adaptive genotype which has also achieved a new epistatic balance. Although this new genotype will contain no new alleles, it may differ enough from the parental stock in epistatic balance so that hybrids with the parental stock are considerably less fit than are progeny from matings within the respective "pure" populations. The case of apparent incipient speciation in the geographically isolated Bogota population of Drosophila pseudoobscura described by Prakash (1972) appears to exemplify this effect. Chromosomal differentiation. Such post-revolution populations

27 10 may survive as biological species if they are well enough adapted to the local ecology to compete successfully with other forms seeking to use it and if they are well enough isolated from the parental stock to keep introgressive gene flow below that which would break down either the new environmental adaptations or the new epistatic balance. However, under most conditions lacking strict geographic isolation, it would seem that occasional spurts of gene flow from neighboring populations would inevitably break down or swamp the new balances (Wallace, 1959; Mayr, 1963). But, on the other hand, chromosomal differentiation during revolutionary speciation in strict geographic isolation (as presumed for the Bogota D. pseudoobscura) would seem to play no role in the process sufficient to account for the apparently frequent fixation of possibly heterozygously semisterilizing chromosomal differences between sister species-especially between sister species 1 of parapatrically dis- tributed swarms as exemplified by cytologically comparatively well known groups such as: the morabine grasshoppers (White et al., 1967, 1969; White, 1973, etc.); the pocket mice, Perognathus (Patton, 1967a, 1967b, 1969a, 1969b); pocket gophers, Thomomys (some populations only--thaeler, 1968); mole rats, Spalax (Wahrman et al., 1969a, 1969b; Lay and Nadler, 1972); the equally fossorial tuco-tucos, Ctenomys (Reig and Kiblisky, 1969); etc.; and especially the cryptic species of the Sceloporus grammicus complex (see below). However, as I will now show, chromosomal differentiation may afford a revolutionary population which is not well isolated geographically from its parental stock with an intrinsic barrier to gene flow that would allow it to survive and differentiate as a species even when exposed to the occasional spurts of potential gene

28 11 Footnote to page 10 1 It must be admitted that the degree of genetic isolation between many of the populations listed is not known. Hybrids between some of them have been found in nature, and, at least in the morabines, the hybrids are sometimes largely fertile in laboratory crosses. However, as Hall and Selander (1973) have shown in the Sceloporus grammicus complex, genetic isolation may still be complete, even in cases of apparently free hybridization and good hybrid fertility, probably because of recombinational breakdown in the backcross generation, serves as a barrier to gene flow between the pure populations. Until similar genie data are available for the examples cited or for comparable cases, these can only suggest patterns. It should also be noted that I do not always interpret published data the same way as do their authors.

29 12 flow that would swamp a karyotypically ancestral population. According to Wright (1941), the conditions required for a genetic revolution are also precisely those which would allow the chance fixation of a chromosomal mutation which reduced heterozygote fertility. He calculated that a mutation not effecting fitness when homozygous, but reducing heterozygote fertility by 50%, still had better than one chance in 10-3 of being fixed after an indefinite number of generations if it occurred in a population with an effective size of 10. Of course, the probability of fixation of such a mutation becomes impossibly low as population size increases; or, more importantly, the probability of fixation becomes greater if heterozygote fertility is higher or the effective population size is lower. However, once a local population becomes fixed for a heterozygously semisterilizing mutation, the reduced fertility of chromosomally heterozygous hybrids and backcrosses, etc. with immigrants from the ancestral stock would reduce the effective introgression of ancestral genes considerably below that which would result from the same immigration rate into a non-chromosomally differentiated population. Therefore, the chromosomally differentiated post-revolution population would have an immediate degree of intrinsic protection against potentially disruptive gene flow lacked by non-chromosomally differentiated populations. Hybridization It might be argued, notwithstanding the partial sterility barrier provided by chromosomal differentiation, that any hybridization and consequent introgression would slow or disrupt the speciation process (e.g., see Mayr, 1963). But the fact that all

30 13 species in some of the swarms of parapatric species cited above seem to be chromosomally differentiated and the fact that this differentiation would seem to play no function in purely allopatric speciation suggests an alternative view. Because the low fertility of chromosomally heterozygous hybrids would function to reduce the effective fitness of hybridizing parents, at least before the post-revolution population grows large, hybridization could serve as an intrinsic selection mechanism to encourage the perfection of any ecological or behavioral differentiation which served to reduce the frequency of hybridization (Fisher, 1930; Dobzhansky, 1940, 1941, etc.). Mayr (1963), arguing against this theory, supposed that premating isolation could not be evolved by reduced hybrid fertility until postulating isolation was essentially complete (which it certainly is not in the present model, at least initially), because he thought that introgression through the reduced fertility barrier would tend to break down whatever genetic difference(s) caused the reduced hybrid fertility. However, hybrid chromosomal semisterility would serve as an intrinsic selective mechanism to maintain itself in the local population against anything up to a 50% frequency of ancestral-type chromosomes (although, if this 50% frequency is exceeded, selection would then work to eliminate the mutant chromosome from the local population). It also seems likely that genie selection within the post-revolution population could maintain new coadaptive and environmental balances against small amounts of potentially disruptive gene flow penetrating the essentially fixed hybrid semisterility barrier (e.g., Bigelow, 1965). Moore (1957) suggested several other reasons why hybrid infertility probably could

31 14 not serve as an effective selective mechanism to promote premating isolation (see below). However, these all rest on the basic assumption that only small fractions of the hybridizing populations occur in contact or overlap zones where they risk hybridization; and, of course, this assumption does not apply to the extremely localized, early postrevolution population where all of its members would bear essentially the same risks of hybridization. Only if the post-revolution population grows large enough so that many of its individuals are geographically removed from the potential dispersal paths of the ancestral stock do Moore's arguments become effective. Possibly much more interestingly, limited introgression might provide an important source of genetic variability which could be selectively incorporated into the new adaptive and coadaptive balances to allow the post-revolution population to differentiate much more rapidly from the ancestral condition than could a strictly isolated and therefore genetically depauperate founder population of Mayr (1942, 1963, etc.). Key (1968) supposes that the semisterility of the heterozygotes will act like a semipermeable barrier to selectively hold back or trap genes or rearrangements that favor hybridization, and to pass those which would reinforce the degree of isolation or improve the local adaptations of the homozygous mutant population. This semipermeable barrier, generally a zone of parapatric hybridization which Key not too appropriately called a tension zone (see below), will be pushed across the countryside to a point of equilibrium where the population pressures of the respective pure populations are equal; i.e., until the populations on either side of the hybrid zone are equally well

32 15 adapted to the local conditions. In the absence of premating isolation, the hybridization will also serve as a barrier to prevent any interpenetration of the populations (Zaslevskii, 1963). Therefore, as long as the new genie equilibria of the chromosomally differentiated postrevolution population are not swamped by too many genes filtering through the semipermeable partial sterility barrier, limited hybridization with the ancestral stock might serve both as a selective mechanism and as a source of genie variability to speed the evolutionary differentiation. If the chromosomally differentiated population achieves a successful adaptation and grows large enough so that many of its individuals no longer risk hybridization themselves, because they are separated from the ancestral stock by a zone of parapatric hybridization, then a further, not-so-obvious effect of the low hybrid fertility will work to reduce introgression into this "protected" population even below that which successfully passes the partial sterility barrier of the chromosomal heterozygotes. If chromosomal heterozygotes formed in the zone of hybridization are less fecund than the chromosomal homozygotes, it then follows that the population pressure competing for limiting resources in the hybrid zone will be reduced by an amount directly related to the proportion of heterozygotes in the total population and the reduction in their fertility with respect to the homozygotes. This reduced population pressure will very likely result in a net dispersal of individuals from areas of pure populations into the hybrid zone. Consequently, the hybrid zone will serve as a partial vacuum, or a "sink," for gene flow; and to get out of the immediate area of the

33 16 hybrid zone, any ancestral genes penetrating its semisterility barrier would still have to migrate out against a net gene flow into the hybrid zone. The effectiveness of this sink effect as a trap for introgressed genes (i.e., those which have successfully backcrossed into chromosomally homozygous individuals) will of course depend on the degree to which the population pressure of the hybrid zone is reduced with respect to pure populations and on the vagilities of the individuals comprising the various populations. This is because these factors will determine the steepness of the diffusion gradient against which the introgressed genes must migrate. It is not intuitively clear to me what values these parameters must have before the sink would become completely effective as a trap for gene flow, but these should be easy to determine mathematically or with computer simulation experiments. However, given a sufficiently reduced heterozygote fitness and an appropriate population structure, it seems evident to me that the hybrid zone could become completely effective as a barrier to gene flow even without complete heterozygote sterility. Therefore, given the chromosomal differentiation of a post-revolution population and a chance for it to spread enough to form a central population protected from the risk of hybridization, this central population might be able to evolve completely independently from the parental stock even without the evolution of complete reproductive isolation in the contact zone. If this situation pertains, further spread and differentiation of the chromosomally mutant population would then depend only on maintenance of a high enough population pressure on its side of the contact zone to

34 17 counterbalance the population pressure of the ancestral stock and any surface tension effects due to concavity of the hybrid zone (Key, 1968; and see below). The sink effect is implicit in Key's (1968) "tension zone" concept, and other discussions of the maintenance of narrow zones of parapatric hybridization by semisterility (Woodruff, 1972), but I do not know that the possible effects of the reduced population pressure resulting from the semisterility have ever been explicitly considered. Perhaps this is because Key's term, "tension zone," is not a good metaphor for this reduced population pressure, although it does signify the disruptive selective effects of the chromosomal discontinuity and the surface tension effects of the hybrid zone. On the other hand, the idea of a "sink" does suggest that the zone of hybridization is an area of comparatively low pressure that would tend to suck in migrants and the genes they carry--genes which would eventually "go down the drain" when they found themselves in lethally unbalanced zygotes formed as a result of malassortment by the chromosomal heterozygotes. When hybrid zones formed as a result of reduced hybrid fitness are thought of as zones of reduced population pressure, it becomes immediately obvious why, as Key (1968: p. 19) suggested but did not convincingly explain, that factors of any kind having negative heterotic effects should interact to stack up in a hybrid zone to "lead eventually to full reproductive isolation, in the sense of complete infertility or inviability of hybrids between the populations on each side of the zone (i.e., to speciation)"--without the evolution of premating isolation. Furthermore, the sink effect of the hybrid zone also seems to provide a

35 18 completely adequate explanation for the puzzling long-term stability of zones of parapatric hybridization between populations which have large ranges in proportion to their hybrid zones, such as are found in Sceloporus grammicus (Hall and Selander, in press; and see below), the grasshoppers cited by Key, etc., and in many other vertebrates (e.g., see Mayr, 1963, 1970; Woodruff, 1972, MS). Not only do genes filtering past the heterozygotes, i.e., genes which might otherwise tend to break down genie differences between the populations, find it difficult to get out of the hybrid zone against the diffusion pressure of a net immigration, but the same problems would also apply to any genes favoring premating isolation (Woodruff, 1972). It seems quite unlikely that any single gene would ever enable its chromosomally homozygous carriers to successfully discriminate among its "conspecifics," homozygous "others," and the host of chromosomally heterozygous F 1, F 2, and backcrosses, because clearly the carriers of the gene would have to avoid mating with the carriers of the alternative chromosome arrangement to avoid eventually losing the gene in lethally unbalanced zygotes. In all probability, any genes allowing only partial discriminatory ability to their carriers would still be trapped before they found their ways into combinations that afforded their carriers with the required complete discriminatory ability. Besides the diffusion gradient tending to keep these "discriminator" genes out of the main population, Moore (1957) suggested two more reasons why they would not spread out of the hybrid zone: 1) any specialization of population A in the hybrid zone which reduced competition with the similar population B would probably be maladaptive

36 19 for A away from the area where B is encountered, because A would not then be making full use of all the resources potentially available to it. 2) Similarly, genes which allowed A in the contact zone to successfully discriminate against mating with the similar B would probably prevent A from mating with some of its conspecifics in the area away from the contact zone. From the discussion above, it should be clear that if a heterozygously semisterilizing chromosome mutation can once become fixed in a local population large enough to be protected by a zone of parapatric hybridization, it will then be comparatively easy for this population to spread further and evolve independently as a new species if it has by chance and/or local adaptive response achieved a genotype more suited to its local habitat than are those of the adjacent populations of the ancestral stock. Also, given a reasonable population structure, mutation rate, and meiotic system in the ancestral stock, it seems likely that local populations within it will occasionally happen to be sufficiently isolated at the right time and for the necessary few generations to allow one of them to become by chance chromosomally differentiated. However, the most critical stage in the speciation process will occur after the initial fixation, when the homozygous mutant population is still small, because to survive and grow it must be able to counterbalance the population pressure of the much larger ancestral stock where the two populations contact. Then, the reproductive fitness of the homozygous mutants must be high enough to push the same absolute number of migrants into the hybrid zone as does the much more extensive ancestral stock, or otherwise the zone will contract

37 20 around the mutant population until it is swamped out of existence. On the other hand, it is also at this early stage when the semipermeability of the semisterility barrier can most effectively change a postrevolution population by simultaneously adding genic variability and selectively removing genes which favor hybridization; and it is at this stage when a major adaptive change may be most critical to the survival of the post-revolution population. For these reasons, in this early stage of differentiation the geographic relationships with the parental stock will be most important for the derived population, because these will determine the effective pressures that the derived and parental populations can bring to bear on the hybrid zone.

38 21 The Geographic Component Although revolutionary speciation involving chromosomal differentiation may occur in rough sympatry, i.e., without prolonged or absolute geographic isolation (at least in comparison to the conservative mode of speciation), in an area historically inhabited by the parental stock, it cannot be over-emphasized that the parental and speciating populations must still be separated a least microgeographically during the first stage of the speciation process to provide the necessary degree of inbreeding for the revolution and chromosomal differentiation. Three ways this separation might occur within the broad geographic range of a parental species may be postulated. Two extremes may be termed the "peripheral" and "interstitial" versions; with a third, "internal periphery," version representing somewhat of an intermediate version. As the name implies, the peripheral version supposes that adequate genetic isolation for the genetic revolution and fixation of a heterozygously semisterilizing chromosome mutation can occur only on the geographic periphery of a species; perhaps as a propagule passes some local environmental barrier or as a local breeding unit becomes isolated from the main population during climatic fluctuations (Callan and Spurway, 1951; Spurway, 1953; Lewis, 1953, 1966; Wallace, 1959; Mayr, 1963, 1969a, 1970; Key, 1968). In this type of speciation, recently derived post-revolution species would be found only peripherally to their parental stocks. Possible examples of this situation are provided by Spalax (Wahrman et al., 1969a, 1969b; Lay and Nadler, 1972), Perognathus (Patton, 1969b), and morabine grasshoppers according to Key

39 22 (1968). The interstitial version supposes that opportunities for revolutionary speciation might occur within the normal range of a species if it has a normally scattered and clumped population structure, as do morabine grasshoppers living on scattered bushes (White, 1968, 1969, in press), many terrestrial and fossorial vertebrates (e.g., burrowing rodents in pockets of loose soil--arnason, 1972; White, 1969, in press), and Sceloporus grammicus in areas where they are found living on scattered fallen logs (Moody et al., in prep,; and see below). If some of the local populations of such a species are effectively small and also effectively (but not necessarily absolutely) isolated for probably a minimum of 10 to 20 generations at a time, then incipient speciation of one of the populations would depend only on the chance fixation of a heterozygously semisterilizing mutation while it was reaching a new adaptive equilibrium by chance drift and local selection. If this interstitial type of speciation is common, and if recent postrevolution populations can be found, some of them should be completely surrounded by populations of the parental stock. This is similar to the "stasipatric" model (White et al., 1967; White, 1968, etc.), which White believes is demonstrated by the "speciation" in the morabine grasshoppers, in that the initiating chromosomal rearrangement becomes fixed in an interstitial population (but see Key, 1968, for an alternative interpretation of the morabine situation). It also seems possible that speciation in the Sceloporus grammicus complex conforms to this pattern (see below). In the peripheral version, only a small fraction of the total

40 23 population of a species is likely to occur in the small peripheral isolates where revolutionary speciation could take place. Furthermore, these peripheral populations would seem to suffer a high probability of going extinct when their ecological tolerance is exceeded by slightly abnormal environmental fluctuations (which is why they are peripheral). On the other hand, their isolation is likely to be more extreme than is that of the interstitial populations, and they would likely be exposed to more extreme disruptive selection for differentiation from the central populations. If revolutionary speciation and ecological differentiation occurred, a nascent species could then easily spread geographically into areas not occupied by its parental stock (Wallace, 1959; Patton, 1969b). And, probably most importantly, because of its peripheral location, the derived population would form a hybrid zone only on the front facing the ancestral stock, thereby avoiding or at least reducing the surface tension effect that would otherwise cause the hybrid zone to have a tendency to close around and extinguish the derived population. In the interstitial version, a comparatively large proportion of a species may live in local inbred populations where revolutionary speciation might occur. Although these local populations may not always maintain sufficient isolation for successful revolutions, they would be far less likely to go extinct for environmental reasons than would peripheral isolates. And, even though an interstitial population is not peripheral, it is quite possible that selection pressures in its local environment would differ from those average for the species. For example, in a species which lives in more than one habitat type (dense

41 24 forest and open woodland, flood plane and hillside, etc.), but most individuals live in only one of the habitats, they would probably be best adapted to this favored habitat. Then, if a genetic revolution provided sufficient reproductive isolation and resulted in a genotype which was more effective in a less favored habitat than the usual genotype of the ancestral stock, this incipient species could then geographically exclude the ancestral population from this less favored habitat. Though less likely, before the nascent species grows large enough to be completely protected by a zone of parapatric hybridization, it might be forced by competition and selective hybridization with the parental stock to differentiate enough ecologically to allow the two species to coexist sympatrically. Given a fortuitous population structure, many more revolutions and initial chromosomal differentiation events would be expected to occur interstitially than would take place on the much more limited periphery of a species which had a more continuous distribution through its range. However, the newly differentiated population would face the possibility of hybridization all around its circumference, and it would therefore require an especially superior local adaptation to counterbalance the tendency for the hybrid zone to contract. An intermediate, and possibly more realistic version of the revolutionary speciation model acknowledges that many species living in complex environments have areas within their broad geographic ranges where they normally do not live, though they may occasionally cross these areas (e.g., different climatic zones on a mountain, forests vs meadows, rocky hillsides vs alluvial flats, etc.). On a microgeographic

42 25 scale within the range of interest for revolutionary speciation, ecotones between these different habitats will be ecologically similar to the periphery of a more continuously distributed species (Key, 1968). The presence of such internal peripheries within the broad range of a species would increase by several orders of magnitude the opportunities for revolutionary speciation in comparison to those available to a species with a more-or-less continuous distribution within its range. As in the external periphery situation, the internal periphery would considerably reduce the extent of the hybrid zone formed with the ancestral stock, and would thereby facilitate the survival of the derived stock. After a species formed on an internal periphery has expanded its range, as in the interstitial version, one would expect to find the derived species completely surrounded by its parental stock. However, in a geographic exclusion situation of the interstitial version, the derived species should be found to occupy a habitat type clearly within the ecological range inhabited by the parental stock away from the area inhabited by the derived stock. Whereas, in the internal periphery version, the derived species should be found in an ecology definitely peripheral to that of the ancestral stock. Whether revolutionary speciation is supposed to be peripheral, interstitial, intermediate, or all of these, it is worth noting that the fixation of chromosomal rearrangements between species seems to be most frequent in groups of organisms of apparently limited vagility; while, on the other hand, highly mobile and presumably outbred groups such as the Cetacea, Pinnipedia, birds, and bats generally show remarkably stable karyotypes (Arnason, 1972; White, in press). Particularly

43 26 notable examples of the former situation are the burrowing rodents cited above (Perognathus, Spalax, Thomomys, and Ctenomys) and probably at least some populations of the karyotypically diverse Sceloporus grammicus (see below). So, it would seem that whatever the geographic details of the revolutionary speciation process, if the above mentioned species have been formed by it, then their limited vagility has probably aided the speciation process, presumably by promoting inbreeding.

44 27 Cascading Revolutionary Speciation The revolutionary speciation model suggests extensions which have interesting implications for systematics. One offers a possible explanation for the occasionally explosive proliferations of species that some groups (such as Sceloporus) form by contrast to their closely related stocks which appear to have had similar ecological opportunities but have speciated much more conservatively (e.g., the other sceloporines). Not only can revolutionary speciation occur without the degree of geographic isolation needed for conservative speciation, but occasionally, in comparison to its ancestral stock, a revolutionarily derived species may show an intrinsically amplified probability of further speciation by revolution; and so on to form further generations of species by a cascading process, until the process terminates, either for intrinsic reasons or because there are no longer ecological niches available for further derived species. In other words, an initial revolutionary speciation event may in some instances start a process which leads to the formation of not just one or two new species, but, rather, to the rapid proliferation of a whole swarm of sister species. The model is subdivided into two categories of conditions and events: those which initiate and accelerate the cascade process, and those which terminate its chains of derivation. The development of the model here is largely intuitive; but, besides the qualitative testing which can be gained from comparative studies of appropriate natural radiations (see below), most aspects should be amenable to mathematical testing and verification by computer simulation (not yet attempted).

45 28 Cascade initiation. The conservative mode of speciation depends mainly on the extrinsic interposition of geological, ecological, or climatic barriers between subdivisions of an ancestral population, and then on the differential effects of extrinsic natural selection on the separated populations. Intrinsic characteristics become important only when (and if) the separated populations come into secondary contact. On the other hand, speciation by revolution depends almost exclusively on intrinsic characteristics of the genetic system of the ancestral stock, e.g.: 1) on genes which influence population structure and vagility; 2) on aspects of the recombination system which determine a) how much genetic variation is carried, b) how this variation is distributed geographically, and c) how fast it can respond to natural selection; 3) on the availability of karyotypic possibilities for rearrangements which reduce heterozygote fertility; 4) on genes which affect mutation rates for rearrangements that reduce heterozygote fertility (Ives, 1950); 5) on genes which affect the frequency of malassortment in structural heterozygotes; 6) etc... These characteristics of the genetic system will vary between species and populations, and hence some species or populations may be genetically more apt to gene-rate new species by the revolutionary mode than will others. Selection may affect all aspects of the genetic system. However, selection would be able to work only extremely slowly on things like mutation rates and the meiotic assortment of the heterozygous mutations, which would occur only rarely, but would nonetheless be of primary importance in determining the rate at which an ancestral stock initiated chromosomal speciation events. The reasons for this slow response to

46 29 selection are twofold: 1) selection on mutation rate would be effective only when a mutation actually occurred, and for meiotic assortment only in the (usually) rare heterozygotes produced by the rare mutations; also 2) these phenomena are likely to be only rarely penetrant pleotropic effects of genes much more stringently controlled for their normal functions. With only infinitesimal rates of selection, characters of this kind will probably show considerable variation by random drift. Then, given: 1) that important characteristics of the genetic system involved in revolutionary speciation may vary among local populations of a species, and 2) that all extrinsic factors relating to revolutionary speciation are held constant, it follows that the essentially chance event of revolutionary speciation will occur most probably within a local population which has the most favorable genetic system for it. And, in turn, because its genes are inherited from the local population which founded it, the post-revolution species may on the average be more likely to produce further derived species by the revolutionary mode than will be the ancestral stock. Then, of course, the same type of selective sampling process which amplified the frequency of favorable alleles in the origin of the first generation reyolutionarily derived species may be repeated in it to produce a second generation of even more derived species, which in turn may be still more likely to produce a third generation of derived species, and so on... until something happens to terminate the cascading process (see below). In other words, this genic amplification process will accelerate the rate of speciation as the cascade proceeds. Two predictions may be made about the phyletic structures of

47 30 recent radiations formed by the cascade process: the process of derivation is likely to result, 1) in the formation of one or a few long sequences of chromosomal derivation in the cascade rather than many short sequences deriving independently from comparatively primitive species; and 2) species formed later in a chain of derivation may be able to occupy a much smaller geographic area and/or ecological range before giving rise to further derivatives than will species formed early in the chain. These predictions will be discussed in turn. Presumably the ecological and/or geographical area available for a radiation will be limited. It is also reasonable to assume that a new species can be formed only peripherally to an ancestral stock, whether this periphery is ecological or geographical. Since any species formed will expand to fill the space for which it is suitably adapted, it is quite likely that a derived species will then expand along the periphery of its parental stock, and thereby limit opportunities for this parental stock to form additional derived species along this periphery. This may be called latera1 inhibition. Furthermore, because of the amplification effect of the derivation process, it is probable that the derived species will form a third species to further saturate the available habitat for speciation before the first (ancestral) species tries to form another derivative. Similarly, because of the amplification, the third species in the sequence is likely to form a fourth species before either the first or second will. Hence, the net effects of the amplification and lateral inhibition will produce a sequence which tends to be linear rather than branched. That is not to say that a cascade will not be branched, because many extrinsic effects

48 31 will influence the speciation and some chains may be terminated for one reason or another (see below); but, in general, the cascade will tend to be more chain-like than fan-like. The second prediction of the cascading chromosomal speciation model follows directly from the amplification effect. Since species formed late in a sequence of derivation will likely form further derived species, much more rapidly than will species formed early in the sequence, these late derivatives in general may have much less opportunity to grow in number or to spread geographically before their "progeny" are formed. These late derivatives may therefore be ephemeral with respect to either early derivatives or to species which terminate their sequences of derivation. As will be seen below, both phyletic predictions of the cascade model seem to be fulfilled by the data from several mammalian genera and the Sceloporus grammicus complex. Chain termination. It should be obvious that cascading speciation cannot go on indefinitely at a high rate. If nothing else happens, the process will end because the available niches become completely saturated with derived species. Or before that, the available chromosomal substrate for a favored class of rearrangements may be used up (e.g., centric fissions are no longer possible when there are no more bi-armed chromosomes in the karyotype), or natural selection may eliminate the factors which account for the reduction in heterozygote fertility; or other, more subtle mechanisms may work to otherwise change the genetic system so revolutionary speciation is no longer likely. The mechanisms for the first two kinds of chain termination are obvious; those for determining heterozygote fertility will be discussed

49 32 in detail, both because they are easily analyzed and because the degree to which fertility is reduced is clearly a controlling variable in the speciation process. However, the mutation rates for semisterilizing mutations must also be discussed, because the extent of the natural selection operating to modify heterozygote sterility is directly related to the frequency of heterozygosity, which, in turn, is related to the mutation rate for the heterozygously semisterilizing rearrangements. For the discussion to follow we will assume that the mutation rate is controlled by "imitator" genes, which Ives (1950) has shown to exist in Drosophila populations, probably more frequently in the wild than in the lab. The critical point in the selective interaction between mutation rate and the degree of heterozygote semisterility is that the mutation rate is likely to be raised by the amplification effect of the cascade process. This is because, all other things being equal, the frequency of speciation in a lineage will be directly related to the rate at which heterozygously semisterilizing rearrangements that might facilitate the speciation are generated, Mutator genes that increased the mutation rate would then probably be concentrated by the amplification effect. All would agree that selection will work to eliminate genes which reduce the fertility of their carriers, and that the magnitude of this selection will be directly related to the degree by which the fertility is reduced. However, even a considerably reduced fitness of the chromosomal heterozygote will have little effect on the mutator gene which caused the chromosome mutation after the first or second generation following the initial rearrangement event (assuming that the gene

50 33 and the rearrangement it "caused" are not closely linked). Furthermore, it is probably reasonable to assume that the mutations caused by mutator genes are the result of rarely penetrant effects of an allele whose normal functions are adaptive. Given this assumption, then selection will probably have little effect on mutation rate, at least during an active cascading sequence, when major shifts in gene frequency and even chance fixations are likely to result from the various sampling processes included under the rubric of the amplification effect. Assuming humans to be "normal," Polani et al. (1965) have estimated that new centric fusions involving chromosome 21 occur at a frequency of 2 x The frequency for all possible fusions in the human karyotype is then probably not over 1 x 10-4, assuming that all acrocentrics show similar probabilities of being involved in fusions. An increase in the mutation frequency by an order of magnitude, to 1 x 10-3, would also increase the rate or probability of chromosomal speciation by an order of magnitude, but even a mutation frequency of 1 x 10-3 will have minimal effects on the frequency of a rarely penetrant mutator gene. The selective disadvantage of a nearly fixed mutator gene would be approximated by the frequency of mutations it caused if heterozygotes had zero fitness. And, as heterozygote fitness improves, the selective disadvantage of the mutator gene diminishes, although in a complex relationship, since the net disadvantage must be figured over several generations. However, we can take the base mutation rate as a reasonable approximation of the selective disadvantage of a common mutator gene. Then, even with a disadvantage as great as s = 1 x 10-3, according to Crow and Kimura (1970), more than 9000

51 34 generations would be required to change a gene with no dominance effect (the simplest case) from a frequency of 0.5 to Given the capability of populations to increase exponentially if their overall fitness is high, a well adapted chromosomally derived new species should be able to saturate the available niche, and hence be ready to form further new species, long before selection would have a chance to greatly change the frequency of the mutator genes controlling the probability of that further speciation. And, it does not seem unreasonable to suppose that other, more subtle aspects of the genetic system will be modified similarly. However, selection for reducing heterozygote sterility will behave somewhat differently. Every chromosomal heterozygote will provide an opportunity to selectively increase fertility. With low mutation rates and no history of chromosomal speciation, these opportunities will be infrequent. However, during each episode involving fixation of a chromosomal difference, there will be a short period when a large fraction of the local population will be chromosomal heterozygotes; and, if a mutation once becomes fixed in a local population, there will be a much longer period of time when chromosomally heterozygous hybrids are formed with the ancestral stock. So, for as long as the population remains too small to benefit from the sink effect of a well established zone of parapatric hybridization, each chromosomally heterozygous hybrid formed will provide an instance of selection favoring the increase of heterozygote fertility. Also, note that if heterozygote fertility in the hybrid zone increases significantly, this will diminish the sink effect of the zone, which will allow longer

52 35 survival and greater dispersal to the genes. If the sink effect is initially strong, this should cause no problem to the speciation process, because the fertility increasing genes would still be lost in the sink. But if the sink is weak, such genes could eventually combine to improve fertility to the point where the sink was no longer functional, and the hybrid zone would decay into a condition of local polymorphism. If such an aborted speciation event occurs after several successful chromosomal speciation events in a sequence, it is quite likely that the basal rate for chromosome mutations in the terminal species would have been boosted to a level where mutations and the consequent heterozygosity were not uncommon. Nor is it unreasonable that there would be more than one aborted speciation attempt in the terminal species. These situations would insure that genes providing full fertility to the chromosomal heterozygotes would spread throughout the terminal species of the chain of derivation, thereby eliminating any further possibility of chromosomal speciation from this species and coincidentally also eliminating the selection pressure that would tend to reduce the rate at which new chromosome mutations were added to the population. Therefore, if the cascade is stopped by this intrinsic mechanism, the chain terminating species would almost inevitably carry many chromosomal polymorphisms of the kind usually fixed between more primitive species of its lineage and those of related lineages. Other aspects of the genetic system may be similarly affected during a cascade, but it is not as obvious what they might be or what changes one might predict as results of the cascade. However, the cascade model does make definite and testable predictions about the

53 36 phyletic relations one should find as a result of a cascade and about some of the changes in the genetic system that should occur during a sequence of derivation. These predictions will be tested by the radiation of crevice-using Sceloporus discussed below. However, it will first be useful to review other comparative data that are available which may also provide tests of these predictions. Evidence. To summarize the preceding discussion, several potentially testable predictions can be made from the cascading speciation model. These assume that chromosomal semisterility provides the initial isolation; other bases for the isolation are theoretically possible, but would be much more difficult to verify in practice so they will not be discussed here. The predictions are: 1) Under favorable circumstances when many niches are available for newly formed species, cascading speciation in a lineage may rapidly proliferate a large swarm of sister species; when otherwise comparable lineages in the same circumstances, but with more conservative genetic systems, form few if any new species. This is a direct result of the amplification process discussed on p ) If the cascading speciation is based on chromosomal semisterility, all species formed by the cascade will be chromosomally differentiated from one another by mutations which might be expected to reduce heterozygote fertility (although species which terminate sequences of derivation may carry apparently stable polymorphisms for these kinds of mutations). This follows from the basic revolutionary speciation model"-otherwise what function is served by the chromosomal differentiation?

54 37 3) Species formed by a cascade will generally have parapatric distributions or may even be sympatric. Species formed originally in a parapatric situation are unlikely to immediately acquire allopatric distributions. 4) Parapatric species formed by a cascade will be genetically isolated, even though no premating isolation may have been evolved and hybrids still show some degree of fertility. This follows from the analysis of the "sink effect" of parapatric hybridization (see p. 15). 5) Sequences of karyotypic derivation will coincide with sequences of phyletic derivation, and these will be more linear than highly branched. This is obvious, and branching patterns are discussed on p ) Sequences of karyotypic derivation will frequently go to extremes in geologically short times (e.g., a completely acrocentric karyotype will be produced if fissions are the main mutation type, or all chromosomes will become bi-armed if fusions are the main mutation type). This follows from the amplification process. 7) If there are gaps in a recent cascade due to extinction, the extinct species will likely have been formed late in the sequence, when species are expected to be rather ephemeral in comparison to either the terminal or relatively primitive species. This also follows from the amplification process (see p. 30). 8) Sequences of derivation which have terminated for other than ecological reasons will end in species which either have used up the karyotypic substrate for chromosomal differentiation or are polymorphic for the kind of karyotypic differences fixed between species

55 38 formed earlier in the sequence (see discussion p. 30). All of these predicted relationships or conditions are logically derived from the revolutionary speciation model, and so far as I know, none of them would be expected from any other model of speciation or would be accounted for by any other cytogenetic mechanism of which I am aware. Therefore, if these conditions and relationships can be demonstrated in suitable natural radiations of species, the demonstrations would tend to verify the model. However, for a radiation to provide useful tests, it should meet several criteria. It should contain many species and be recent enough that only few species formed by it are likely to have become extinct. The taxonomy of the species, in the radiation and their biogeographic relationships should be very well known. Most importantly, a great deal must be known both about the comparative population cytogenetics and the phylogenetic relationships of the species. And, finally, there should be available similar information from proper control groups against which the patterns found in the test radiation can be compared. Needless to say, very few groups have been well studied in any of these respects, let alone all. However, as outlined in the introduction of the present report, Sceloporus--and especially the radiation of creviceusing Sceloporus--are well enough known in most of the required aspects to provide nearly ideal tests of most of the predictions of the cascading revolutionary speciation model. However, before turning to my analysis of the crevice-using radiation of Sceloporus, it will be useful to review some of the non-reptilian radiations which might provide good test cases if more were known about them.

56 39 The now classic case of chromosomal "speciation" in insects is provided by the radiation of the viatica group of morabine grasshoppers (White et al., 1964, 1967, 1969; White, 1968, in press). Although the radiation is cytogenetically well known and some studies of hybridization have been made, both in the field and in the lab, there is no consensus regarding the genetic relationships of the chromosomally different populations or the details of their phylogenetic derivations (e.g., see White, 1968; Key, 1968). Since these differences in interpretation can best be resolved by those with first-hand knowledge of the organisms and their biologies, I will restrict my review to some of the better known mammalian radiations, But, unfortunately, even here the data are scattered and incomplete. And, unlike the lizard radiation to be discussed below, there are usually no good comparative backgrounds available for the studies to be analyzed against. The most detailed studies have all involved rodents, and the ones I will review have all been based on more than 100 specimens, though generally less than 500. In general, all rodent genera are karyotypically diverse, which correlates very closely with their limited vagility, as Arnason (1972) and White (in press) among others have noted. Therefore, there seem to be no really good rodent genera or groups which can serve as chromosomally conservative controls for the karyotypically variable stocks. However, among the better known groups and in terms of whole genera, possibly the most conservative of the large genera is Peromyscus, at least with respect to change in chromosome number. According to Hall and Kelson (1959), this genus contains approximately

57 40 40 continental species, although it is one of the ecologically most diverse groups--extending from the arctic tundra through all possible terrestrial habitats in North America almost to South America. Yet, among 20 species karyotyped (Hsu and Arrighi, 1968) only the 2n=52 nuttali, questionably placed in Peromyscus, deviates from a 2n=48. However, in other respects of the karyotype, Peromyscus is anything but chromosomally conservative, as the number of chromosome arms (i.e., NF) varies at least from NF=56 to NF=96 because of a great deal of interspecific and intrapopulational variation resulting from pericentric inversions (e.g., see Ohno et al., 1966; Sparkes and Arakaki, 1971; Te and Dawson, 1971; Bradshaw and Hsu, 1972; Lee et al., 1972). Presumably these polymorphisms are adaptive, as are the paracentric inversions of Drosophila, and may account for the great ecological plasticities of its species. On the other hand, Peromyscus is not notably speciose, considering its great ecological and geographic range. An interesting comparison with the conservative 2n Peromyscus is provided by the ecologically much more restricted and more-or-less deserticolous Perognathus (Heteromyidae), which has probably more than 35 species concentrated in the xeric areas of North America (Hall and Kelson, 1959; Patton, 1969b). Patton has karyotyped 11 of the species recognized by Hall and Kelson and found 2n's fixed among them ranging from 34 to 56 as a result of Robertsonian rearrangements, with other differences due to inversions fixed between populations (Patton, 1967a, b, 1969a, 1969b). One species is polymorphic for inversions. The most striking karyotypic variation was found fixed between six parapatrically distributed cryptic species conventionally included in P. goldmani

58 41 (Patton, 1969b). These cryptic species all differed by fixed centric fusions and/or inversions. Based on geographic relationships and minimal karyotypic derivations between species, Patton reconstructed a phylogeny in which the longest chain of derivation had five steps, with only the hypothetical ancestral karyotype not surviving in a present population. Two other one-step derivations trace from this ancestral stock, and a third one-step derivation branches from the second step of the chain of five species. The cryptic species are all now separated by ecological barriers (narrow flood plains inhabited by other Perognathus species). However, these barriers are not absolute, as three chromosomally hybrid individuals were taken in a sample of 221 from near these contact zones. By comparison with Peromyscus, Perognathus is much more speciose in proportion to its ecological and geographic range. Furthermore, the cryptic speciation in the goldmani complex appears to have been recent and rapid, and conforms to the generally linear pattern predicted by the cascade model. The mole-rats. Spalax (Spalacidae) show a similar picture, although the genus contains only two or three taxonomically recognized species. Chromosome numbers in this genus range from 2n=48 to 2n=62 (Matthey, 1959; Walknowska, 1963; Soldatovic et al., 1966a, 1966b, 1967; Raicu et al., 1968; Wahrman et al., 1969a, 1969b; Lay and Nadler, 1972). Detailed collecting in Israel and on the Golan Heights revealed 2n s of 52, 54, 58, and 60 in parapatrically distributed populations of S. "ehrenbergi." Unlike Perognathus, there seem to be no ecological barriers separating the populations, although they do show a clinal distribution corresponding to annual precipitation (Wahrman et al.,

59 a, 1969b; Lay and Nadler, 1972). Searches for contact zones revealed five hybrids between the 58 and 60 chromosome forms on Mt. Hermon, although these populations appear to be separated by the upper Jordan River at lower elevations. Two hybrids were found between the 52 and 54 chromosome forms, but none were found between the 52 and 58 chromosome forms, in spite of detailed searches in their contact zone. Nevo (1969) reports indications of behavioral isolation between the karyotypically different forms in laboratory crosses. On the other hand, Nevo and Shaw (1972) in an electrophoretic survey of 13 proteins controlled by 17 loci found no fixed genie differences between any of the chromosomal types, indicating a very close relationship between the populations and providing no test for barriers to gene flow between the populations. The lack of differences between populations may correspond to the generally low variability within populations. Wahrman et al., (1969a, 1969b) suggest that Spalax with a low 2n radiated into Israel from a center to the north or east which had a mesic climate, and that higher chromosome numbers were then fixed in a linear sequence as the animals encountered increasingly xeric conditions. Lay and Nadler (1972) accept this sequence of derivation and attempt to date the separation between the Israeli 2n=60 stock and a now disjunct but apparently related 2n=60 stock which has spread along the Mediterranean coast as far as central Libya, Assuming that the 2n=60 stock is actually the most recently derived, Lay and Nadler suggest that the two disjunct populations were separated for at least 10,000-25,000 years and that the sequence of karyotypic derivation must therefore have been older than that separation. However, at least until the Jordanian, Syrian,

60 43 and Lebanese populations are sampled, it seems that the alternative possibility, i.e., that the 2n=60 population is primitive in the Israeli radiation, cannot be ruled out. However, in either case, the Robertsonian sequence is essentially linear and, except for the possible extinction of the 2n=56 population (if it is not found on the east side of the Jordan River--possibly below the Sea of Galilee), the parapatrically contacting populations can be traced in a linear sequence. This is of course fully consistent with the cascade model, and, if the 2n=56 population is extinct, this could easily be due to an acceleration of chromosomal differentiation during the cascade. Probably the most striking example which might be ascribed to the acceleration effect of cascading speciation is the karyotypic variation among the cotton rats (Sigmodon: Cricetidae). Until the revision of Zimmerman (1970), based largely on karyology, the genus was thought to contain only five species: hispidus, ranging from South America through all of mainland Mexico and throughout the southern United States; ochrognathus, found in the SW United States and NW Mexico; leucotis, found on the Mexican plateau; alleni, found in southern Mexico; and fulviventer, found in the Sierra Madre Occidental of Mexico and extending northward into Arizona and New Mexico. Of these five taxa, three are karyotypically monomorphic and have identical 2n's: ochrognathus, 2n=52, FN=66; alleni, 2n=52, FN=64; and leucotis, 2ns=52, FN=52 (Zimmerman, 1970). The other two taxa are karyotypically variable: fulviventer, with an FN of 34, shows a Robertsonian polymorphism for a 2n range of 28 to 30 (Lee and Zimmerman, 1959); while different hispidus populations show fixed karyotypes of 2n=52, FN=52

61 44 (identical to leucotis); 2n=28, FN=28; 2n=24, FN=38; and 2n=22, FN=38 (Zimmerman and Lee, 1968; Zimmerman, 1970). Based on these striking differences among hispidus, Zimmerman (1970) applied the names mascotensis to the 2n=28 population, found in the Mexican Pacific coastal states of Jalisco through Oaxaca, and arizonae to 2n=24 and 2n=22 populations found west of the Continental Divide from Jalisco through central Arizona. The revised hispidus, then, has a range from South America to the southern United States to the east of the mascotensis and arizonae populations. Karyotypic samples of hispidus, sensu stricto were from localities as far separated as Panama, Florida, North Carolina, Kansas and SE Arizona and showed essentially no variation, Based on this wide distribution, and the fact that three other species have the same 2n, one of which also has the same FN, there is little doubt that the 2n=52 pattern is primitive in the hispidus complex. Zimmerman (1970) would then derive all of the reduced 2n karyotypes in Sigmodon from this 52 chromosome pattern in a single sequence of derivation, more or less in the sequence fulviventer (2n==30-28), mascotensis (2n=28), arizonae (2n=24), arizonae (2n=22). However, if the cascading speciation model is correct, the polymorphism of fulviventer should be a chain termination situation, and this species should not have given rise to any further derivatives. In this respect it is significant that Johnson et al. (in press), in an electrophoretic study of protein variation in Sigmodon, provide convincing evidence that fulviventer has evolved quite independently of the hispidus complex derivation. Presumably, then, two independent sequences of derivation are involved, one from a 2n=52 ancestor to terminate in the polymorphism

62 45 of fulviventer, which has apparently left no surviving intermediates; and the second within the hispidus complex, again which has left few intermediates until low 2n's were evolved and presumably the possibilities for further reduction in 2n's were considerably reduced. The lack of intermediate forms suggests that these populations either never grew very large and were completely displaced by the most highly derived end products, which is of course a prediction of the cascading speciation model. Many other mammalian radiations could be analyzed in a similar manner, but all suffer from the same defects found in the cases discussed above: 1) sample sizes are generally too small and poorly distributed to allow one to safely assume that all of the chromosomal variation within the group has been detected; 2) in most cases the degree of isolation between the populations has not been determined--i have assumed for the discussion above (as have most other workers) that the chromosomally differentiated allopatric populations are in fact good species, but this has rarely been proven; 3) very few of the workers are adequately trained in both cytogenetics and systematics; 4) only very rarely have attempts been made to derive phylogenetic relationships independently of the karyotypic relationships; and 5) mammals as a group are so chromosomally diverse that there appear to be no radiations which provide otherwise closely comparable groups which can serve as a natural experiment in chromosomal variability versus chromosomal conservatism (at least not to someone who is not an expert in the systematics of the groups concerned). However, as will be seen below, the radiation of Sceloporus suffers from few of these defects. Although, before this is

63 46 described, there remains one extension of the speciation model to be discussed.

64 47 Karyotypes as a Phyletic Tool If fixations of chromosomal rearrangements which potentially induce partial sterility when heterozygous are in fact closely involved in some speciation processes, then analysis of this type of interspecific chromosomal variation in a highly variable group such as Sceloporus can provide a potentially extremely powerful indication of phylogenetic relationships. If a primitive karyotypic configuration can be determined for the variable group, and sequences of karyotypic derivation can be traced within it, these sequences can be used to trace sequences of phylogenetic derivation of the species characterized by the different karyotypes. One would predict, in general, that chromosomally identical species have usually speciated according to the more conservative allopatric mode, while, on the other hand, observed karyotypic differences that were not adaptive as balanced or transient polymorphisms are likely to have been fixed during episodes of revolutionary speciation which were aided by their fixation. Furthermore, the karyotypically derived population will have been the one to undergo the genetic revolution, and because the incipient post-revolution species will have been numerically small in comparison to the parental species, in any early competition between the two, the incipient species will be ecologically and behaviorally displaced with respect to the parental species. Therefore, the sequence of karyotypic derivation may serve as a guide to the evolution of the species from primitive to advanced or specialized conditions. Then, karyotypically conservative species may be expected to be more like the primitive stock from which the radiation began than will karyotypically highly derived species.

65 48 Also, if genetic revolutions in a sequence of karyotypic derivation have played important roles in the evolutionary differentiations of the species in the sequence, then the species' place in the progression of karyotypic derivation probably also will indicate its place in the sequence of phylogenetic derivation, The karyotypic variability of Sceloporus provides an excellent test of the validity of this approach. Will an analysis of the karyotypic variation in Sceloporus according to the principles developed above provide useful and hopefully unsuspected insights for reconstructing the phylogenetic history of the genus, which can then be tested using independent lines of evidence? If it does, then we may begin to have some faith in the principles used in the reconstruction. However, the genus Sceloporus is far too large and far too much karyotypic data are available for it to allow the whole genus to be treated in this manner in one paper. Therefore, in the present report the treatment will be restricted to Smith's (1939) megalepidurus, grammicus, and torquatus (= poinsettii) species groups, plus the species asper (currently placed in the formosus group) and clarkii and melanorhinus (currently placed in the spinosus group), which I believe and will attempt to show below form a complete natural grouping within Sceloporus.

66 49 METHODS AND MATERIALS Cytological Procedure Cytological material used in this study was prepared over several years, ranging from 1964 to 1971, and under a variety of laboratory conditions, therefore preparation techniques have varied considerably during the course of the study. However, most chromosome spreads were made using basic techniques similar to those of Evans et al. (1964), Patton (1967a), Bianchi and Contreras (1967), Hsu and Patton (1969), and Ford and Evans (1969). A few preparations were made with the temporary aceto-orcein squash technique (Darlington and Le Cour, 1962) and in rare instances these were preserved with the dry-icequick-freeze procedure. Tissues examined were generally testes, from sexually active males, and bone marrow and spleen from females. In nonreproductive males all three tissues were generally used. To minimize preparation time and maximize the number of individuals examined, all tissues from a given animal were generally mixed together in a single cell suspension. To further maximize productivity, two to four preparations were processed in parallel and earlier and later stages of three parallel sets were overlapped. By using these expedients, the average time used in processing an individual from sacrifice to preliminary karyotype determination was usually kept to less than a half hour. To allow rapid feedback to the collecting program and to insure that the animals were in close to optimal condition when karyotyped, most preparations were made in the field, generally in convenient motels or in laboratory space kindly provided by local institutions (see

67 50 acknowledgments). Unfortunately, since some preparations were made under primitive conditions and the microscope used to control their quality was not especially good, not all are adequate to resolve the details of microchromosomal morphology. Also, Sceloporus grammicus from the Teotihuacán area of the Valley of Mexico were generally so wary that many had to be killed for capture. These were immediately placed on crushed ice in a Styrofoam container and were usually processed within six hours. In all cases the killed lizards could be unequivocally scored as to which karyotype population they belonged, However, because they did not benefit from a colchicine pretreatment, and because of the inevitable (but surprisingly slow) cell degradation, the microchromosomes could not always be counted, nor was it always possible to tell which of the macrochromosomes in fissioned karyotypes retained the metacentric condition. Where these technical problems are pertinent they will be mentioned. Most preparations were preliminarily scored as aceto-orcein stained wet mounts in the field. Counts of chromosome number were generally made from at least three good spreads from each animal. In reproductive males, counts were usually made from diakinesis figures, but frequently at least one mitotic figure was found to check for obvious shifts in centromere positions. Slides were then washed clean with methanol and stored dry and unmounted until they were permanently mounted in the home laboratory. For cases where preliminary determinations were not made, were questionable, or were particularly critical (as in the contact zone study by Hall and Selander, in press), the slides were re-examined in the home lab and usually at least three more

68 51 good figures checked and their coordinates on the slide recorded. In many cases slides from other animals were also re-examined, and frequently more than the minimum number of figures were inspected. For detailed comparisons of karyotypes and for preparation of the figures presented here, photographs were taken with either the planapo-brightfield or -phase contrast 100X objectives of a Zeiss Ultraphot II, generally using 4" x 5" Kodak 4154 Contrast Process Ortho film developed in D-11. All photographs used for karyotypes were printed at 3000X (as determined by calibration with a stage micrometer) on Kodabromide paper chosen and processed to provide maximal use of the grey scale. Where feasible, karyotypes were photographed for several individuals of each species and usually represent all of the available subspecies in my material. For some individuals, several karyotypes were prepared to allow intra-individual variation to be compared with inter-individual variation. This is especially important for comparisons of microchromosomal morphology, where much of the observed variation is clearly preparation artifact.

69 52 Material Examined Detailed locality data for all karyotyped specimens except Sceloporus grammicus are presented in the Appendix. Detailed localities for grammicus will be found in Hall and Alvarez (in prep.) and are mapped there and in Hall and Selander (in press). Here I simply list the number of grammicus belonging to each karyotype population examined from each state. Species are arranged in the Appendix by species groups following Smith and Taylor (1950) as emended by more recent publications and the present work. All species and subspecies are listed, whether karyotypic data exist or not. The following format is used for each locality entry: state name in caps, airline distance from a principal town listed in an excellent topographic road atlas (Caminos de Mexico, 3rd ed., Compania Hulera Euzkadi--B. F. Goodrich, Mexico, D.F. 1967), the elevation in meters, and the Universal Transverse Mercator grid reference coordinates taken directly from the 1:500,000 sheets of the Carta Geografica de la Republica Mexicana, Primera Ed. (pub. by the Comision Intersecretarial Coordinadora del Levantamiento de la Carta Geografica de la Republica Mexicana, Tacubaya, D.F.). This is then followed by the number of specimens examined from that locality. In general, particularly where they are near roads or other surveyed features, the grid references are probably accurate to ±1 kilometer. Compass distances given are only approximate. In the Valley of Mexico, most localities were plotted on the 1:25,000 topographic maps of the Departamento Cartagraphico Militar, in some cases checked by reference to stereoscopic aerial photographs, both kindly made available through the Departamento de Prehistoria of the

70 53 Institute Nacional de Antropologia e Historia. For work in the Archeological Zone at San Juan Teotihuacan, Dr. Rene Millon kindly allowed the use of his exceptionally detailed 1:6000 topographic maps (Millon, 1970). Species were identified following Smith and Taylor (1950), Smith (1936, 1939), or by more recent taxonomic works where appropriate. In some cases identifications have been confirmed by Smith himself. Specimens not preserved frozen for future biochemical studies or used by Hall and Selander (in press) are entered in the herpetological collections of the Museum of Comparative Zoology or Southern Illinois University, Edwardsville.

71 54 RESULTS The "Standard" or S Karyotype All species reported here except those belonging to the megalepidurus group, S. asper, the clarkii group, and the karyotypically derived populations of the grammicus complex have indistinguishable karyotypes, which will be termed the "standard" or S pattern (Figs, 2, 3, 4 (a, b, c, d), 5a, 6b, and 8a). This is the karyotype described by Cole et al. (1967) for S. jarrovii (see also Axtell and Axtell, 1971) and poinsettii, although my arrangement of the chromosomes differs somewhat from theirs. Females with the standard karyotype have 32 chromosomes, including 12 macrochromosomes and 20 microchromosomes; while males have only 19 microchromosomes due to a sex chromosomal heteromorphism of the x 1 x 2 y type. Ordered in sequence from largest to smallest, macrochromosome pairs 3 and 4 are almost exactly metacentric and are approximately identical in size. These cannot be reliably distinguished from one another, although in most mitotic spreads the remaining macrochromosomes can be. Pairs 1, 5, and 6 are all slightly submetacentric but conspicuously differ in size from one another and from pairs 3 and 4. Pair 2 is only slightly smaller than 1, but it is conspicuously submetacentric, with the long arms about 1.5 times the length of the short. In particularly good preparations, minute satellites can be seen at the tips of the long arms of pair 2. These are presumably homologous with the satellites reported for the similar chromosomes of other Sceloporus (Lowe et al., 1967; Cole and Lowe, 1968; Cole, 1970, 1971a, 1971b, 1972; Jackson and Hunsaker, 1970), some other iguanids (Jackson and Hunsaker, 1970), and possibly even with

72 55 those reported in primitive teiids (Gorman, 1970). In diakinesis arrays (Figs. 9a, b) four size classes may reliably be determined among the macrochromosomal bivalents. Pairs 1 and 2 are close enough in size that their identification is equivocal, 3 and 4 are indistinguishable from one another, but are distinctly smaller than 1 and 2, and 5 and 6 differ enough from each other and from the larger bivalents to be readily identifiable. The microchromosomal morphology of the S karyotype is less certain because of preparation artifacts and the fact that the sizes of the smaller ones approach the limits of optical resolution. Yet, a careful examination of the clearest preparations from several species reveals an apparently consistent pattern of microchromosomal morphology (Figs. 2, 3, 4a, b). Although the microchromosomal structure of the remaining species assumed to have the S karyotype cannot be seen as well, their microchromosomes can be arranged in the same size sequence, and I have no reason to believe that they differ from the clearer preparations. In males the y chromosome is the largest microchromosome and it is submetacentric (short arm more than half as long as the long arm). The unpaired x 1 (the original x chromosome see discussion below), designnated #8 in the male karyotype (Fig. 2), is acrocentric and is about the size of the second or more probably third smallest pair of acrocentric to subacrocentric microchromosomes. However, exactly which of the individual microchromosomes in this size range is unpaired cannot be determined with certainty in any mitotic spread. The x 2 chromosome, designated #7 in the male karyotype (Fig. 2), is clearly one of three

73 56 largest subacrocentric microchromosomes, and is probably slightly larger and slightly more conspicuously subacrocentric than the other two, which are designated pair 9. These sex chromosomes form a clear trivalent in male diakinesis figures (Figs. 9a, b; 10). Besides the sex chromosomes, the microautosomes then include 5 pairs of acrocentric to subacrocentric chromosomes and 3 pairs of submetacentric to metacentric chromosomes. The acro's are arrayed in decreasing order of size from pairs 9 through 13. Pair 9 is subacro, 10 acro, 11 subacro, and 12 and 13 probably acro. Pair 14 is submetacentric and probably between pairs 9 and 10 in size. Pairs 15 and 16 are probably metacentric and somewhat smaller than 14. All karyotyped species of the torquatus group (Figs. 2, 3) appear to have this microchromosomal pattern, as do all species and populations of the grammicus group except the FM2 population of the grammicus complex (see below).

74 57 The "Enlarged Micro" or Em Karyotype In species with the S karyotype, the y chromosome is distinctly larger than the three large subacrocentric microchromosomes which include the x 2 chromosome (#7 and pair 9). Three species: megalepidurus (Figs. 4f, 9e) and pictus (Figs. 4e, 10h), which form part of the present megalepidurus group (Smith and Lynch, 1967), and asper (Figs. 4g, 8d), currently placed in the formosus group (Smith and Taylor, 1950), all have karyotypes indistinguishable from S except that one of the larger microchromosome pairs (probably 9) is replaced by a much larger pair of subacrocentric chromosomes which are intermediate in size between the y and pair 6. These enlarged microchromosomes are similar to or slightly shorter than the long arms of pair 6. The chromosome pattern distinguished by these comparatively enlarged microchromosomes is hereby designated the Em karyotype, and will be discussed further below. Other karyotyped species in the megalepidurus and formosus groups (as these are presently defined) are karyotypically quite different from the Em pattern, indicating that these groups probably are not natural as they stand (Hall, in prep., and see below).

75 58 Karyotypic Diversity of Sceloporus grammicus Introductory comments. By far the greatest karyotypic variation found among the species characterized by the microchromosomal pattern of the S karyotype occurs between and within populations now placed in Sceloporus grammicus. Chromosome numbers of the approximately 1300 grammicus studied range from the 2n=31 of standard males to a maximum of 2n=46 in some females of the FM2 population. Fig. 5 clearly shows that most of this variation is Robertsonian and results from centric fissions. Table 1 lists the number of species of each lizard family known to have karyotypes with 12 approximately metacentric macrochromosomes (see second to last column). Most of these species also show a relatively constant pattern of macrochromosome size and centromere placement. Thus pairs 1 and 2 are usually similar in size with 1 nearly metacentric and 2 clearly submetacentric and sometimes satellited on its long arms. Pairs 3 and 4 are usually indistinguishable-- both are nearly exactly metacentric and are distinctly but not greatly smaller than 2. Pair 6 is usually rather small and slightly submetacentric, and pair 5 is generally intermediate in size between 4 and 6 but usually closer to 4 and also slightly submetacentric. Fig. 6 compares the agamid, Agama caucasica (6a), and a standard grammicus (6b) to demonstrate this point. More importantly, all 12 karyotyped species of live-bearing and crevice-using Sceloporus also have exactly the standard grammicus macro chromosomal pattern. It would patently be absurd to suppose that all of the species with 12 metacentric macrochromosomes listed in Table 1 (and in the present report) independently

76 59 evolved their similar macro chromosomal patterns from ancestors with 46 or 48 chromosome karyotypes. And it would be even more implausible to suppose that this supposedly primitive karyotype survives in Sceloporus only as one end of a polymorphic series in a small fraction of one species (FM2 grammicus) and in one other species (S. merriami--cole, 1971a) which is very distantly related to grammicus. It is much more reasonable to suppose that the many species with 12 metacentric macrochromosomes retain a primitive pattern. A complete fissioning of the standard 12 metacentrics would always give 24 similar appearing acrocentric chromosomes, whatever the sequence of the fissioning; but, on the other hand, the various acrocentrics in a 46 or 48 chromosome karyotype could be fused in numerous combinations, and only a few would produce the size and arm ratios observed in the standard karyotype. Furthermore, whatever opinions have been held in the past about the improbability of centric fissioning as a mode of karyotypic evolution (e.g., Matthey, 1949; White, 1954, 1957; Cole, 1970, 1971a); Webster et al. (1972) have clearly documented a case of centric fissioning in the Iguanidae. The conclusion is thus inescapable that the female 2n=32, male 2n=31 standard karyotype is primitive in the grammicus radiation, and that karyotypes with higher numbers of macrochromosomes must have been derived from the standard karyotype by centric fissioning. These derived karyotypes in grammicus are described in order of the increasing amount of their fissioning. Ecological and geographic distributions of the various grammicus populations will be analyzed in detail in Hall and Alvarez (in prep.), and are summarized here in Tables 2 through 8.

77 60 Collection localities are mapped in Hall and Selander (in press) and in more detail in Hall and Alvarez (in prep.). "S" or standard grammicus. The grammicus populations characterized by the S karyotype (Figs. 4b, 5a, 6b) are themselves designated S, or standard, and have the widest distribution of any of the karyotypically distinct grammicus populations. These S populations range geographically from northern Coahuila, discontinuously, to southern Oaxaca (Fig. 7a, 7b, and see also Figs, 2 and 3 in Hall and Selander, in press), and ecologically from the upper edges of the Chihuahuan Desert in southern Coahuila and Zacatecas to mountain rain forest in Oaxaca and Veracruz (Table 2). Most of the karyotypically derived populations of grammicus are found across the middle of the range defined by standard grammicus and appear to replace them geographically, thereby separating northern and southern populations of standard grammicus. Also, though geographically close to the continuous southern population of standard grammicus, intervening populations of chromosomally derived forms appear to separate the standard population on the floor of the Valley of Mexico from the standard populations to the south and east of the Valley. P1 or polymorphic--1 grammicus. Populations given the P1 designation are polymorphic for a fission of chromosome 1 (the FIS-1 mutation,. Fig. 5b) and are found only above 3200 m elevation on the three mountains, Tlaloc, Ixtaccihuatl, and Popocatepetl, that form the eastern divide of the Valley of Mexico (Table 4). The P1 population appears to be completely surrounded by the population of F6 grammicus (described next, below) which encircles the mountains at intermediate elevations

78 61 (Figs. 7a, 7b), and see also Fig. 3 in Hall and Selander, in press). Most individuals in the P1 population have S karyotypes, but some are characterized by heterozygosity for the FIS-1 mutation, and three individuals were found to be homozygous for the fission. The frequency of this mutation is in a total of 301 individuals from throughout the P1 range above the F6 grammicus. The FIS-1 mutation was found in all areas where reasonably large samples were taken (18 was the largest number of individuals sampled from any 1 km area which did not include at least one FIS-1 heterozygote). There are hints that the frequency of the fission may exhibit geographic or microgeographic variation within the range of P1, but present sample sizes and localities are poorly chosen for exploring this question. Any treatment of this possibility must depend on further collecting and should be designed to distinguish between differences in frequency due to drift in local breeding populations vs systematic differences due to selection along geographical or ecological gradients. However, it is clear that the FIS-1 polymorphism is well established and widespread in the P1 population. Clearly, the FIS-1 polymorphism is not selectively disadvantageous, or it would not have spread over the approximately 2,500 km 2 range of P1. I have not yet thoroughly studied meiotic assortment in FIS-1 heterozygotes, but a check of 500 second meiotic metaphase cells from one Individual showed a frequency of malassortment products from the fission trivalent (3 spreads showed duplications and 8 showed deficiencies for arms of the fissioned chromosome 1). No cells were scored that were aneuploid for only one other macro-

79 62 chromosome. If these counts represent an actual rate of malassortment and are not an artifact of preparation, and they are typical for all heterozygotes, and if the aneuploid sperm function in fertilization, then there should be a selective disadvantage for the heterozygous condition in males. Of course, I have no information on meiotic assortment in female meiosis. If the fission was genetically neutral, and the single specimen examined was typical, one would think that the presumed anti-heterozygote selection should have been enough to eliminate it from the population soon after its origin. Four counter possibilities are offered: 1) aneuploidy for such a large block of chromatin (one chromosome arm of pair 1 approximates 10% of the haploid genome) may be gametically lethal so that aneuploid-1 sperm do not compete in fertilization, and the mutation is therefore effectively neutral in males; 2) the fissioned condition exhibits meiotic drive (White, 1968); 3) some adaptive gene mutation is closely linked with the fissioned centromere; or 4) fission heterozygosity alters chiasma formation or localization in some adaptive fashion (but I have seen no obvious disturbances in chiasma localization in the FIS-1 heterozygotes). No attempt has yet been made to test any of these questions experimentally.

80 63 F6 or fission-6 grammicus. Populations designated F6 are all characterized by fixation of a centric fission of chromosome pair 6 in the standard karyotype (the FIS-6 mutation. Fig. 5c). F6 populations have a much wider distribution than does the P1 population. As indicated by my collections, F6 are continuously distributed in the humid forests of the central section of the Sierra Volcanica Transversal from western Michoacan to the eastern side of the Valley of Mexico (Figs. 7a, 7b; see also Figs. 2 and 3 in Hall and Selander, in press). On the east side of the Valley of Mexico, F6 occupy the humid forest belt between the S population found on the Valley floor and east and south of the Valley below about 2400 m, and the Pi population which is found above about 3200 m along the eastern divide of the Valley. An apparently disjunct population of F6 was found on the Nevado de Colima in Jalisco, to the west of the continuous range; while to the north, similarly disjunct populations occur along the intermediate eastern slopes of the Sierra Madre Oriental in San Luis Potosi, on a high peak above the town of Marcela in Tamaulipas, and near springs in three deep canyons in north central Nuevo Leon. All of these F6 populations inhabit what seem to be the most humid forest associations in their respective areas (Table 3). Oaks seem to be an important component of most forests where F6 were collected. Aside from the obvious hybrids discussed below and by Hall and Selander (in press), only two FIS-6 heterozygotes have been karyotyped. Both were from the north side of Cerro la Malinche in southeastern Tlaxcala, but were separated by 12 km distance and 900 m in elevation. Both heterozygotes came from collections that contained

81 64 respectively 7 and 8 karyotypically standard individuals. A third sample of 5 standard individuals was taken at an intermediate locality and elevation. Three possible explanations for these heterozygotes are offered: 1) the heterozygotes resulted from separate, fairly recent fission events, or 2) the heterozygotes represent a rare polymorphism of that area, or 3) the heterozygotes represent surviving hybrids with an almost completely swamped F6 population that used to extend east from Cerro Tlaloc through the comparatively humid forests of Tlaxcala to La Malinche during more humid times. More sampling and/or biochemical and morphological studies of the heterozygotes will be required before any of these possibilities can be assessed. Meiotic assortment has not yet been studied in any of the F6 hybrids or in these heterozygotes. F5 or fission-5 grammicus. The fission-5 karyotype is standard except for fixation of a centric fission of chromosome pair 5 (the FIS-5 mutation. Fig. 5d). Ten individuals from two localities in western Chihuahua, separated by about 30 km, had this karyotype (Fig. 7a, see also Fig. 2, Hall and Selander, in press). The F5 lizards were found living on fallen logs in the pinon-juniper woodland association (Table 5). Presumably these samples are representative of a population of unknown extent on the northwestern border of the distribution of grammicus. F5+6 or fissions 5+6 grammicus. The F5+6 karyotype is characterized by fixations of both the FIS-5 and FIS-6 mutations (Fig. 5e), Two widespread and possibly connected populations are characterized by this karyotypic pattern (Fig. 7a, see also Fig. 2, Hall and

82 65 Selander, in press). One is a high elevation population, centered on the dry plateau lands of San Luis Potosi, Guanajuato, Queretaro, and northern Hidalgo, Ecologically this population ranges from the upper Chihuahuan Desert, where the lizards inhabit Yucca, Agave, Opuntia, and occasionally mesquite trees, to comparatively dry oak and pine woodlands at higher elevations and in the southeastern parts of its range, where the lizards are found on logs and trees (Table 6). The second, a coastal population, was found sporadically from the lower Rio Grande Valley of Texas and Mexico south along the coastal hills and foothills of the Sierra Madre Oriental. I do not know if these sampled populations are now connected, as the area has been too poorly collected to tell. These lizards were found only on large mesquite trees along arroyos or, in one case, on scrub oak at the top of the coastal Sierra San Carlos in Tamaulipas (Table 6). It also seems possible that the coastal and plateau populations either are now or were in the past united through connections in the area of southern Tamaulipas, northern Veracruz, and northern Hidalgo. A few grammicus in museum collections were taken from these areas, but I do not yet have any karyotypic data from them, nor have I determined the validity of the localities. An almost certainly introduced F5+6 population is found within the city of Kingsville, Texas. Its ancestors were probably brought in on "ironwood" logs esteemed by the local ranchers for fence posts, which were presumably cut in the once extensive wooded areas along the flood plain of the lower Rio Grande River. Grammicus were seen on these

83 66 mesquite-like trees in the Santa Ana Wildlife Refuge, Alamo, Texas, which is one of the few areas where the trees still survive in any number. Since grammicus from near Rio Grande City, upstream, were F5+6, I assume that the lizards from the Alamo locality also belong to this population, although I was not able to karyotype any. FM or multiple fissions grammicus. The populations given this designation are cytogenetically by far the most complex of any lizard material I have examined; and most unfortunately, the average quality of the available preparations is poorer than that of most of the other forms treated here, even without considering material from animals which were dead for several hours before being processed. Furthermore, laboratory analyses of this material are not yet complete. However, some tentative generalizations about the distribution and cytogenetics of these populations can be made at this time. The FM designation is given to karyotypes in which most or even all of the macrochromosomes have fissioned and to the populations characterized by these karyotypes. FM grammicus have been collected only in an area approximately 100 km square straddling the Hidalgo-Mexico state line (Fig. 7a, 7b, and see Figs. 2 and 3 in Hall and Selander, in press), and all of the collection localities were either from semicultivated areas where the lizards lived on Opuntia, Agave, or occasionally on Yucca or trees and/or from edificarian habitats (Table 7 and Table 8). Except for unquestioned hybrids and backcrosses with the Standard grammicus in the hybrid zone described below, all FM individuals are homozygous for fissions of macro chromosome pairs 2 (FIS-2), 5 (FIS-5), 6 (FIS-6), and one of the two pairs 3 and 4, which cannot be

84 67 reliably distinguished in the S karyotype (the fissioned pair is arbitrarily designated 3 and the mutation FIS-3). Given the limitations of the available material, it appears that the FM grammicus may actually be subdivided into two cytogenetically and geographically distinguishable populations. Ten grammicus from the western and two northernmost areas where FM individuals were collected, besides being homozygous for the four fissions, FIS-2, FIS-3, FIS-5, and FIS-6, were polymorphic for fissions of the two remaining pairs of metacentric macrochromosomes: FIS-1, ~ 50% frequency and FIS-4, ~ 10% frequency (Fig. 5f). Although I have not been able to count the microchromosomes of all of these individuals with certainty, where I am confident of the counts, all had the standard 19 or 20 micros, depending on their sex. Lizards from these northern and western localities are designated FMl to distinguish them from the bulk of the FM specimens, described below and designated FM2, which all seem to show an even higher state of chromosomal fissioning. All of the grammicus falling into the FM2 category were collected in an area about 55 x 20 km 2 between Pachuca (state of Hidalgo) and San Juan Teotihuacan (state of Mexico). These FM2 differ most importantly from FMl and all other crevice-using Sceloporus by having an extra pair of microchromosomes, presumably generated by centric fissioning of one of the metacentric micros (the FIS-m mutation) (Fig. 5g). Other differences of FM2 from FMl are a higher frequency of FIS-4 (~50% frequency, rather than ~10%), and the complete or nearly complete fixation of FIS-1 (frequency 50% in FMl). Whether the metacentric-1

85 68 condition is present at a low frequency in the FM2 population or completely absent will be impossible to determine without better material than I presently have available. The arm ratios of pairs 1 and 4 in the Standard karyotype are similar, and when only one or two metacentric chromosomes are present in a somewhat fuzzy mitotic spread from FM2, it is difficult to decide to which of the two pairs the metacentric(s) should be assigned. However, as nearly as can be determined, the majority of FM2 with only one metacentric were heterozygous for the metacentric of Standard pair 4, although a few of the metacentrics might have been large enough to fall in the pair 1 size range. Among the individuals with two metacentrics, two reproductively mature males each showed two macrochromosomal trivalents in diakinesis spreads, a clear indication that their two metacentrics were non-homologous. However, both individuals came from near the contact zone with Standard grammicus, and in one of them both trivalents seemed to fit in the 3-4 size class, suggesting that this might be a backcross individual (see discussion on hybridization below). The second doubly heterozygous male, although one of its metacentrics is almost certainly in the pair 1 size range, showed one or two extra microchromosomes beyond the extra pair typical of all FM2 in mitotic spreads and one or two extra "bivalents" in diakinesis spreads. This lizard can also be reasonably interpreted as a backcross, with the extra microchromosome(s) resulting from meiotic malassortment in the hybrid parent (it should be remembered that Standard grammicus have 16 microautosomes while the FM2 have 18). Finally, two FM2 females which could not be definitely identified as backcrosses showed

86 69 two metacentrics in the pair 3-4 size range and one in the pair 1 size range. However, both did come from near the contact zone so that the possibility that they are backcrosses cannot be discounted. Hopefully, the electrophoretic analyses of these specimens being carried out by Sheldon Guttman will eventually allow them to be precisely allocated. We may then tentatively conclude that there are actually two cytogenetically distinct FM populations: FMl, which has the standard grammicus microchromosomal pattern (19 in the male, 20 in the female), a polymorphic FIS-1 condition with the metacentric present at about a 0.5 frequency, and a low frequency (0.1) polymorphism for FIS-4, and therefore with 2n's in the female usually ranging between 40 and 42 but possibly going as low as 38; and FM2, with 21 microchromosomes in the male, 22 in the female, a fixed or nearly fixed FIS-1 condition, and a polymorphic FIS-4 present at about a 0.5 frequency, and therefore with 2n's in the female usually between 44 and 46. Further reports on the cytogenetics of the FM populations will be deferred until more collections and better preparations are available.

87 70 Contact Zones, Hybridization, and Reproductive Isolation in grammicus General. The six or seven cytogenetically distinctive "races" of grammicus show a mosaic distribution in Mexico (Fig. 7a, 7b) see also Figs. 2 and 3 in Hall and Selander, in press). Some form of grammicus can be found, at least in scattered local populations, almost anywhere in Mexico above 1000 m elevation north of the Isthmus of Tehuantepec where their preferred escape cover, wood or plant crevices can be found. On the plateau they seem to be completely excluded only from the hottest deserts; and, on the other hand, some grammicus populations extend almost to sea level to the east of the plateau. Furthermore, grammicus do not always seem to be strictly limited by the absence of their preferred cover, since I have occasionally found them using rock crevices. Therefore, because of their wide distribution on the Mexican plateau, most of the chromosomal races must at least occasionally contact neighboring populations, if they are not in fact constantly in contact. Yet, despite these probable contacts, I have found neither overlaps nor wide zones of polymorphism between karyotypically distinctive populations. (Although it is barely possible that the FMl population represents the latter situation.) For example, in the northern half of Mexico, based on karyotyped specimens, two distinctive races have been found within about 50 km of one another in several areas: F5+6 and S, in central Nuevo Leon and also near Ciudad Zacatecas; F6 and S, at the southern border of Nuevo Leon and Tamaulipas; and F6 and F5+6, in eastern San Luis Potosí. Similarly, F5+6 and FMl, and FMl and FM2 collections are separated by no more than about 70 km. In none of these areas is there evidence for

88 71 geographic or ecological barriers which would separate the respective populations. There simply has not been an opportunity to do the necessary collecting in the intervening areas to establish the nature of the contacts. Also, although the intervening terrain is some of the most inaccessible to collectors in all of North America, there seem to be no vegetational, physiographic, or climatic barriers separating the F5 grammicus of Chihuahua from the S population of Durango. The failure to find any indication of gradual transition from one cytogenetic system to another in any of these cases must be significant. However, in several areas of the Valley of Mexico (see Fig. 3, Hall and Selander, in press), intensive collecting has pinpointed geographic contacts between three sets of karyotypically distinct populations: 1) between Pi and F6 in five separate transects on the eastern divide of the Valley-and in two other transects the separation is no more than 2-4 km (all these are described in Hall and Selander, in press, and will be further discussed by Moody et al., in prep.); 2) between S and FM2 in two areas of the valley of San Juan Teotihuacan (see below; these will also be described in detail in later works); 3) between S and F6 north of Cuernavaca (not yet studied in detail). Lizards heterozygous for the chromosomal conditions fixed between their respective "pure" populations were recovered from each of these contacts and are presumed to represent hybrids between the pure populations. In each of these contacts (except the incompletely examined S x F6) hybrids were found in belts of parapatric contact no more than 500 meters wide. This width does not exceed a reasonable dispersal distance for single individuals. The hybridization is therefore parapatric according to the

89 72 nomenclature of Woodruff (MS). Hybridization between P1 and F6 populations has been analyzed in Hall and Selander (in press) and will be summarized here. One fixed chromosomal difference (FIS-6) and two fixed isozyme differences (LDH-2 and GOT-1) were used as genetic markers to determine the ancestry of 153 individuals collected from a transect through the zone of parapatric hybridization on Cerro Potrero near the town of Rio Frio (see Fig. 3b, Hall and Selander, in press). In the sample from this transect there were 13 presumptive F 1 hybrids, as indicated by their heterozygosity for all three genetic markers. Such F 1 hybrids were apparently quite fertile, as shown by the many presumptive backcross individuals, heterozygous for only one or two of the genetic markers. Twenty-seven of these were backcrosses with F6 and another 29 were P1 backcrosses. Yet, there was no evidence of either an F 2 generation or of introgression into F6 populations beyond the first generation of backcross; and, if there was any introgression into P1 beyond the first generation backcross, it was very slight and the evidence for it equivocal at best. The apparent deficiency of certain marker combinations in the backcrosses suggested that some recombination products survived poorly, and the lack of evidence for any second or later generation introgression suggested that those backcrosses surviving to adulthood were effectively sterile. Hall and Selander (in press) therefore concluded, notwithstanding the unquestionable evidence for both hybridization and backcrossing, that the P1 and F6 populations were genetically isolated from one another, and were therefore good biological species--at least with respect to one another, even though individuals of the two populations

90 73 did not appear to behaviorally recognize this fact. This conclusion was supported by the electrophoretic evidence that other aspects of the genetic systems of the two species were more different in samples collected only two km apart on either side of the zone of hybridization than they were between two FIS-6 samples collected some 500 km apart. A similar but less complete analysis is possible for the contact between Standard and FM2 in the valley of San Juan Teotihuacán, using as genetic markers the four (or five) macrochromosomal differences fixed between the two populations. Seven-presumptive F 1 hybrids were found in some 150 karyotyped lizards from the mapped area of the Teotihuacan Archeological Zone (Millon, 1970). An eighth F 1 hybrid was found in a contact area about 12 km E of the Archeological Zone. Again, as in the F6 x P1 hybridizations, F 1 hybrids were not completely sterile, as revealed by the recovery of unquestionable backcrosses: one to Standard and three to FM2. Subject to the limitations in the data discussed above (pp. 50, 68), four more individuals might also be backcrosses to FM2 on the basis of being double heterozygotes or because they have three metacentric macrochromosomes. By comparison with the F6 x Pi hybridization, it would seem that there are fewer hybrids here and that these hybrids apparently have a much lower fertility in backcrossing. However, this might be artifact from differences between the population structures of the grammicus in the two areas and our sampling of it. In the Rio Frio area, lizards are concentrated on more-or-less randomly dispersed logs and dead trees, while in the Archeological Zone they are concentrated along the gridwork formed by the ancient walls of the old city (Millon, 1970).

91 74 Details of the microgeography of hybridization between S and FM2 will be reported when biochemical studies of the collections are completed by Sheldon Guttman [2003 note: unfortunately the biochemical studies were not completed]; however, some general conclusions from the preliminary mapping of the collection localities of the specimens and our karyotypic determinations of them can be given here. Intermediate scale mapping of the two populations indicates that they meet along an approximately east-west line following the valley of San Juan Teotihuacan (see Fig. 3a, Hall and Selander, in press). Within the Archeological Zone, as mapped by Millon (1970), our most intensive collecting was done in his quadrangles N2W1, N3W1, and N4W1 west of the "Street of the Dead" (see Fig. 1 in Millon, 1970). Apparently, in the area between the "Pyramid of the Moon" and the "Explorations of 1917," the "Street of the Dead" relatively effectively impedes local dispersal, as pure FM2 are found on the east side. (Three FM2 were taken from an ancient wall leading away from the "Puma Mural" and one F, hybrid was taken in the "Compound of the Four-Temple Complexes.") Most of the hybrids were found in N2W1 and N3W1 along a line running nearly NNW from the region of the "Explorations of 1917." The pure FM2 population was more-or-less restricted to the wedge-shaped area of lava and tufaceous soil figured by Mooser (1968) between this line and the "Street of the Dead." Standard grammicus are then found to the west of this line and to the east of the "Street of the Dead" mostly on alluvial soil. In this area of rather favorable habitat for grammicus, the hybrid zone was no wider than about 300 m. As in the case of hybridization between P1 and F6 grammicus,

92 75 though the hybridization between FM2 and S definitely involves backcrossing, there is no evidence from chromosomal markers for genetic exchanges beyond the first generation backcross. Therefore, by the criteria used by Hall and Selander (in press) to show that Pi and F6 are good biological species with respect to one another, FM2 and S are also good species-at least with respect to one another. By extension, if the contacts between the remaining chromosomal races prove to be similar to those described above (and there is no reason to think otherwise), then these other cytogenetically distinctive populations of grammicus are also probably good species with respect to one another. Triploidy and possible incipient parthenogenesis. During the present study, several karyotypically novel grammicus were found, including four with unquestionably triploid karyotypes. Three were morphological males: respectively, a Standard from southern Coahuila, a P1 from near Río Frío, and an FM2 from the Teotihuacán Archeological Zone, These and other novelties will be described in future works, but the fourth triploid lizard defies easy categorization and deserves special mention here, as it may have a bearing on possible consequences of hybridization between FM2 and Standard. This animal is a morphological female with two standard chromosome sets plus one FM2 set and was collected in or near the zone of hybridization between FM2 and Standard. With respect to this unique triploid female, it is worth note that six of eight F 1 hybrids from the valley of San Juan Teotihuacán were female. The sample is too small for the female bias to be statistically significant, but these observations suggest the interesting speculation

93 76 that the triploid individual resulted from fertilization of a parthenogenetically developing unreduced hybrid ovum. Hybrid origins have been suggested for most parthenogenetically reproducing animal populations (Maslin, 1968; Asturov, 1969; Schultz, 1969), and in almost every case where diploid parthenogenones are sympatric with closely related bisexual species, sterile polyploid hybrids and/or polyploid parthenogenetic species have been found (previous refs., Neaves, 1971; Lowe et al., 1970), indicating that parthenogenetically developing ova may easily be fertilized. Furthermore, parthenogenetic populations are known or are suspected to occur in the Agamidae and Chamaeleontidae (Hall, 1970), the two families most closely related to the Iguanidae. It is therefore not completely unreasonable that an incipient case of parthenogenesis was found in the S x FM2 hybrid zone. However, other explanations for the triploid female cannot be discounted, as triploidy has clearly occurred in grammicus in cases where a hybrid origin is not suspected. But, conversely, if these four cases of triploidy occur randomly in my sample of 1300 grammicus, the probability that one of them would be included in a sample of nine hybrids, such as that taken from the Teotihuacan hybrid zone, is about This situation suggests the further speculation that incipient parthenogenetic clones induced by hybridization may be not uncommon in natural zones of hybridization. However, if as is the case in both of the grammicus hybrid zones studied, the population density of males is high enough and the reproductive isolation low enough that such parthenogenetic clones would be bred into genetic difficulties within a few generations by increases of polyploidy because of the partheno-

94 77 genones inability to prevent males from fertilizing already developing ova. If this speculation pertains, it may be possible to find and isolate cases of such incipient parthenogenesis under conditions of controlled hybridization as Schultz (1973) has done in an analogous situation.

95 78 KARYOTYPE EVOLUTION Karyotype Evolution in the Grammicus Complex Caveat. Clearly the extraordinary karyotypic variability and cryptic speciation in the grammicus complex will provide material for testing chromosomal speciation hypotheses and other ideas reputing to explain the association between chromosomal diversity and prolific speciation. However, to be used effectively as a test, the sequence of karyotypic evolution in grammicus should be derived as accurately and as independently as possible from any hypothesis it will be used to test. On the other hand, if the molecular mechanism(s) of Robertsonian mutations were well understood, the cytological evidence might still be a powerful constraint on the phylogenetic interpretation independent of the hypotheses to be tested. Unfortunately, this understanding of Robertsonian change does not exist. Thus, Cole (1970, 1971a), accepting the postulated translocation mechanisms of White (1954) and Matthey (1949), which predict that centric fissions should be much less probable than centric fusions, reconstructed the chromosomal phylogeny of the spinosus group of Sceloporus on the assumption that its chromosomal evolution was unidirectional from primitively high chromosome numbers to low numbers by centric fusions. On the other hand, Todd (pers. comm.) suggested that chromosomal evolution in Sceloporus, and particularly within grammicus, can readily be interpreted by his hypothesis of the simultaneous fissioning of all metacentrics by "centric misdivision" (Todd, 1970), followed by the sorting out of the fissions as polymorphisms through the populations of the species. Yet again, White, while now accepting

96 79 the comparative data which suggest that centric fissioning (= "centric dissociation"--white, 1969) is not uncommon as a mode of Robertsonian mutation, dismisses Todd's thesis of the simultaneous fissioning of all metacentrics because this is "equivalent to a belief in miracles, which has no place in science" (White, in press, p. 501). And, to show that the possibility of simultaneous fissioning cannot be discounted out of hand, I can suggest at least two "mechanisms" which might produce just White's "miracle:" 1) some viruses are known to induce high frequencies of occasionally localized chromosome breakage (sometimes several mutations in a single cell) in cell cultures (Moorhead and Saksela, 1963; Kato, 1967), so it is easy to suppose that with certain viruses this breakage may be limited to the specialized regions of the centromeres; 2) it is also easy to suppose that specific mutator genes may exist, which, in some rare combinations, may cause several or all of the centromeres to break or "misdivide" by effecting some molecular constituent common to all centromeres. Certainly such mutator genes are known which affect other chromosome loci (Ives, 1950). Todd's (1970) thesis rests on the a priori assumption of a mechanism for "centric misdivision," while White's (1973) counter argument seems to be based on equally a priori assumptions from his mechanism for "centric dissociation." It seems to me most unwise to use any of these coined terms "with a very precise implication as to the mechanism," as White (1969) suggested; when, in plain fact, the mechanisms which generate the mutations are far from being demonstrated and are much farther from being understood in the details of their biochemical and molecular modes of action.

97 80 Instead, I consider that at this stage in our understanding of the molecular mechanics of Robertsonian mutation (e.g., see Brinkley and Stubblefield, 1970; Comings and Okada, 1970) it will be far better in the analysis of grammicus phylogeny which follows to use the terms fission and fusion as simple descriptions of what can be determined from nature about the direction and sequence of the Robertsonian mutations. Possible phyletic sequences. I have shown above that karyotypic variability per se in the grammicus complex cannot be used safely as an independent guide to sequence(s) of karyotypic derivation. Also, since neither morphological nor biochemical data are yet available which could contribute to this analysis, discussion of the phyletic sequences must depend largely on what is known about ecology and biogeography for the various chromosomal races of the complex. Although these limited data are not conclusive, they suggest the tentative rejection of the more complex, and intuitively unlikely, of the possible phyletic sequences. The apparent possibilities fall in four categories: 1) Simple fissioning sequences, in which each fission occurred and was fixed either locally in revolutionary speciation events or in a large population after a stage of balanced or transient polymorphism. In this category, a probable sequence for the derivation of FM2 would be: S F6 [F5+6] [F3+5+6] [F ] FMl FM2. 2) Independent simultaneous fissioning events, in which each population derives independently from Standard by the fissioning of those chromosomes now fixed in the various populations. 3) Todd's (1970) fissioning model, in which all chromosomes fission in one event and then sort out through the range of the species

98 81 to eventual elimination or fixation in various subpopulations of the parental species. In this case, each of the present grammicus races with a derived karyotype would have been derived through intermediate populations polymorphic for the now fixed mutations. Presumably, in this case the derived population most central to the karyotypic radiation would be the FM2. 4) Complex sequences, which may include any of the three preceding cases, plus the possibility of fusion events following fission events. The probable phyletic sequence. Figs. 7a and 7b illustrate the approximate geographical ranges of the karyotypically distinctive populations of the grammicus complex, and Tables 2, 3, 4, 5, 6, 7, and 8 summarize their ecological distributions in these ranges. A logical and consistent simple fissioning sequence can be developed which readily accounts for this biogeographic distribution. Judging by their present range and ecological diversity, the Standard grammicus must have been widespread on the Mexican Plateau before their karyotypic evolution began. Then, sometime during the history of Standard, the FIS-5 mutation became fixed on its northwestern periphery. However, since the F5 population probably has always been separated from other chromosomally derived populations, either by intervening standard populations or by the hottest part of the Chihuahuan Desert; and since it presumably was derived independently from the other populations, there is no information which would allow an estimate of F5's age with respect to the other derived populations. Therefore, no profit will derive from considering this population

99 82 further in the present report. The derivation of P1 is least certain. Two possibilities were suggested by Hall and Selander (in press): 1) that P1 derives from a standard population cut off from the remaining standard populations by an encirclement of F6 at intermediate elevations, followed by establishment of the FIS-1 mutation; or 2) that P1 was derived by a fusion of the acrocentric-6 elements to reconstitute the metacentric-6 morphology, followed by establishment of FIS-1. The reconstitution of a metacentric-6 would not be an intrinsically unlikely mutation if fusions between chromosomes of greatly different lengths (e.g., fusion with an acrocentric-6 element and a micro chromosome) are especially disadvantageous; since in the F6 karyotype, the acrocentric-6 elements could then fuse successfully only with one another. Either model is geographically plausible. Of the remaining derived populations, F6 seems the oldest. Its present disjunct range in dry areas, where local populations are found only in comparatively mesic local habitats, and the restriction of its continuous distribution only to more mesic parts of the Sierra Volcánica Transversal together suggest that F6 achieved a maximum distribution during a Pleistocene pluvial period. Then, F6 probably ranged throughout the entire Sierra Volcánica Transversal-Sierra Madre Oriental system. The next largest distribution (assuming that the coastal and plateau populations have a single ancestry) is that of the F5+6 population, It seems likely (though not yet confirmed) that the F5+6 population bisects F6 in the area of northern Guanajuato and Hidalgo,

100 83 where F5+6 inhabit areas almost as mesic as those inhabited by F6. From this possible center, F5+6 has spread into much more xeric areas in San Luis Potosi and along the coast. This range suggests a possible spread coinciding with increasing aridity in the post-pleistocene period. An alternative hypothesis would derive the coastal and plateau populations independently from an F6 ancestry. However, the ecological derivations would remain the same, with both F5+6 independently spreading into more xeric habitats. FMl and FM2 also occupy xeric habitats, which extend south of the F5+6 range, with FMl occupying the geographically intermediate region. The apparently very small ranges of these populations suggest that they may be phylogenetically the most recently derived. The fact that they have not been found anywhere except in close association with human agriculture suggests that they may have evolved contemporaneously with the cultivation of primitive crops in the area of central Mexico. The area between the northern FMl and southern F5+6 has not been sufficiently sampled to rule out the possibility that populations karyotypically intermediate between F5+6 and FMl may be found there. Furthermore, if the FM populations did evolve in close association with agriculture, the more primitive intermediate populations may have been eliminated by man-induced habitat modifications. It is also quite possible that these populations did not grow large before giving rise to the later derivatives. These known distributions are clearly compatible with the simple fissioning sequence, as geographical and ecological derivations can be traced in the same sequence as the karyotypes; whereas, on the

101 84 other hand, it seems unlikely that independent simultaneous fissioning events would involve just these chromosomes and would be arrayed geographically in just such a fashion that this clear linear sequence could be traced. Superficially, the observed distributions also seem compatible with the Todd model. This would assume that the original fissioning occurred in a single event, and the fissioned chromosomes then diffused outward as polymorphisms from a center until they became either fixed in local populations or lost. By this model, all of the fissions were either fixed or retained as polymorphisms in the FM2 population, with increasingly fewer of the fissions having reached or having been retained by the more peripheral populations to the north. However, if the actual events did follow this model, it seems strange that the fissions have spread only to the north. Furthermore, it is awkward that the probable "central" FM2 population seems to have differentiated so much more recently than either of the more "peripheral" F6 or F5+6 populations. Finally, it seems even more strange that the polymorphic populations (P1, FMl, and FM2) occupy such small parts of the total range of grammicus, if we are to believe that at one time polymorphisms for all of the macrochromosomes were spreading throughout the range of grammicus now occupied by the chromosomally derived races. Therefore, the simplest phyletic model, that of a direct sequence of fissions from Standard through F6 to FM2, seems most in accord with the available evidence, although until more is known about the biochemical and morphological relationships of the various grammicus complex populations, the Todd model cannot be completely discounted.

102 85 Except for the possibility that the P1 population is derived from F6 by a centric fusion, there is no evidence supporting either independent multiple fissioning or more complex patterns of derivation. And, except for the single derivation branches leading to F5 and P1, and a possible single derivation branch to one of the F5+6 populations, all of the remaining chromosomal races can be traced in a single linear sequence of derivation, which can still be traced through a sequence of parapatric contacts.

103 86 Clarkii Group Chromosomes and Origin of Em Karyotypes Although evidence from the crevice-using Sceloporus is sufficient to determine the primitive macrochromosome morphology and to suggest sequences of macrochromosomal fissioning away from it in the grammicus complex, to fully interpret the phylogenetic significance of the Em chromosomal condition found in asper and the megalepidurus group, the karyology of certain non-crevice-using Sceloporus must also be considered. The crevice-using species all belong to Smith's (1939) large-sized, large-scaled division of the genus; most species of which have been karyotyped either by Cole or myself. Besides the creviceusers, only two other species in this division, S. clarkii and melanorhinus (= the clarkii group), have female karyotypes with 20 microchromosomes. In fact, the common microchromosomal pattern in clarkii group females is indistinguishable from that of the standard karyotype crevice-using females, although the male microchromosomal patterns in the clarkii group species differ from one another and from standard (Figs. 8 and 9). Also, both clarkii group species differ from the standard karyotype by being fixed for fissions of macrochromosome pairs 1,3,4, and 5. (Cole, 1970, and Lowe et al., 1967, postulated that this pattern is primitive relative to the standard macrochromosomal pattern, but this idea is indefensible for the same reasons that the FM2 cannot be considered primitive in the grammicus radiation.) However, here we are concerned with the similarities of the microchromosomal patterns between the clarkii group species and the species with the standard karyotypes, which suggest that all may have a fairly close common ancestry within the large-scaled Sceloporus division.

104 87 One aspect of the microchromosomal variation in the clarkii group is especially interesting. Cole (1970) reports that both clarkii and melanorhinus are polymorphic for an enlarged microchromosome found in his KB or patterns. I find this enlarged microchromosome indistinguishable from the Em chromosome fixed in asper and the megalepidurus group (Figs. 8, 9). Cole quite reasonably suggests that this Em condition is derived and may be due to a tandem duplication resulting from unequal crossing over. Cole found the Em chromosome in clarkii sampled from two areas (Santa Cruz Co., Arizona, and near La Concha, Sinaloa) which represent two of three subspecies and the opposite extremes of clarkii's geographic range. In the Santa Cruz Co. sample, the Em chromosome was present at a frequency of 0.25 (N=51); and in the total of two individuals collected at the Sinaloa locality, one was heterozygous for the mutation. The remaining clarkii karyotyped by Cole (12 lizards from 7 other localities and 16 from an 8th) did not show the Em chromosome. Of 12 clarkii I have karyotyped, including eight from the Playa Escondido area of Mazatlan, Sinaloa, none conclusively showed the Em chromosome, although two individuals might have been homozygous for the mutation based only on diakinesis figures (see Fig. 10. Many of my clarkii preparations were made during my 1966 expedition and did not benefit from a colchicine pretreatment, and hence have few if any serviceable mitoses). Of seven melanorhinus karyotyped by Cole (1970), two of three individuals from one locality near Acapulco, Guerrero, were heterozygous for the Em mutation, while the third individual from that locality

105 88 lacked it, as did the remaining four from two other localities. Of the six melanorhinus I have karyotyped, only one from Rio Maria Basio, west of Manzanillo, Colima, was heterozygous Em; while all of the remaining specimens, representing a second locality near Manzanillo and two localities near San Bias, Nayarit, lacked the Em chromosome. It seems most unlikely that the two clarkii group species, the two megalepidurus group species, and asper would have independently evolved these very similar appearing Em chromosomes. On the other hand, particularly since the other microchromosomal similarities between these species also suggest that they are closely related, the Em mutation probably originated in the common ancestor of these five species. If so, this common ancestor must have been the progenitor of both the clarkii group and all of the crevice-using Sceloporus as well. The survival of the Em chromosome as a polymorphism in both clarkii and melanorhinus is surprising enough, but its presence in the crevice-using assemblage as well indicates that it must have survived as a polymorphism in the clarkii group from before it and the crevice-users diverged from one another. Furthermore, the polymorphism must also have been retained for a while as such by the primitive crevice-users to explain its fixation in two rather different sections of this radiation. Analysis of the sex chromosomes of the clarkii group and the crevice-users provides additional evidence on their relationships.

106 89 Sex Chromosome Evolution As described above, all crevice-users are characterized by identical X 1 X 2 Y sex trivalents in males, as originally noted by Cole et al. (1967) for jarrovii and poinsettii and Axtell and Axtell (1971) for another jarrovii population. On the other hand, Lowe et al. (1967) and Cole (1970) did not identify sex chromosomes in the clarkii group karyotypes. However, my studies have revealed distinguishable sex chromosomal heteromorphisms in both species of the clarkii group. This is most obvious in melanorhinus and will be discussed first. The four females of the seven melanorhinus karyotyped by Cole (1970) had 2n=40 karyotypes identical to the common clarkii karyotype, which has two pairs of metacentric macrochromosomes (2 and 6 of the standard karyotype), eight pairs of acrocentric macrochromosomes, and 20 microchromosomes. However, Cole's three males had only 39 chromosomes in their karyotypes: one micro was "missing" and there were five instead of the usual four metacentric macros, with the anomalous meta-centric in the pair 6 size range. This replaced one of the acrocentrics in the female karyotype. Cole called this 39 chromosome pattern the KC karyotype and compared it to the similar but not sex-correlated 40 chromosome KC karyotype of S. clarkii. However, he also noted that the apparent sex correlation in melanorhinus was "suggestive." Unfortunately Cole's material did not provide meiotic figures to clarify the situation. Four of the six melanorhinus I have karyotyped were males, and each of these also had the 2n=39 KC karyotype as described by Cole (Fig. 8c), while my two females had the 2n=40 normal pattern. (Actually, as can be seen in Fig. 8c, the anomalous "metacentric" chromosome is more

107 90 submetacentric than the pair 6 chromosomes, and Cole's indication that it was metacentric is due to a poor pairing of the small metacentrics, as can be seen by inspection of his Fig. 4c.) Additionally, one of my males (incidentally, the individual heterozygous for the Em mutation) yielded several diakinesis figures, all of which showed clear trivalent involving the anomalous metacentric (Fig. 9d). The strict sex correlation of this KC karyotype (all 7 males karyotyped had it, while all 6 females lacked it) and the trivalent formation in diakinesis both clearly indicate that this metacentric must be the Y of an X 1 X 2 Y sex chromosome system--but clearly the Y here is different from that found in the crevice-users (Figs. 8 and 9). Presumably this metacentric Y was generated by the centric fusion of one of the 5b acrocentrics and a more primitive Y chromosome, which seems to have been a comparatively large acrocentric microchromosome (Fig. 8). As a result of the fusion, the second 5b macrochromosome thereby became linked with sex as an X 2 chromosome. The X 1 or the more primitive X, is in the size range of the second or third smallest pair of microchromosomes, as it is in the crevice-users, and it is probably homologous with the X 1 of these species. Furthermore, in comparisons of the karyotypes of melanorhinus and the crevice-users (Fig. 8), it can be seen that the short arm of the melanorhinus Y is comparable in size to the long arm of the crevice-user Y. Therefore, these arms are probably homologous and approximate the more primitive acrocentric Y from which the biarmed Y chromosomes of the two taxa were independently derived by centric fusions, respectively with a macro chromosome in melanorhinus and with a microchromosome in

108 91 the crevice-users. As noted above, no sex chromosomal heteromorphism was seen in clarkii by Lowe et al. (1967) or Cole (1970). However, the close relationship between the clarkii and melanorhinus karyotypes in other respects, and the size relationships in melanorhinus between its X 1 and the short arm of its bi-armed Y (presumably homologous to the whole of a more primitive Y in clarkii), suggest that a sex chromosomal heteromorphism should exist in clarkii as well. Since male clarkii have an even 2n, this heteromorphism would most likely be of the XY type. Unfortunately none of my preparations from male clarkii yielded useful mitotic figures, but the predicted XY heteromorphism is detectable in the diakinesis figures. In most cells, one of the larger microchromosomal bivalents is distinctly unequal (Fig. 9c). (Cole's, 1970, Fig. 3a also demonstrates this heteromorphic pairing in diakinesis: the unequal bivalent can be seen near the periphery of this figure at about 4:30 o'clock.) Fig. 10 shows additional arrays of the microchromosomes from representative diakinesis figures of several individuals, which again demonstrate the unequal bivalents. Although I cannot confirm the morphological relationships of the components of the heteromorphic pair with mitotic preparations from my material, the size relationships of these chromosomes with one another and with the other microchromosomes are consistent with the idea that the Y chromosome is a large acrocentric micro and the X is a smaller micro, equivalent to the X 1 of the crevice-users and melanorhinus, as would be expected. Furthermore, in Cole's (1970) Fig. 1c, illustrating a male karyotype of clarkii, it appears that the second chromosome in the microchromosomal array is

109 92 improperly paired. This is a large acrocentric and may be the Y. However, Cole paired it with a submetacentric micro, which more likely pairs with the fourth micro in his karyotype. So, to recapitulate, clarkii probably retains the primitive sex chromosomal condition in the clarkii-crevice-user radiation; i.e., an XY condition, where the Y is a large acrocentric microchromosome equivalent in size to pair 9 or 10 in the standard karyotype; and where the X is a smaller acrocentric, equivalent in size to pair 11 or 12 in the standard karyotype. The melanorhinus-type Y would then be formed by a fusion between the clarkii-type Y and one of the pair 5b acrocentric macro chromosomes. The standard-type Y would then be formed by a fusion of the clarkii-type Y and another large acrocentric or subacrocentric microautosome, probably homologous to pair 7 in the clarkii group karyotypes (Figs. 8a, b). In comparison with the sex chromosomes of the primitive 2n=34 sceloporine karyotype (e.g., Pennock et al., 1969; Cole 1971a, 1971b), even the sex chromosomes in clarkii are derived. The X chromosome may be homologous to this more primitive sceloporine X, but the sceloporine Y is minute, being by far the smallest micro in the 2n=34 karyotype. With the present data it is unclear how the large Y and 18 microautosomes of the clarkii karyotype derive from the minute Y and 20 microautosomes of the primitive 2n=34 karyotype, particularly if their X chromosomes are homologous. The frequency with which complex sex chromosomal systems have evolved independently within Sceloporus and in other iguanid genera suggests that it may be selectively advantageous for them to increase

110 93 the amount of sex linkage. Many iguanids show no cytologically distinct sex chromosomal heteromorphism. This is presumably the primitive condition (Gorman et al., 1967). Several Anolis species (Hall et al., unpub.) and most of the sceloporines show an XY heteromorphism involving the smallest microbivalent. So, whether the primitive sex chromosomes are cryptic or not, they are very small microchromosomes. This cytological evidence and the facts that sex is frequently indeterminant or determined by only a single "gene" in fish and amphibia (Ohno, 1967), and that at least one lizard (Chevalier, 1969; Gorman, in press), snakes (Ohno, 1967; Gorman, in press), and birds (Ohno, 1967) are female heterogametic all suggest that sex determination in reptiles and lizards was based primitively on only very slight amounts of heterogamety and that the differentiation of distinctive sex chromosomes has occurred within the reptilia in several independent lines. In Sceloporus several lines also show increases in the chromosomal linkage of sex, as indicated by distinctive heteromorphisms; 1)the X 1 X 2 Y heteromorphism of maculosus (based on only three specimens, Cole, 1971a); 2) the XY heteromorphism of clarkii; 3) the X 1 X 2 Y heteromorphism of melanorhinus; 4) the X 1 X 2 Y heteromorphism of the crevice-users; 5) the large XY microchromosomes of lundelli (Cole, 1970), and 6) the large XY microchromosomes of aeneus (Hall, unpub.). The increases in some of these lines must have been at least partially independent. Also, in Anolis sex linkage has increased independently in at least three lines (Gorman, in press). Finally, the genus Polychrus also shows at least one line with increased sex linkage. Many iguanid species show pronounced sexual dimorphisms, and it

111 94 is interesting to speculate that the increased sex linkage is adaptive in that it simplifies developmental control of the dimorphic characters. But, at least the sceloporine species with increased sex linkage are not conspicuously more dimorphic than are those that have the primitively small amount of sex linkage. However, this is a possible correlation that deserves more attention than I have given it to date.

112 95 EVOLUTION OF THE SPECIES GROUPS Karyotypic Phylogeny The karyotypic results suggest certain phyletic relationships among the species treated in the present work. These are summarized by Fig. 11. Of all these species, S. clarkii has the most primitive microchromosomal pattern. S. clarkii and melanorhinus are clearly more closely related to one another than either is to any other Sceloporus, as evidenced by their common possession of the same four macrochromosomal fissions (i.e., FIS-1, FIS-3, FIS-4, FIS-5), which appear to have been fixed independently of fissions in any other Sceloporus. Based on their karyotypic similarities, these two species form a natural group (i.e., they have a common ancestry, and the group includes all of the species deriving from this common ancestry), which is hereby designated the clarkii group. Subsequent to the fixation of the fissions in the primitive clarkii group stock, a derived sex chromosomal condition became fixed in melanorhinus (i.e., the macrochromosomal X 1 X 2 Y). It also seems evident from the karyotypic data that both the clarkii group species and the crevice-using species with the standard and Em karyotypes were derived from an ancestral species with standard macrochromosomal and a 20 microchromosome pattern identical to that of clarkii (i.e., heteromorphic XY and polymorphic Em). This primitive stock is probably now extinct. The apparently identical X 1 X 2 Y heteromorphisms of all crevice-users, which are derived with respect to the XY condition of clarkii, indicate that the crevice-users are phylogenetically closer to one another than they are to either the clarkii group species or any other Sceloporus. This is in spite of the great morpho-

113 96 logical divergence (see below) of the torquatus group on one hand, which Is specialized for the use of rock crevices, and the grammicus group species, megalepidurus and pictus of the megalepidurus group, and asper of the formosus group, which are all specialized for the use of one form or another of plant crevices. Then, to turn this around, the great morphological divergence between the rock crevice-users and the plant crevice-users indicates that their divergence, both from the clarkii group and from one another, must have taken place comparatively early in the phylogeny of the genus, or at least in that of the large-sized, large-scaled branch (see below) to which they all belong. Based on karyotypic data to be presented in a later paper of this series, S. cryptus is hereby transferred from the megalepidurus group to the formosus group. Although S. subpictus has not yet been karyotyped, it is morphologically sufficiently close enough to cryptus to indicate that it also should be transferred to the formosus group. Similarly, asper's karyotype clearly indicates that it should be removed from the formosus group. However, asper is sufficiently distinctive that it will be treated as a monotypic group, at least in the present analysis. The grammicus group remains unchanged. Karyotypic data provide no information on the phylogenetic relationships between these four crevice-using species groups. In contrast to the apparently early karyological divergence of the clarkii and crevice-using stocks, the very small degree of morphological divergence among the cryptic species of the grammicus complex suggests that the chromosomal differentiation within this radiation is quite recent.

114 97 However certain these karyotypic relationships may seem, at this early stage in our understanding of the chromosomal differentiation of species, it is especially important that the relationships suggested by the karyotypic similarities should be tested by independent lines of evidence. Independent confirmation is even more important if we wish to use the karyotypic data to test theories of chromosomal speciation.

115 98 Alternative Phylogenies Some workers familiar with Sceloporus (e.g., R. W. Axtell, pers. comm.; Larsen, 1972 and pers. comm.) would dispute my contention that the karyotypically similar crevice-using species form a natural grouping within Sceloporus. They base their arguments on the abovementioned great morphological disparity between the rock-crevice-using species and the plant-crevice-users. The rock-crevice-users are considerably larger and larger-scaled than are all but asper among the plant-crevice-using species. The rock-crevice-users are generally robust and dorsoventrally quite flattened, their scales tend to be strongly mucronate, and most have bold black nuchal collars highlighted with white borders. By contrast, the plant-crevice-using species, excepting the intermediate asper, are smaller than the smallest rockcrevice-users and generally have smaller and smoother scales--which in some of the grammicus become almost granular. Furthermore, the plantcrevice-users are generally more gracile and are less flattened, Their nuchal collars, if they can be considered that, are much less developed and tend to blend into the generally cryptic dorsal patterns typical of these species. To determine the phylogenetic significances of these morphological differences, it will be useful to review Smith's (1936, 1939) phylogenetic scheme for the genus, as well as other forms of evidence and other phylogenetic interpretations. The Smith phylogeny. Figure 12 summarize Smith's (1939) ideas regarding the grouping of species in Sceloporus (as emended by subsequent taxonomy) and the phylogenetic relationships of these groups. When these ideas were being developed, Smith (pers. comm.)

116 99 believed that the ancestral stock from which Sceloporus derived was to be found in the tropical South American tropidurine radiation--a not unreasonable idea based on what was then known about iguanid evolution. According to this concept, Sceloporus diverged early into generally large-sized, large-scaled (Fig. 12 left) and small-sized, smallscaled branches (Fig. 12 right), with the latter eventually giving rise to the well differentiated variabilis group. In turn, Smith (1936) believed that the variabilis group gave rise to the xeric adapted but otherwise generalized sceloporine genera Uta, Urosaurus, and Petrosaurus (all then included in Uta). He also thought (Smith, 1939) that the tropical formosus group was primitive relative to the large-sized, large-scaled branch and that the groups radiating out to the north and/or into more xeric areas were derived from it and more advanced in their ecology. Smith (1939) placed the grammicus group origin within the formosus group, and derived the megalepidurus group from within the grammicus group. On the other hand, torquatus were thought to derive from within the spinosus radiation, which, in turn, were derived from another part of the formosus radiation. According to this concept, the crevice-using assemblage could not be natural. If the compositions of Smith's species groups are accepted and it is also accepted that the primitive Sceloporus was tropical, then this treatment is not unreasonable. And, in this regard, it should also be noted that, before the present work and Larsen's (1972) preliminary report, no one has directly challenged Smith's groups or their phylogenetic treatment (e.g., see Cole, 1970, 1971a, 1971b, 1972; and Carpenter, 1972). However, recent

117 100 work outside of the genus Sceloporus suggests that some of Smith's phylogenetic assumptions are untenable. Problems with the Smith phylogeny. Comparative osteology (Etheridge, 1964; Presch, 1969) clearly shows that Sceloporus is derived with respect to other sceloporine genera, and that the closest relationship between the sceloporines and tropidurines is actually through the xeric adapted genus Petrosaurus. Even a casual inspection of other suites of characters among the Iguanidae, such as the development of the femoral pores and surrounding scalation, the elimination of the gular fold, the development of enlarged head shields, the development of mucronate and keeled dorsal scalation, etc., will all show that Sceloporus is advanced or derived with respect to other sceloporine genera, including its osteologically closest relatives Uta, Urosaurus, and possibly Sator (which otherwise might be derived from within Sceloporus). These relationships imply that the primitive Sceloporus were xeric adapted ground lizards and radiated from the northern deserts southward and/or into more mesic habitats. Significantly, except for Sceloporus, all of the sceloporine genera are largely or entirely xeric in their distributions. Also, except for the rock-dwelling Petrosaurus (which have been tested) and the insular endemic Sator (which has not been tested), all other sceloporine genera regularly "shimmy bury" (Axtell, 1956) in loose sand or soil for sleeping and escape (Hall, pers. obs., see also Stebbins, 1943, 1944, 1948; Pough, 1969a, 1969b), which is clearly an adaptive behavior in a xeric habitat. The "sink trap" type of nasal apparatus, characteristic of all sceloporine genera (Savage, 1958), appears to have

118 101 evolved in conjunction with the "shimmy burial" trait as an adaptation for excluding sand from the nasal passages (Stebbins, 1948). Both are developed to extremes in the "sand swimming" Uma (Stebbins, 1943). Even the mesic tropic species of the formosus group have the "sink trap" type nasal apparatus (Hall, in prep.[see attached material]) and S. formosus collected from the high mountain (3600 m elevation) rain forest of the Sierra de Juarez in Oaxaca will "shimmy bury" almost as readily as do S. magister taken from desert sand dunes of the southwest United States (Hall, pers. obs.). Needless to say, the formosus have little chance to "shimmy bury" in their present habitat (because of a lack of a suitable substrate) and it is highly unlikely that such a trait would have evolved in the present habitat. Except for Urosaurus, bush and rock users, and the rock dwelling Petrosaurus, the remaining sceloporine genera are all basically ground dwellers, and presumably loose soil may frequently be the only escape cover readily available for many of them. Similarly, many of the small-sized, small-scaled Sceloporus are ground and rock dwellers. From this analysis, it appears that the basic assumptions of Smith's phylogeny are false. I do not dispute either Smith's concept that the variabilis group of Sceloporus (especially S. couchii) is closely related to Uta and Urosaurus, or that the southern species of the spinosus group are very closely relatetd to the formosus group. But, on the other hand, it is clear from the evidence cited above 1) that the formosus group is a highly derived and only recently differentiated group in Sceloporus, rather than being one of the most primitive groups in the genus, and 2) that the variabilis group, as

119 102 [Smith] defined it, contains some of the oldest derivatives of the genus, rather than being comparatively recent. This analysis would suggest that the large-sized, large-scaled radiation in Sceloporus is a comparatively recent proliferation growing out of the older small-sized, small-scaled radiation (Hall, in prep.) and that evolution in Sceloporus has been away from xeric habitats and the invasion of mesic environments is comparatively recent. If these assumptions are correct, it then becomes more reasonable that the grouping of crevice-using species is a natural one. The Larsen phylogeny. Although Larsen (1972, pers. comm.) accepts the conclusion that Sceloporus derive from the primitively xeric adapted sceleporine genera, he subdivides the genus into three major divisions based on a multivariate analysis of 80 characters (40 skull ratios + 40 external characters which are also mostly ratios). His phylogenetic conclusions are summarized in Figure 13. Except that the plant-crevice-users were placed in his division II, Smith's large-sized, large-scaled branch was left intact in Larsen's division III. The small-sized, small-scaled branch was split into divisions I and II. Larsen then subdivided each of these three major divisions into species groups. Note that the rock-crevice-using torquatus group remains intact in Larsen's analysis and that the species of the grammicus group, plus megalepidurus, pictus, and asper, were all placed together in a single species group of division II. Placement of cryptus, a truly cryptic sibling of formosus, in this group is probably due to an erroneous identification of a plant-crevice-user as cryptus. Larsen's analysis fully supports the ideas that the wood-crevice-users and the

120 103 rock-crevice-users each form natural groups, but, if this analysis is representative of the true phylogenetic relationships of these species groups, the crevice-using assemblage as a whole cannot be natural. Nor, does Larsen's analysis indicate a close relationship between the clarkii group and either of the crevice-using groups. Problems with the Larsen phylogeny. Although I have no desire to review the whole controversy revolving around the philosophy of numerical systematics, since this has been adequately discussed by Mayr (1965, 1969b) and others, it should be noted here that there are several problems inherent in the phenetic approach itself: 1) The choice of the characters to be used is at least as subjective as is the choice of characters for more "subjective" approaches. 2) The equal weighting of many characters, when some may be determined by single genes, others by many genes, and still others may be covariant with only one or a few genes, practically insures that the estimate of phenetic similarity will not be a good estimate of genetic similarity (Inger, 1958). 3) The requirement that many different characters be examined limits the number of individuals that can be examined, and therefore greatly increases the danger that the individuals examined do not truly represent the species from which they were sampled. 4) Even if it can be assumed that the comparisons of the characters examined gives an unbiased estimate of genetic similarity, there is no justification that these estimates will allow the true phylogenetic relationships of the organisms to be determined. In sum, a more reliable phylogeny can probably be constructed through the analysis of only a few characters, the evolution of which is thoroughly understood,

121 104 than can be constructed from the computerized analysis of many characters picked at random. Besides my general objections to the use of numerical systematics as an "objective" approach to constructing a phylogeny, there are some specific problems in Larsen's application of the multivariate technique to Sceloporus: 1) The skull is the major trophic structure used in prey capture and in feeding, and the scalation is a major defense against the environment. And, as will be seen below, both character complexes appear to be differentially specialized in the crevice-users to serve additional important functions. It is then reasonable to expect that these character complexes will be quite plastic in their ecological adaptations and that they would therefore not be good indicators of genetic ancestry, especially if the various lineages in question show important divergences or convergences in ecology. Only five of Larsen's 80 characters are independent of the skull or scalation. 2) Most of Larsen's characters are based on measurements and almost all of the measurements are expressed in ratios. Ratios were chosen to reduce dependence on the absolute size of the organisms measured. But, on the other hand, many such measures in vertebrates show important allometric changes with growth (= size). In taxa where this occurs, the allometric relationship frequently extends across related species which differ in size. Algometry in skull ratios with age (= size) has been established in Iguana (Costelli, ms.), and therefore seems likely in Sceloporus. Larsen does not seem to have checked this possibility, which is especially critical for his

122 105 analysis, because many of his variables may be changing in a coordinate fashion with size alone. Such chances could, of course, greatly bias his conclusions. 3) In addition to the size allometry, there is a considerable variation among Sceloporus in the degree of dorsoventral flattening. Many characters will vary in a coordinate fashion with this genetically possibly simple change; and again it seems possible that Larsen's approach may be giving undue weight to this one change. Now, to look at Larsen's results (Fig. 14), I note that there is a strong tendency to group similar sized species, even when their genetic relationships are probably not close (e.g., see Smith's phylogeny, Fig. 12). This is particularly evident in his group II. Also, the great separation of the plant-crevice-users and the rockcrevice-users in Larsen's analysis could mainly be a function of three characters: the degree of dorsoventral flattening, the size of the scales, and absolute size. By these criticisms I do not mean to denigrate this massive work of Larsen's, or even many of his conclusions. However, I do think it wise to emphasize that his conclusions should not be blindly accepted simply because they are the result of an "objective" procedure. They should be used with common sense.

123 106 Reconciliation and Phyletic Synthesis There are now before us three different phylogenetic schemes for the clarkii group plus the crevice-using Sceloporus; the purely karyotypic one presented above. Smith's (1939) treatment, and Larsen's (1972) multivariate analysis. Each suggests different derivations for the taxa treated in the present paper. Can these differences be reconciled? Additional phylogenetic analyses of independent suites of characters are either now under way (isozyme variation by Sheldon Guttman, Miami Univ. [note added not completed], and microdermatoglyphics by Hobart Smith, Colorado Univ.) or are planned (immunological distance) which should eventually serve to unequivocally eluci-date the phylogenetic relation- ships of the different species. But, even now, some lines of evidence exist which seem to support the karyological phylogeny and which may also account for the great morphological differences between the rock-crevice-users and the plant-crevice-users. The biology of crevice-using. I have alluded to the propensity of the standard and Em karyotype species to use crevices for escape and sleeping cover. This may be an especially significant aspect of their biology, both indicative of a common ancestry and sufficient to account for the morphological differences seen between the users of different kinds of crevices. The comments which follow on escape behavior and the use of cover by the various species of Sceloporus, although not the result of a planned series of studies, are based on observations from five full summers of field work and many observations of captive lizards under suitable conditions.

124 107 When chased in the field, most Sceloporus will attempt to escape by running away into a bush, up a tree, or into open burrows in the ground. However, if no other cover is present and loose soil or sand is available, such as in a holding pen, most species will quickly "shimmy bury" (Axtell, 1956) to escape being caught. Also, when kept in sand bottomed cages most Sceloporus species, including even the mountain rain forest dwelling formosus, will usually "shimmy bury" for sleeping, even if other cover such as loose boards or cardboard is available. In the field I have night-collected magister and clarkii sleeping in the dry sand under stone bridges that provide them cover and perches during the day. The only Sceloporus which I have observed under suitable conditions and which I have never seen "shimmy bury" or found sleeping buried are species given the crevice-using designation. However, only the plant-crevice-users pictus and standard grammicus (from Oaxaca) and the rock-crevice-users torquatus and mucronatus have been specifically tested for "shimmy burial" under controlled conditions. Also, unlike the species which run or "shimmy bury" for escape, the crevice-users are always found closely associated with some kind of crevice into which they will retreat at the slightest disturbance. The torquatus groups species, excepting serrifer which is reputed to be at least semiarboreal (Smith, 1936), and cyanogenys which are not infrequently found on Joshua trees (Yucca), are almost always restricted to areas of rock outcropping (or stone fences) where they take cover in the crevices under exfoliating chips or in the cracks formed along bedding or fracture planes. On rare occasions, individuals of most of these species can be found on other substrates,

125 108 such as split logs or hollow trees, that provide suitable crevices for cover. But, in many thousands of observations, I have never seen torquatus group species use burrows in the ground for cover. Nor have I ever found them very far away from the crevices they normally use for cover. The plant-crevice-users exploit all varieties of crevice-like spaces in plants, such as spaces within the macerated and dried remnants of prickly pear (Opuntia) pads, spaces under and between the dead and dry thick sword-like leaves of both Yucca and Agave, the cracks in split logs and trees, and under all classes of loose bark on Yucca, Opuntia, dead trees, fallen logs, and stumps. The various grammicus types, especially standard, are found in all of these habitats, and even occasionally in rock crevices; while megalepidurus and pictus, which occur syntopically with standard grammicus, seem to be much more restricted to Yucca and Agave and spend much more time on the rocky ground close to these plants than do the grammicus inhabiting them. The conclusion from these observations is that crevice-using is a derived character. The apparently complete loss of the behavioral routine for "shimmy burial" also appears to be derived, and its loss in the crevice-users, which frequently live in areas where loose soil is present in close juxtaposition to their usual perches and cover, is probably indicative that the crevice-users have made a major qualitative change in their use of the environment. The facts that the plantcrevice-users will occasionally use rock crevices for cover and that the rock-crevice-users will in turn use plant crevices indicates that the availability of crevices of whatever form is probably more important to

126 109 the lizards than is the specific substrate in which the crevice occurs. By contrast, formosus, which probably have less potential opportunity to use the trait in their present habitat than do most crevice-users, will still readily "shimmy bury" when put in a sand-bottomed cage. Many other Sceloporus use trees, rocks, or other plants as preferred substrates and for cover, but they are not restricted to them. For example, besides formosus, spinosus (a rock and Yucca user), olivaceus (a tree user), and melanorhinus (a tree user) will all "shimmy bury" almost as readily as do magister collected from sand dunes. It therefore seems reasonable that the crevice-using habit and concomitant loss of the "shimmy burial" behavior were evolved only once in Sceloporus. If so, by this criterion, the crevice-user grouping clearly would be natural. Given that the original adaptation of the ancestral creviceuser was a use of crevices in whatever substrate, an early split into rock- and plant-crevice-using stocks seems entirely adequate to account for the evolution of the obvious morphological differences between the two stocks. Crevice-users are found only in areas where crevices provide them with cover, with their population densities in these areas generally roughly proportional to the available cover. They are never found in areas where the habitat seems to otherwise provide an abundance of food and perches but is deficient in suitable crevices. Clearly, the availability of cover is a major factor limiting their populations. It then follows that selection pressure for effective use of the preferred crevice type must be rather high. The obvious morphological differences between the two stocks are clearly adaptations to the physical charac-

127 110 teristics of the kinds of crevices exploited. S. megalepidurus and pictus exploit the complex crevice systems found between the leaves of Yucca and Agave. They generally escape a predator by moving away through the complex system of spaces and only occasionally do they wedge themselves in the crevices of the leaf axils or under the loose bark (on dead Yucca branches and trunks). Specialization for this class of crevice-using is mainly behavioral, but their scales are intermediate in size between those of the grammicus and torquatus groups and are comparatively smooth for any large-scaled Sceloporus, presumably so as not to impede passage between the thick leaves. In general, the megalepidurus are cryptically striped, as are some of the grammicus populations which primarily inhabit Yucca away from the range of the megalepidurus group. Grammicus group species are generally wood-crevice-users, and achieve their greatest population densities in areas where there are many logs or dead trees with loose bark. Up to 15 adults may live on a single large log, where all take cover in the space between the bark and wood. In these habitats the grammicus are particularly good at wedging themselves into an opening crevice as the bark is being pulled away by a predator or herpetologist. If the lizards had large spiny scales, as do the rock-crevice-users, they would be trapped in the crevice once the tension on the bark was released. However, the small, almost granular scales of the grammicus allow them to work their way out of the crevice once the tension on the bark has been released. It also seems likely that the kinetic skull may be used as a wedge in helping the lizard to force its way out of a crevice. These lizards are inter-

128 111 mediate in their degree of dorsoventral flattening, presumably because the juveniles are small enough to use the round holes bored in the solid wood by insect larvae, and selection for a cylindrical body form at this age prevents dorsoventral flattening from becoming extreme in the adults. Most grammicus have dorsal patterns of undulating bars, which are exceptionally cryptic against bark. The torquatus group species use rock crevices which are in general quite rough-textured and completely rigid to the normal predator. These lizards escape predation by wedging themselves into a crevice facing away from the predator and then arching their back and inflating their body to set the stiff spines of their large scales against the rough rock surface. The thick, generally spiny tail is then waved back and forth to prevent the predator from grabbing a leg or the body. And, even if a leg is successfully grabbed, in my experience it is frequently easier to pull the lizard's leg off its body than it is to get the animal unstuck from its crevice. Yet, when the predator leaves, the lizard deflates and easily extricates itself. Large and small rock-crevice lizards exploit structurally identical cover, so the pronounced flattening observed in these species is clearly adaptive at all ages. Coloration in the rock-crevice-users is generally not especially cryptic, but most species live in habitats where vegetation is sparse and the animals can usually see a predator coming from a great enough distance so that they can easily reach cover. From my experience, in contrast to some grammicus populations where the animals may be relatively easily caught if they are seen, all torquatus group species will take cover at the slightest provocation and clearly do not

129 112 attempt to rely on being cryptic. Size differences between the various species groups of crevice-users are probably at least partially due to trophic competition as constrained by the physical characteristics of the classes of crevices used. In many areas of the Mexican Plateau, two torquatus group species and a grammicus group species are sympatric (syntopic where habitats overlap), and within the range of the megalepidurus group, this group may add a fourth species to the sympatry or replace a torquatus group species. The size distribution of adults in areas of sympatry is always in the sequence: megalepidurus group < grammicus group < torquatus group A < torquatus group B. The physical limitations of the crevices exploited would seem to allow only this sequence of sizes, as 1) the biomass of prey in the vicinity of an Agave or small Yucca must be comparatively small, as it is rare to see more than one individual or pair on a plant, 2) logs and dead trees seem to provide an abundance of prey, but there is a definite upper limit to the size of lizard that can be accommodated in the under-bark crevices, and 3) rock crevices come in an essentially unlimited range of sizes, accommodating crevice-users such as the herbivorous iguanids, Sauromalus and Ctenosaura, which are many times the size of the largest Sceloporus. Presumably the first crevice-using Sceloporus exploited a wide range of crevices or at least had this potentiality available. After speciation began in the complex and derivatives became sympatric, trophic competition or competition for cover would demand ecological differentiation, perhaps most readily in size, as suggested by Williams (1972) for Anolis. However, the availability of different size distri-

130 113 butions of crevices in structurally rather different substrates would inevitably lead to specialization for the use of different substrate classes as well, leading in time to just those morphological differences observed between the different groups of crevice users. Virtually nothing is known about the behavioral ecology of asper, except that all specimens seem to have been taken on live trees. Based on subjective comparisons, it seems to be morphologically intermediate between the other crevice-users and the other large-sized, large-scaled Sceloporus, perhaps being closest to shannorum (omitted from Larsen's work) and heterolepis among the crevice-users, as indicated by Larsen (1972). It may well be that asper is a comparatively unmodified derivative of the ancestral stock from which the other crevice-users evolved. Reproductive biology. The reproductive biologies of the crevice-using species of Sceloporus are also derived and similar enough to indicate a possible common ancestry. Published observations on their reproductive biology (Mulaik, 1936; Axtell and Axtell, 1970; Axtell and Axtell, 1971; Goldberg, 1970, 1971) and my own observations on pregnancy and testicular activity when preparing karyotypes show that mating, testicular activity, and oogenesis in these lizards is maximal in the fall; that pregnancy begins in the fall or winter, and that birth takes place in the spring. Some grammicus populations and megalepidurus may have a second litter during the summer, but there is little doubt that most mating takes place in the fall. By comparison, most of the egg laying species show maximal testicular activity and mating in the spring, with their young hatching generally during the

131 114 summer. Other live-bearing Sceloporus belong to the formosus and scalaris groups. S. malachiticus (formosus group) from Costa Rica show maximal testicular activity during the summer, are pregnant during the fall and early winter, and most have given birth by early March (Marion and Sexton, 1971). From my own observations, formosus from Oaxaca were also pregnant early in the fall, S. aeneus and possibly some populations of the related scalaris are also live-bearing, but they are so distantly related to the crevice-users that this reproductive system has unquestionably been evolved independently. Live-bearing is a derived condition in lizards and is generally associated with adaptation to high elevation (Greer, 1967, 1968; Greene, 1970), and all of the live-bearing Sceloporus belong to basically high elevation assemblages; but the details of the reproductive cycles within the crevice-using species seem more similar to one another than they do to the formosus group species. Excepting asper, which Smith placed in the formosus group, but which Larsen and I agree is actually closely related to the grammicus and redefined megalepidurus groups, the formosus are clearly allied to the egg laying southern spinosus group radiation. Presumably the formosus have also independently evolved the live-bearing habit. On the other hand, the closely similar reproductive biologies of the two classes of creviceusers suggests a common origin, although these similarities taken alone certainly are not sufficient to prove a common ancestry. Biogeography. Again, the geographic relationships of the species treated in the present work are fully consistent with their genetic relationships as suggested by their karyotypic resemblances,

132 115 although other geographic derivations may be proposed with some justification. Figs. 15, 16, 17, & 18 illustrate the approximate ranges of these species. As background information, the only species in the largesized, large-scaled branch known to have the primitive 2n=34,XY male karyotype of the other sceloporine genera are three species included in orcutti by Smith (1939), Cole (1970), Hall (1969, and in prep.) [Hall and Smith, 1979]). These are restricted to the Baja California Peninsula (Fig. 14). Two of them are rock dwellers in comparatively xeric habitats, and the lizards of the third species [S. hunsakeri - Hall and Smith 1979)] are found mainly in the trees of the relict oak-conifer woodland of the Cape Region, south of the Isthmus of La Paz. Their closest main-land relatives are the small-sized, small-scaled species nelsoni, with the primitive 2n=34, XY male karyotype, and pyrocephalus, whose karyo-type differs from the primitive by a macrochromosomal inversion (Cole, 1971b; Hall, in prep.). Nelsoni and pyrocephalus are mainly ground and rock dwellers, with nelsoni being found in dry thorn scrub habitats, and pyrocephalus living in moderately open areas of the generally more mesic tropical deciduous forest. These two species appear to be separated by the lower reach of the Rio Grande de Santiago, which drains the Lago Chapala-Rio Lerma system. Note that the chromosomally primitive species occur around the periphery of the hottest parts of the Colorado-Sonoran Desert, north of the Río Grande de Santiago. This, of course, coincides with the supposition that Sceloporus are primitively xeric adapted. The distributions of the two clarkii group species are similar

133 116 to, though more extensive than, those of nelsoni and pyrocephalus. S. clarkii the chromosomally more primitive species, occurs north of the Rio Grande de Santiago, in generally more xeric habitats, while the chromosomally more derived melanorhinus are found to the south of the river (Fig. 15), in generally more mesic habitats. The clarkii are tree and rock users during the day, although they frequently retreat to the ground at night to sleep buried in sand or in burrows (pers. obs.). In Sonora and Arizona the species is generally found in the foothills below 1800 m in dry oak woodland situations. In Sinaloa and Nayarit the clarkii spread down onto the coastal plain, where they are more completely arboreal than they are to the north, although they can still be seen migrating to burrows in the ground at dusk. The melanorhinus occur to the south of the Rio Grande de Santiago in more mesic forest and woodland situations along rivers, etc., where they are highly arboreal. S. asper, possibly the earliest derivative of the crevice-user radiation, are found at intermediate elevations ( m) in the valley of the Rio Grande de Santiago and on the western parts of the Sierra Volcanica Transversal (Fig. 16). Asper, like melanorhinus and the southern clarkii, appear to be highly arboreal. Although asper are generally found at higher elevations, in cooler and more mesic areas than generally inhabited by clarkii, I would not be surprised to find these two species sympatric in some areas of the valley of the Río Grande de Santiago, nor would I be surprised to see some sympatry with melanorhinus in the area south of the river. The species megalepidurus and pictus are probably early

134 117 derivatives of the radiation leading to grammicus, to judge by their morphology. These species are found in the xeric basins at the eastern end of the Sierra Volcánica Transversal (Fig. 16). S. shannorum and heterolepis are two other well differentiated species which are more closely related to standard grammicus than are megalepidurus (Fig. 17). (I do not accept here Webb's 1969 lumping of these species, because the morphological differences between them--the conspicuous rows of enlarged paravertebral scales in heterolepis are completely lacking in shannorum -- are far more obvious than differences separating the proven cryptic species of grammicus. And, as will be seen below, there is no reason to think that these populations are currently connected by gene flow.) Somewhat as in clarkii and melanorhinus and in nelsoni and pyrocephalus, the deep, hot, and sometimes xeric barrancas of the Río Grande de Santiago and/or its major tributaries, the Ríos de Huaynamota and Bolaños, would appear to completely separate heterolepis and shannorum, with the morphologically more derived species, heterolepis, occurring to the south of the barrier. North of the barrier, shannorum seems to be restricted to intermediate elevations ( m), while to the south of the barrier, heterolepis seem to be found only above 2000 m. Although standard grammicus are probably parapatric with shannorum at present, the grammicus generally occur at higher elevations and along the crest of the Sierra Madre Occidental and generally on the east facing slopes of the Sierra. It seems likely that the distributions of shannorum and standard grammicus would have been separated by the crest of the mountains during the coldest Pleistocene periods. Although sequences of speciation can be deduced with some

135 118 confidence in the plant-crevice-users, because the torquatus group species are so well differentiated from other Sceloporus, it is difficult to say that one species is any more primitive than another. However, it may be significant that Larsen indicated that jarrovi, mainly a species of the western edge of the plateau north of the Lerma- Río Grande de Santiago system, is comparatively primitive by his analysis. It also appears (Fig. 18) that somewhat more speciation has occurred on the west side of the plateau, perhaps indicating the greater ages of these populations. Synthesis. If it is assumed that the clarkii group and all of the crevice-users derive from a close common ancestry in the largesized, large-scaled Sceloporus, then a simple phylogeny may be constructed which provides a straightforward account of the derivations of the modified karyotypes and of all other biological details summarized above. This phylogeny is summarized in Fig. 19 (cf. also Fig. 11). As can be seen from figures 16, 17, 18 & 19 and the discussions above, the major stocks we are considering in the present paper have rather linear ranges along the west coast of Mexico, which follow the major vegetational zones of that area as they are determined by elevation and humidity. Five major zones of climate and vegetation structure may be distinguished: 1) hot desert, generally with a floor of sand and bare rock, no trees, and scattered thorn bushes; 2) thorn scrub, generally with a floor of adobe or rocky soil, scattered thorn bushes, succulent xerophytes, and trees along drainages; 3) dry woodland, generally with deeper and more friable soil, ranging from oak woodland with juniper and Agave in the north to thorn forest and tropical

136 119 deciduous forest in the south (it should be noted that zones 2 and 3 interdigitate to a considerable degree); 4) cool mountain forest, with oak and mixed conifer; 5) xeric plateau, with few or no trees, scattered bushes and grass, much exposed rock, less equable and cooler climates. These vegetational zones are indicated in Fig. 19 by appropriate tone bands. Early divergences in the radiation under examination seem to have been restricted to the western coastal slopes of mainland Mexico north of the present Río Grande de Santiago, and seem to have involved ecological shifts outward from the thorn scrub community. The present ecologies of two of the three surviving karyotypically primitive species of the large-sized, large-scaled radiation (i.e., the orcutti complex species) and the related nelsoni and pyrocephalus indicate that the prototypical large-sized, large-scaled Sceloporus were probably ground and rock using inhabitants of the thorn scrub community. In Baja California, the orcutti complex species have survived in this community with probably little modification. However, on the mainland, competition with the related nelsoni stock and a host of other predominantly terrestrial sceloporines living in the hot desert and thorn scrub pushed the clarkii-crevice-use progenitor into the dry woodland habitat, where it began to make use of tree trunks as well as rocks for perches. Presumably this shift took place approximately when the Em polymorphism spread and the modified sex chromosome and reduced micro number (which survive with no further modification in clarkii) became fixed in the line. The divergence between the 40 chromosome clarkii group lineage and the primitive 32 chromosome crevice-using lineage seems to have

137 120 involved the fixation of the four macrochromosomal fissions in the ancestral clarkii. The clarkii stock then probably fully invaded the arboreal niche as a result of this speciation, thereby restricting its ancestral 32 chromosome stock more to the ground at lower elevations and/or pushing it into more mesic forested situations at higher elevations. As more chromosomally derived and predominantly terrestrial species of the large-sized, large-scaled radiation (these will be discussed in a later work) began to invade the area at low elevations, the clarkii stock was pushed even further up into the trees, and the 32 chromosome (prototypical crevice-user) line then became completely restricted to the slopes of the volcanic Sierra Madre Occidental. This cooler mountainside habitat is now occupied by the crevice-using asper, shannorum, and bulleri. Here, the evolution of live-bearing habits would be favored by the generally lower temperatures, shorter growing season, high humidity, and a probable scarcity of suitably loose and sun-warmed soil for egg incubation (there are certainly many areas in the present habitat where soil is scarce because of the precipitous slopes). It also seems likely that crevices were the most abundant class of cover readily available in this generally rocky zone, which would undoubtedly encourage the evolution of specializations for using this cover. The standard X 1 X 2 Y sex chromosomal fusion probably spread through this prototypical crevice-using stock some time before it became geographically subdivided. Once forced to high elevations, the ancestral crevice-user became specially adapted to them and thereby during climatic fluctuations became increasingly liable to geographic disruption, isolation,

138 121 and speciation on or between different mountain masses. Subsequent contacts and sympatry during climatic optima would inevitably lead to size and/or ecological displacement within the broad range of creviceusing niches, as described in the preceding sections above. Also, once the crest of the Sierra Madre was breached by the crevice-using radiation, the whole of Mexican highlands became available for its proliferation. This is not the only possible phylogenetic reconstruction which could account for the observed geographic relationships of the different chromosomal stocks treated in this report, but it does have the advantage of simplicity to support a plausibility arising from its complete consistency with present geography. And, more importantly, it is completely consistent with the relationships predicted from the karyotypic data alone (compare Figures 11 and 19). On the other hand, Larsen's (1972) phylogeny, for example, requires rather extensive longitudinal shifts in some of the faunas to derive present day species from their supposed ancestors. Hopefully, data from the analyses of additional suites of characters will provide a more definite picture.

139 122 PATTERNS OF SPECIATION Introductory Comments In a future work of this series I will attempt a detailed comparative study of speciation in the whole sceloporine radiation to test the revolutionary speciation hypothesis presented in the beginning of the present report. However, now that a phylogeny of the creviceusing and clarkii group radiation has been synthesized, a preliminary analysis of speciation patterns in it may be made. In this analysis one should note that the conclusions drawn from it are no better than the phylogeny on which it is based. However, if the phylogeny is realistic and the chromosomal speciation hypothesis valid, one should find several distinct phenomena in this radiation: 1) In conservative or classical geographic speciation, species will be formed where the range of an ancestral population has been disrupted by a geographic barrier. In this situation one does not expect heterozygously semisterilizing chromosomal differences to be fixed between sister species. Nor does one expect to find indications of rapid ecological differentiation between the sister species--unless the differentiation obviously has been forced by differences in the other species competing for the respective environments of the sister species. 2) In revolutionary speciation, species will be formed in situations where there is no indication that the ancestral population was subject to obvious geographic disruption. This should be in clear contrast to the disruption that can be seen in cases of conservative speciation. In revolutionary speciation one also expects to find fixed

140 123 between the sister species chromosomal differences which are potentially semisterilizing when heterozygous. Sister species probably will be parapatric, or even sympatric, and either will show considerable ecological differentiation (in the sympatric situation) or geographic exclusion, if ecological differentiation is not possible in the environmental framework of the competing species. In general, the chromosomally derived species will show the greatest ecological displacement from the ancestral condition. 3) Revolutionary speciation may form cascades of species. Some instances of revolutionary speciation may initiate a cascade process (see discussion at the beginning of this report) which will lead to the rapid and sequential formation of several species by the revolutionary mode. These sister species will show more linear than highly branched sequences of increasing chromosomal derivation, which may terminate with neutral polymorphisms or exhaustion of the chromosomal possibilities that provided the mutations that facilitated the speciation. All three of these phenomena seem to have occurred in the radiation of the clarkii group and the crevice-users. However, before they are examined, the biological background against which the speciation of the radiation occurred should be reviewed, as this speciation did not take place in a vacuum. At the beginning of the radiation, there were a host of related and unrelated lizard species competing for xeric and subxeric habitats along the NW coast of Mexico. Minimally there must have been at least one small-sized, small-scaled Sceloporus (i.e., the nelsoni prototype), at least one Uta, at least one Urosaurus,

141 124 at least one sand lizard (Callisaurus, Holbrookia, or Uma), probably a Petrosaurus, and at least one Phrynosoma among the sceloporines; and a Crotaphytus or two, probably at least three iguaninines, and possibly some Anolis (in less xeric areas) among the other Iguanidae; not to mention several representatives of at least two other lizard families. Later, as the present lineage radiated, several other Sceloporus species besides those resulting from the radiation would be added to the fauna of competing lizards. In such a complex community, one might expect all Sceloporus to evolve rather sharp niche specializations, as have Anolis species in complex faunas. However, many Sceloporus, such as clarkii for example, still show surprisingly great ecological plasticity, perhaps because they have evolved especially good "general" adaptations that allow them to function in a variety of habitats. This background should be kept in mind in the discussion to follow.

142 125 Conservative Geographic Speciation There are several clear examples of conservative geographic speciation in the present radiation. At least five species pairs appear to have formed across the lower Río Grande de Santiago. This river, though not impressive during the present, comparatively dry climatic regime, drains almost all of the high plateau enclosed by the Sierra Madre Oriental, the Sierra Zacatecas, the Sierra Volcanica Transversal, and the southern half of the Sierra Madre Occidental. Although the lower reach of this drainage now meanders gently across the short stretch of coastal plane, it must have been an important barrier during Pleistocene pluvial periods. Furthermore, for species distributed at higher elevations along the western fronts of the Sierras, the deep, hot barrancas of the Rio Santiago and its major tributaries would add major ecological barriers to the river itself. The river approximately divides at least five pairs of sister species: nelsoni/pyrocephalus, clarkii/melanorhinus, possibly the primitive rockcrevice-user/primitive plant-crevice-user, asper/shannorum, shannorum/heterolepis, and possibly jarrovi/dugesii. To review these cases, both pyrocephalus and melanorhinus are karyotypically derived with respect to their sister species, but in melanorhinus the difference is due to the spread of a Y-autosomal fusion which clearly could not have had a net adverse effect on fitness or it would have spread through the population. In pyrocephalus, the fixed inversion difference which differentiates it from nelsoni may have been adaptive as a balanced or transient polymorphism, as indicated by Cole's (1970) report of a similar inversion polymorphism in clarkii. In many

143 126 organisms such inversion heterozygosity does not appear to reduce fertility, apparently because chiasma formation in inhibited in the mutually inverted region. The remaining species pairs separated by the Río Santiago are chromosomally identical, and except for the important divergence between the primitive rock- and plant-creviceusers, the ecological differences of the species pairs are slight and clearly the result of climatic differences in their respective ranges. Also, even the early divergence between the rock- and plant-creviceusers can be explained by climatic differences. North of the Río Santiago, the slopes of the Sierra Madre Occidental are much more xeric than are the slopes of the Sierra Volcanica Transversal to the south of the river. Presumably rock crevices would be the most common cover on the xeric mountainsides then, as it is now (pers. obs.), while tree holes and crevices would be more available in the humid habitats to the south. Any specializations for the different crevice types would be enhanced when the sister species again became sympatric. Other conservatively evolved species pairs appear to have been formed by the divide of the Sierra Madre Oriental. These divisions probably resulted from separations during Pleistocene cold periods. Examples are: shannorum/standard grammicus and possibly jarrovii/bulleri. All speciation in the torquatus group appears to be conservative. Once the primitive torquatus became specialized for the exclusive use of rock crevices, populations could survive only in areas of rock outcropping. On the highlands of the Mexican Plateau, there are many rocky mountains surrounded by "seas" of alluvial soil. Although colonists can probably cross these seas easily enough to found new

144 127 populations, the "seas" should provide enough isolation to allow abundant opportunities for conservative speciation, with or without a genetic revolution in the founder population. And in this respect, it is worth noting that, excepting the cases of sympatry which prove that the overlapping populations are good species, the present species taxonomy of the torquatus group is arbitrary to a high degree. Each mountain range rising above the alluvial floor has its own torquatus group populations which can usually be taxonomically distinguished from those of other mountains if someone wishes to look at enough characters. The origin and speciation of the megalepidurus group are less clear. First, although megalepidurus and pictus are treated as full species in the present work, samples collected in the area where they meet are morphologically intermediate. This problem is still under study, but it seems likely that these two "species" are only the geographic extremes of a single intergrading population. Secondly, it is difficult to determine where and when the ancestral megalepidurus originated from the standard grammicus that probably gave rise to it, because standard grammicus have probably displaced megalepidurus from a good part of its former range.

145 128 Revolutionary Speciation Likely examples of revolutionary speciation involving chromosomal differentiation are shown by the divergence of the 2n=32 neo XY stock [now assumed to be extinct] from the primitive 2n=34 lineage (assuming that the sex chromosomal modification does not account for the reduction in 2n) and by the split between the 2n=40 clarkii group and the 2n=32 neo XY stock. Then, of course, the grammicus complex seems to provide an ideal example of cascading speciation. However, the two older cases must be mentioned first. Of these, the 2n=34 XY/2n=32 neo XY speciation event is so old that it has been greatly obscured by subsequent speciations and extinctions, and therefore analysis of it would not be profitable; but the 2n=32 neo XY/clarkii speciation seems less obscure, though it must also be rather old. If the 2n=32 neo XY stock was basically ground and rock dwelling (see page 119), then the shift of clarkii up into the trees represents a major ecological shift which might be associated with its chromosomal derivation and revolutionary speciation. However, if the ecologies of the present day clarkii and the crevice-using derivatives of the 2n=32 stock are any indication, the primitive clarkii was sufficiently plastic in its ecology to exert a continual pressure on the more conservative ground dwelling 2n=32 XY stock. This pressure, and other competitive pressures from a series of species in the more xeric area north of the Rio Grande de Santiago would tend to force the still ground dwelling 2n=32 XY stock up the slopes of the Sierra Madre, where it evolved the live-bearing and crevice-using habits and the Y-autosome fusion spread throughout the primitive crevice-using stock.

146 129 It should be noted that, in the present approximately parapatric distributions of the clarkii and crevice-using stocks along the whole length of the Sierra Madre, there is no evidence for a past geographic separation of the stocks. However, it should be recalled that the derivation of the 2n=40 clarkii lineage from the 2n=32 ancestral stock must have been older than separations of the species pairs discussed in the preceding section. From this we may conclude only that the derivation of clarkii is too old to provide any evidence for or against speciation involving the geographic isolation of the primitive and derived stocks. As will be seen in the following section, the evidence from grammicus is considerably clearer, presumably because the speciation in this group is much more recent than that which gave rise to clarkii.

147 130 Cascading Revolutionary Speciation From this distance in time, and since no intermediates survive, it is impossible to say whether the four fissions fixed in the clarkii group karyotypes were fixed simultaneously in a single event or in a sequence of speciation events. On the other hand, there is no doubt that a sequence of karyotypes exists in grammicus, and that in at least some instances the karyotypically different populations are good species. Furthermore, the karyotypic sequence in grammicus has most of the characteristics predicted by the cascading speciation model. And, in comparison to the obvious geographic differentiation of the torquatus group species which live on comparative isolated and geologically stable rock "islands," the grammicus live in the much more continuous vegetational phase of the environment. Within this vegetational phase, at least standard and F5+6 grammicus seem plastic enough in their physiological tolerances that they can live anywhere from desert to rain forest, where they can find suitably large plants or plant products which have crevices that are not occupied by other grammicus races. And even areas that are largely uninhabitable by grammicus because they contain no large plants may still be crossed along water courses: for example, grammicus have been taken from trees along the Río Nazas near the Coahuila-Durango border, which is well out in the otherwise completely uninhabitable area of the Chihuahuan Desert (Bogert, 1949). From this it should be obvious that grammicus will have had much less opportunity for conservative speciation than the torquatus have had. And where apparently conservative speciation has occurred in the grammicus group, i.e., between the pairs

148 131 shannorum/heterolepis and shannorum/standard grammicus, this speciation seems to have been associated with obvious geographic barriers and is not associated with chromosomal differentiation. On the other hand, there are no obvious geographic barriers associated with the speciation in the grammicus complex (which does not prove that such did not exist-- there is just no evidence for them). And, most significantly, it is precisely these grammicus populations, and not the populations of the torquatus group, etc., which are differentiated by the chromosomal mutations likely to reduce heterozygote fertility. The ecological shifts predicted by the revolutionary speciation model have not occurred in the grammicus complex. However, any major ecological differentiation between species would seem to be precluded because all of the ecologically adjacent habitats seem to be fully occupied by other Sceloporus or other lizards. On the other hand, the grammicus complex species exclude one another geographically, which is the alternative prediction of the hypothesis. The phyletic structure of the species cascade in grammicus is adequately described in the section on the phylogeny of karyotype evolution (page 119) and need not be described again here. It should be sufficient to note that it is the terminal FM populations which are highly polymorphic for fissions, and that the polymorphic P1 may also represent the end of a sequence (S F6 Pl). Also, it is pertinent that these polymorphisms involve only the largest and most symmetrical chromosomes, which might not function well in speciation because assortment from their trivalents would be most nearly regular (White, 1973) and because their malassortment products may be gametically rather than

149 132 zygotically lethal. In other words, in the FM populations the substrate for further speciation by fissioning has essentially been exhausted; and in the P1 population, the chromosome involved in the polymorphism is the one least likely to be successful in initiating further speciation, and therefore most likely to be involved in an aborted speciation event. These situations are, of course, completely consistent with the cascading speciation model. Another coincidence with the cascade model is the failure to find populations karyotypically intermediate between F5+6 and FM, which may indicate that these intermediates had little chance to spread before they, in turn, spawned chromosomally more derived species by the revolutionary mode. These missing intermediates then suggest that the rate of speciation may have accelerated as the sequence of derivation progressed. Finally, although I have treated them as one species above--on only flimsy evidence--it is entirely possible that the coastal and plateau F5+6 populations independently differentiated from an F6 ancestry. This would provide another short branch to the cascade; but, even so, the cascade is much more linear than branched. In short, where relevant data are available, the observations are fully consistent with the cascading speciation model and cannot be readily explained by anything other than some kind of chromosomal speciation model.

150 133 Geography of Revolutionary Speciation As discussed in the previous sections, there are three somewhat different and not necessarily exclusive geographic circumstances under which chromosomal speciation may occur: one which supposes that speciation can occur only on the geographic periphery of the parental stock if it has a continuous distribution within that range (the "peripheral version"), one which supposes that speciation can occur anywhere within the normal range of a parental stock whose population is subdivided into effectively very small semi-isolated breeding pools (the "interstitial version"), and an intermediate one which supposes that a species may be formed on one of the many "internal peripheries" of a parental stock which is restricted to habitats which show mosaic distributions within the broad geographic range of the population. Of the three, the interstitial speciation would seem to deviate roost extremely from classical allopatric models. At least in some parts of its range, grammicus seem to have ideal population structures for interstitial speciation. In areas where P1 and F6 occur on the eastern divide of the Valley of Mexico, most individuals live on dead trees and fallen logs, and only a few grammicus are found on live trees or near other classes of cover. Based on many anecdotal observations (Moody et al., in prep.), where the logs are widely scattered (as they are in many places), these logs would seem to serve as fairly well isolated "islands" until they decay, which to judge from our observations over a two-year period, probably takes ten to twenty years. That is not to say that no grammicus disperse past these logs, but only that the populations on the logs will

151 134 probably remain inbred because dispersing individuals will find it difficult to establish residency on already inhabited logs. Although grammicus appear to be territorial when living in areas with dispersed cover (e.g., in Agave or on Yucca), when five or ten adults live on a log they appear to shift into a social hierarchy with despotic males, as do other iguanids when exploiting concentrated resources (Hunsaker and Burrage, 1969). Since most grammicus found in habitats unsuitable for overwintering have been young-of-the-year, it is likely that these young do most of the dispersing. Since adults actively chase young, once a juvenile is chased away from its home log--where it was probably familiar with cover and other resources--it seems unlikely that it could establish residency on another well-populated log where the cover and other resources would be initially unfamiliar. Presumably, then, once a newly fallen log is colonized, the population on it will be fairly effectively isolated by its communal defense of the log until it decayed to the point where it was no longer habitable. Since the maximum populations of these logs range from five to usually no more than 15 adults plus their young, it would seem that grammicus living in this type of habitat have an ideal population structure for interstitial speciation. A mark-and-observation analysis of dispersal and population structure on the east side of the Valley of Mexico was begun to support the anecdotal observations and proved to be completely feasible, but the time demands for other aspects of this study were too great for the analysis to be very useful. It is hoped that an effective study of this nature can be completed in the future. Since the basic population structure of grammicus seems ideally

152 135 suited for interstitial revolutionary speciation, the grammicus radiation might serve to test whether the speciation is strictly a phenomenon of the species border or whether it actually can occur interstitially. If such speciation were recent enough, a derived population should be completely surrounded by its ancestral stock. In this situation, the derived population will have either excluded its ancestral stock from that part of its former range, or the derived stock will have diverged enough to live sympatrically with the ancestral stock. However, for ecological differentiation to occur, an adjacent niche must have been available to the differentiating species; and, as we have seen above, the niches adjacent to grammicus are probably sufficiently saturated with other Sceloporus to completely preclude this possibility. Therefore, the only test would be to find encircled populations that exclude the ancestral stock from areas that it would otherwise inhabit. If Pi derives from F6, which is at best a questionable possibility, this condition would be met (excluding the possibility that speciation could have occurred along the sharp edge of the treeline, which would function much like a continental margin, etc.). But, even here. Hall and Selander (1973) were able to propose reasonable allopatric models to explain the speciation (but, on the other hand, the allopatric models do not account for the chromosomal differentiation between the various grammicus). The other grammicus races are either too poorly mapped or appear to have spread to the geographical limits of the climatic or ecological zones within which they can exclude other grammicus from the plant crevice niche, and therefore do not provide unequivocal tests either. However, it does seem significant that all of

153 136 the derived populations except F5, and perhaps the coastal F5+6, occur well within what must have been the historical geographic and ecological range of standard grammicus. This suggests that differentiation occurred either on an internal periphery or was strictly interstitial. Tending to support the interstitial alternative is the great ecological latitude shown by standard grammicus in the southern part of its range, where an apparently continuous population occurs in all wood- and plant-crevice habitats from rain forests (over a minimum altitudinal range of from 1500 m to 3600 m-confirmed by karyotyped specimens) to Yucca and Agave in the most xeric parts of the southern (and northern) plateau. All derived grammicus occur well within the ecological range inhabited by standard populations. But, to take the contrary view again, it is impossible to estimate the importance of the major climatic fluctuations of the Pleistocene in the speciation of grammicus. So, to summarize this section, while the data presented do not prove that speciation in grammicus has occurred interstitially, they do suggest the possibility. And, as is true for most evolutionary questions, it is unlikely that field data will ever provide an absolutely unequivocal proof of the possibility. However, sufficiently detailed studies of enough other radiations comparable to that of grammicus may eventually provide enough evidence to be convincing. Furthermore, many aspects of the speciation model should be susceptible to modeling by computer simulation, which would allow values to be estimated which would be required of various parameters of population structure and genetic system before interstitial or other kinds of chromosomal speciation could become likely. Once these values have been calculated, it should

154 137 then be relatively simple to go into the field to see whether natural populations conform to them. Much work can be done in this area.

155 138 SUMMARY AND CONCLUSION In this work I have developed a chromosomal speciation model for animals of limited vagility or subdivided population structure which combines and extends aspects of Mayr's (1942, 1954, 1963) genetic revolution, the chromosomal fixation model of Wright (1941), and the chromosomal speciation models of White (1968, 1969; White et al., 1967) and Key (1968). The theory suggests that speciation may occur in certain species under a variety of geographic conditions without the requirement for prolonged or absolute allopatric isolation if these species have suitable population structures and genetic systems. Basic requirements for chromosomal speciation are: 1) A meiotic system in which there is an appreciable frequency of malassortment from structureally heterozygous pairing units in the first meiotic division, which thereby reduces the reproductive fitness of the heterozygotes without affecting fitnesses of either homozygous condition. 2) An appreciable mutation rate for such heterozygously semisterilizing chromosomal rearrangements. And 3), a population structure such that many local populations become effectively small enough and well enough isolated so that such mutations may occasionally become fixed by chance in local populations. Given the initial fixation event in a non-optimal environment for the ancestral stock (which may likely be associated with a Mayrian genetic revolution, since the conditions required for the fixation of the chromosomal difference are also ideal for the revolution), the chromosomally differentiated local population will occasionally be endowed with a superior local adaptation. If this happens, the

156 139 chromosomally differentiated population may survive its early interactions with the ancestral stock. Then the chromosomal semisterility of the heterozygotes will 1) considerably reduce gene flow into the "pure" population of homozygous mutants, and thereby provide partial intrinsic protection to the new adaptive balance of the population of homozygous mutant individuals; 2) it will act as an intrinsic selective mechanism to weed out genetic variability which promotes hybridization; and 3) it will act as a "semipermeable filter" (Key, 1968) to allow the passage of genie variability which integrates well with the epistatic balance, thereby speeding the differentiation. If the chromosomally derived population has achieved a superior adaptation and/or is assisted by chance events so that it can grow large enough to form a defined zone of parapatric hybridization with the parental stock (assuming that premating isolation has not been evolved), the reduced fecundity of chromosomal heterozygotes in the hybrid zone will lower the population pressure in the zone, thereby encouraging net migration--and hence gene flow--into the hybrid zone. This will add considerably to the effectiveness of the heterozygote semisterility as a barrier to gene flow into the "pure" population beyond the hybrid zone, because the zone will function as a "sink" to trap, even many of those genes which may once or twice successfully introgress into chromosomal homozygotes. Once the "sink" effect becomes operative, it would also tend to prevent the further evolution of premating isolation by the parapatrically hybridizing populations by trapping genes that were only partially effective. Therefore, notwithstanding the lack of effective premating isolation or complete

157 140 hybrid sterility, the populations removed from the hybrid zone could be sufficiently isolated genetically to evolve independently as good biological species. Since 1) such chromosomal speciation is largely a function of intrinsic aspects of the genetic system and population structure, which themselves are genetically determined; since 2) speciation will be most likely in those areas of the ancestral stock's range where the genetic conditions are most favorable for it; and since 3) the derived species is likely to perpetuate those favorable genotypes: such chromosomal speciation may initiate a cascading process which will amplify the probability that derived species will in turn give rise to even more derived species. Because of this amplification effect and the fact that the first species to be derived from an ancestral stock is likely to fill the ecological niches adjacent to the ancestral stock, either by itself or by its own further derivatives, before the ancestral stock can give rise to a second derived species (= "lateral inhibition"), cascades of species will tend to show sequences of karyotypic derivation that are more linear than fan-like. Also, because of the genetic inertias of the larger ancestral populations, the karyotypically derived species will either be ecologically derived or specialized with respect to its ancestral stocks. Karyotypically diverse rodent radiations were reviewed which seem to exemplify the predicted phenomena of the chromosomal speciation theory (Perognathus: Heteromyidae, Spalax: Spalacidae, and Sigmodon: Cricetidae, among the best known), and in which these phenomena cannot be readily explained by other cytogenetic mechanisms. These were

158 141 compared with a karyotypically diverse radiation (Peromyscus: Cricetidae) which can be readily accounted for by other mechanisms. Unfortunately, the published information on these radiations is too limited to provide more than suggestive evidence that they have resulted from chromosomal speciation as described in the present model. However, the much more detailed analysis of the karyotypic variation and cryptic speciation in the Sceloporus grammicus complex, which is presented in the present report, exemplifies almost all of the phylogenetic and phenomenological predictions of the chromosomal speciation model; and, unlike the rodent radiations discussed that lack good comparative backgrounds against which the evolutionary significance of their karyotypic variability can be judged, the radiation of the other crevice-using Sceloporus provides an ideal background for comparison. As far as can be determined from the present speciation in the grammicus complex seems to be strictly associated with chromosomal differentiation, And, in comparison to the rock-crevice-using torquatus group, which has radiated in a habitat which provides ample opportunity for geographic speciation, and which is diverse enough to allow size differentiation for sympatry, the grammicus complex has proliferated almost as many species in its much more limited and geographically continuous habitat. Relative to the torquatus radiation, it would then seem that the grammicus have proliferated many more species than would be expected if speciation could occur only by the classical allopatric models. Also, data to be presented in later reports indicate that similar but older radiations of chromosomal speciation have occurred in other branches of Sceloporus, which in large part account for the great

159 142 species diversity of Sceloporus in comparison to the chromosomally conservative and conservatively speciating remaining sceloporines. Clearly, much work still needs to be done before the details of all of the speciation in Sceloporus are fully understood, but at least the present data seem to provide more than suggestive evidence for some mode of chromosomal speciation that depends more on intrinsic characteristics of the species than does classical allopatric speciation. However, as is true for most evolutionary studies, the evidence does not conclusively prove that the speciation has occurred exactly as I have reconstructed it.

160 143 ACKNOWLEDGMENTS I dedicate this work to Don Hunsaker, II, who suggested the initial problem; to R. W. Axtell, who got me off to a good start on it; and to E. E. Williams, who saw it through to at least a partial completion. Without their assistance and moral support throughout, the work would never have been done. Ticul Alvarez-S. assisted with all aspects of the work in Mexico. A. Ornstein, S. M. Moody, T. Dickinson, S. Reichlin, R. B. Stamm, R. Garcia-L., A. Ocana, P. Huerta-M., and H. Pedraza-M. all provided extensive help in the field. Valuable specimens were provided by K. Larsen, L. J. Bussjaeger, R. F. Clarke, R. W. Axtell and others. A. Russell and H. F. Londe assisted with the cytology. Drs. R. Hernandez-C. and B. Villa-R., both of the Direccion General de la Fauna Silvestre, SAG, arranged for collecting permits and assisted in other respects. Laboratory space for cytological work in the field was provided by Texas A & I University, AMNH Southwest Research Station, California State College-San Diego, Escuela de Ciencias Biologicas of the Universidad de Nuevo Leon, Escuela Nacional de Ciencias Biologicas of the Institute Politecnico Nacional, and the Departamento de Prehistoria of the Institute Nacional de Antropologia y Historia. The Departamento de Prehistoria also provided maps, surveying equipment, and a jeep for much of the field work in the Valley of Mexico. Many of the issues in this study were discussed with E. E. Williams, E. Mayr, R. K. Selander, R. W. Axtell, and H. M. Smith. I thank Williams, Mayr, and R. C. Rollins for critically reading the manuscript and making many useful suggestions for improvement. Any

161 144 faults that remain are mine. I also thank Catherine McGeary for her assistance in preparing the manuscript. This research was supported by grants from the National Geographic Society Committee on Research and Exploration, the Society of the Sigma Xi (both to Hall), and the National Science Foundation (GB and B X to E. E. Williams, Harvard; and GB-3167, 7346, and to R. C. Rollins, Harvard).

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180 Zimmerman, E. G Karyology, systematics, and chromosomal evolution in the rodent genus, Sigmodon. Publ. Mus. Michigan State Univ. Biol. Ser. 4: Zimmerman, E.G. and M. R. Lee Variation in chromosomes of the cotton rat, Sigmodon hispidus. Chromosoma 24;

181 162 APPENDIX: SPECIMENS EXAMINED NOTE; Catelog numbers refer to the MCZ karyology catelog, except that the "SIU ser" numbers refer to specimens retained by Southern Illinois University, Edwardsville. However, the karyotype preparations from these specimens have been entered in the MCZ karyology collection. Clarkii group Sceloporus clarkii (n=94): Specimens karyotyped by Lowe, et al. (1967) and Cole (1970); (n=81). Specimens reported herein: S. c. clarkii: ARIZONA: Cochise Co., 1 mi W AMNH Southwest Research Sta. [near Portal] (SIU ser 40). SONORA: 24 km S Hermosillo, 180 m, 12RWG0493 (SIU ser 125). SINALOA: 6 km NNW Mazatlan, 5 m, 13QCR5272 (F ; SIU ser 95/96, 97, 104/105, 110, 135/136). NO LOCALITY: (Y44325). S. c. boulengeri: NAYARIT: 35 km SE Tepic, 1200 m, 13QEP3855 (Y44245, Y44246). S. c. uriquensis; no specimens karyotyped. Sceloporus melanorhinus (n=13): Specimens karyotyped by Cole (1970); n=7. Specimens reported herein: S. m. calligaster: NAYARIT: San Bias, 5 m, 13QDP7083 (Y14693, Y44299, Y44301); at Rio Grande de Santiago, 3 km W Yago, 20 m, 13QDQ8914 (SIU ser 103). COLIMA: 10 km E Manzanillo, 100 m, 13QEM8205 (Y44250); "Rio Maria Basio, W of Manzanillo" (Y44249). S. m. melanorhinus; no specimens karyotyped. S. m. stuarti; no specimens karyotyped. Asper group Sceloporus asper (n=6): NAYARIT: 35 km SE Topic, 1200 m, 13QEP3855 (F5809, Y44247, Y44248). JALISCO: 16 km S Tuxpan, 1390 m, 13QFM7247 (F5166, F5133). MICHOACAN: NE quadrant of Uruapan, 1700 m, 13QHM0949 (F5201).

182 163 Megalepidurus group Sceloporus megalepidurus (n=12): MEXICO: 9 km ESE Otumba, 2600 m, 14QNS3376 (F6289). PUEBIA: 5 km NE Entronque Zacatepec, 2600 m, 14QPS6439 (F6156, F6157, F6230, F , Y25556); 5 km NW Atenco, 2550m, 14QPS5013 (F ). Sceloporus megalepidurus x[?] pictus intergrades (n=14): PUEBLA: 8 km SE Cuidad Serdan, 2570 m, 14QPQ6994 (F , F6206, F6207, F6247, F6248, F6252, F6276, F6305, F6307, F6309, Y25591). Sceloporus pictus (n=2): PUEBIA: 15 km ESE Amozoc de Mota, 2260 m, 14QPS1400 (Y25568, Y25569). Grammicus group Sceloporus shannorum (n=5): SINALOA: 14 km ENE Copala, 1950 m, 13QDS1302 (F5065-7, F5069, F5070). Sceloporus heterolepis (n=4): JALISCO: Cerro Tequila, 10 km S Tequila, 2400 m, 13QPF1900 (F5827, F ). Sceloporus grammicus complex (n=1217) [detailed locality data will be found in Hall and Alvarez, in prep.]: Standard grammicus (n=339): TAMAULIPAS (9), NUEVO LEON (42), COAHUILA (72), ZACATECAS (1), DURANGO (16), DISTRITO FEDERAL (3), MEXICO (94), TLAXCALA (34) 1, MORELOS (10) 2, PUEBIA (38), VERACRUZ (5), OAXACA (15). Polymorphic-1 grammicus [exclusive of C. Potrero transect sample] (n=286): MEXICO (275), PUEBLA (11). Pi X F6 heterozygotes [hybrids and backcrosses exclusive of C. Potrero transect sample] (20). Cerro Potrero hybrid zone transect sample (167). Fission-6 grammicus [exclusive of C. Potrero transect sample] (n=226): NUEVO LEON (7), TAMAULIPAS (2), SAN LUIS POTOSI (2), TLAXCALA (10), PUEBLA (20), MEXICO (129), MORELOS (19), DISTRITO FEDERAL (7),

183 164 MICHOACAN (22), JALISCO (8). Fission-4+5 grammicus (n=33): TEXAS (6), NUEVO LEON (2), TAMAULIPAS (6), ZACATECAS (2), SAN LUIS POTOSI (11), GUANAJUATO (4), QUERETARO (1), HIDALGO (1). Multiple fissions (FMl) grammicus (n=10): MEXICO (8), HIDALGO (2). Multiple fissions (FM2) grammicus (n=110): HIDALGO (35), MEXICO (75); Backcrosses to S (n=2) 3 ; F 1 hybrids (n=8) and probable and possible backcrosses to FM2 (n=6). Torquatus group Sceloporus jarrovii (n=55): Specimens karyotyped by Cole et al. (1967): (N=11). S. j. cyanostictus; specimen karyotyped by Axtell and Axtell (1971): (n=1). Specimens reported herein: S. j. oberon [?]; COAHUILA: 13 km SSE Arteaga, 2200 m, 14RLD2005 (F5415). S. j. minor: NUEVO LEON: 12 km ENE Iturbide, 950 m, 14RMC1737 (Y25474, Y25476, Y25482); 13 km WNW Galeana, 2000 m, 14RLC8052 (F5966-8); 27 km S Galeana 2150 m, 14RLC9919 (F , F5677); 36 km S Galeana, 2180 m, 14RMC0011 (F5425, F5473-5); 19 km NNW Ascencion, 2200 m, 14RLC9808 (F5960-3); 10 km NNW La Escondida, 2050 m, 14RMB0157 (F5383, F5384, F5390). TAMAULIPAS: Marcela, 20 km N Miquihuana, 3000m, 14QMB1927 (Y44295). SAN LUIS POTOSI: 14 km S Matehuala, 1575 m, 14QLB3403 (F5469); 4 km NNE Cuidad de Maiz, 1400 m, 14QMV4081 (Y25528, Y25543); 4 km WSW Charcas, 2200m, 14QKA7958 (Y25539, Y25541); 10 km N Ahualuico, 2000 m, 14RKV7688 (Y25517, Y25538); 17 km WNW Santa Catarina, 2650 m, 14QLV3144 (F5683, F5684); 16 km SSE Cardenas, 1000 m, 14QMV3817 (F5992). S. j. immucronatus; MEXICO: 24 km ESE Acuico, 2750 m, 14QMT3516 (F6006, F6007). S. J. sugillatus; no specimens karyotyped. S. j. jarrovii; DURANGO: 7 km SSW Empalme Purisima, 2750 m, 13QDS8840 (F5051, F5081). ARIZONA: Cochise Co., Chirichaua Mts. (SIU ser 38, 126).

184 165 Sceloporus ornatus (N=7): S. o. ornatus: COAHUILA: 24 km NNW Santa Cruz, 930 m, 14RKD8565 (Y25500, Y25501); 18 km NNW Santa Cruz, 1000 m, 14RKD8368 (SIU ser 240/241, 242/243/244); 3.5 km S Ramos Arizpe, 1600 m, 14RLE8918 (Y25498, Y25499, Y25505). S. o. caeruleus: no specimens karyotyped. Sceloporus dugesii (n=6); S. d. dugesii: JALISCO: 9 km W Juchitlan, 1375 m, 13QEN8422 (F5125, F5126, F5128). S. d. intermedius; MICHOACAN: 7 km E center of Morelia, 2000 m, 14QKS7778 (F5164, F5166, F5167). Sceloporus bulleri (n=l): SINALOA: 14 km ENE Copala, 1950 m, 13QDS1302 (F5080). Sceloporus insignis: no specimens karyotyped. Sceloporus mucronatus (n=31): S. m. mucronatus: HIDALGO: 23 km ESE Pachuca, 2640 m, 14QNT4815 (F6034, F6035). MEXICO: 5.5 km ESE Otumba, 2750 m, 14QNS3177 (F6287); 9 km ESE Otumba, 2750 m, 14QNS3472 (F6290, F6291); 5 km WSW Rio Frio, 3200 m, 14QNS3037 (Y25585); 13 km WSW Tenancingo, 2600 m, 14QMR2593 (Y25571). VERACRUZ: 4 km ESE Las Vigas, 2200 m, 14QQS0670 (F6202, F6304, F6308); 8 km ESE Las Vigas, 14QQS0570 (F6300). S. m. aureolus x[?] mucronatus intergrades: 5 km NW Atenco, 2550 m, 14QPS5013 (F6289, F6299); 10 km ESE San Salvador El Seco, 2530 m, 15QPS5112 (Y25570). S. m. aureolus; VERACRUZ: 5 km W Acultzingo, 2035 m, 14QPR7569 (F6453, F6454). S. m. aureolus x[?] omiltemanus intergrade: OAXACA: Jicotlan, 19 km NNW Tamazulapan, 2250 m, 14QPQ6270 (Y14688). S. m. omiltemanus: OAXACA: 4 km WSW Tiaxiaco, 2200 m, 14QPQ3608 (F6431-3, F6436, F6437, F6440, F6441, F6443); 16 km NE Oaxaca, 2520 m, 14QQP5398 (F6412); "Sierra Madre del Sur, 65 mi S Oaxaca on Hwy 131" (F5163).

185 166 Sceloporus torquatus (N=42): S. t. binocularis; NUEVO LEON: 14 km E Iturbide, 700 m, 14RMC2337 (F5957); 10 km ENE Iturbide, 950 m, 14RMC1735 (Y25478, Y25483). S. t. melanogaster: HIDALGO: 16 km SSW Jacala, 2200 m, 14QMU7508 (Y25516). MICHOACAN: 7 km E center of Morelia, 2000 m, 14QKS7778 (F5165); 7 km W Jacona, 2000 m, 13QGN7509 (F5147-9). JALISCO: N side of Nevado de Colima, 3000 m, 13QFM4470 (F5138). S. t. torquatus; MICHOACAN: 10 km N Los Reyes, 1500 m, 13QGM6277 (F5207); 17 km ESE Los Reyes, 1850 m, 13QGM8063 (F5123); 3 km ENE Uruapan, 1700 m, 13QHM1250 (F5210); NE quadrant of Uruapan, 1700 m, 13QHM0949 (F5205); 16 km ENE Uruapan, 2200m, 14QJS9258 (F5209); 4 km WNW Patzcuaro, 2050 m, 14QKS2362 (F5198). DISTRITO FEDERAL: 7 km ENE Tres Cumbres [Morelos], 3050 m, 14QMS7912 (F5681). MEXICO: 24 km ESE Acuico, 2750 m, 14QMT3516 (F5997-9, F6008); 5.5 km ESE Otumba, 2600 m, 14QNS3177 (F6286); 3.5 km NE Otumba, 2415 m, 14QNS2981 (F6318); Archeological Zone, San Juan Teotihuacan, 2300 m, 14QNS1677 (F5982-4, F6002, F6004, F6069, F6070, F6089, F6091, F6096, F6103, F6104, F6I10, F6115, F6120-2, F6302, F6303). TLAXCALA: 3 km SE Calpulalpan, 2600 m, 14QNS4664 (Y09763). Sceloporus poinsettii (n=21): Specimens karyotyped by Cole et al. (1967): (n=6). Specimens reported herein: S. p. poinsettii; NUEVO LEON: 5 km W Bustamante, 560 m, 14RLE4637 (Y25510). COAHUILA: 19 km N Guadalupe, 1000 m, 14RKE6500 (Y25507, Y25508); 21 km NNW center of Saltillo, 1230 m, 14RKD8836 (Y25504); Hwy 50, 0.7 km WSW Nuevo Leon border, 1200 m, 14RLD2137 (Y25544); "15 mi S Saltillo" (F5168, F5180); hillside NW Carenros, 2150 m, 14RKC8781 (Y25542); 7 km SW General Cepeda, 1650 m, 14RKD4604 (F5942); 12 km SW General Cepeda, 1740 m, 14RKD4301 (F5939); 18 km SW General Cepeda, 1900 m, 14RKC3792 (F5853).

186 167 S. p. polylepis x[?] poinsettii intergrades: CHIHUAHUA: 25 km ENE Allende, 1580m, 13RDV8293 (F5111). DURANGO: Nieves, 1700m, 14RDV==L? (F5030); 37 km SSE Entronque La Zarca, 1850 m, 13REU4120 (F5105, F5106). S. p. macrplepis; no specimens karyotyped. Sceloporus cyanogenys (n=14): TEXAS; "Rio Grande Valley" (Y23785, Y23786, SIU ser 247/248). NUEVO LEON: 3 km NNW China, 170 m, 14RMD7846 (Y25452, Y25454, Y25455); 15 km SSE Sabinas Hidalgo, 380 m, 14RLE8918 (Y25497); 10 km ENE Dr Gonzalez, 410 m, 14RMD1463 (Y25491); 5 km NE Dr. Gonzalez, 600 m, 14RMD1162 (Y25488). TAMAULIPAS: Sierra San Carlos, 3-5 km S San Scarlos, m, 14RNC0514 (F5947, F5950-2, F5965). Sceloporus serrifer: no specimens karyotyped. Sceloporus macdougalli; no specimens karyotyped. Notes 1 Sample includes 2 individuals heterozygous for the FIS-6 mutation. 2 Sample includes 2 individuals heterozygous for FIS-6, presumed to be F6 x S hybrids. 3 Sample includes triploid "backcross" individual.

187 168 TABLE 1 SUMMARY OF KARYOTYPED LIZARD GENERA AND SPECIES, BY FAMILY 1 Data from Bezy (1972). 2 Karyotypes may be derived with a few pericentric inversions in addition to Robertsonian rearrangements. The table is based on Gorman (in press) N.B. Of 15 lizard families for which karyotypic data exist, 6 contain species having karyotypes with 12 approximately metacentric macro chromosomes and 24 microchromosomes. Four more families have species whose karyotypes differ from this 12 and 24 pattern only by Robertsonian rearrangements. Only 3 families contain no species whose karyotypes can be derived from this 12 and 24 pattern with relative ease.

188 169 TABLE 2 GEOGRAPHICAL AND ECOLOGICAL DISTRIBUTION OF STANDARD GRAMMICUS

189 170 TABLE 3 GEOGRAPHICAL AND ECOLOGICAL DISTRIBUTION OF F6 GRAMMICUS TABLE 4 GEOGRAPHICAL AND ECOLOGICAL DISTRIBUION OF P1 GRAMMICUS TABLE 5 GEOGRAPHICAL AND ECOLOGICAL DISTRIBUTION OF F5 GRAMMICUS

190 171 TABLE 6 GEOGRAPHICAL AND ECOLOGICAL DISTRIBUTION OF F5+6 GRAMMICUS TABLE 7 GEOGRAPHICAL AND ECOLOGICAL DISTRIBUTION OF FM1 GRAMMICUS

191 172 TABLE 8 GEOGRAPHICAL AND ECOLOGICAL DISTRIBUTION OF FM2 GRAMMICUS

192 173 Figure 1. Distribution of karyotypic variation in the family Iguanidae. The sets of numbers associated with the generic names provides a summary in the following format: the known range of 2n's in the genus / the number of species karyotyped from that genus / the total number of species in the genus (where it has been possible to determine this). An asterisk after the number of species indicates that many species in the genus appear to result from isolation on oceanic islands.

193 174 Figure 2. The standard karyotype; as illustrated by a male Sceloporus jarrovii. The bar below pairs 12 and 13 represents 10 micrometers.

194 175 Figure 3. Standard karyotypes of the torquatus species group. A. S. cyanogenys (Y25455), B. S. poinsettii (Y25504), C. S. torquatus (F5415), D. S. jarrovii (F56006), E. S. mucronatus (F6433), F. S. ornatus (Y25499), G. S. dugesii (F5166), H. S. bulleri (F5080). The bar in the center of the figure between pairs 4 and 5 represents 10 micrometers.

195 176 Figure 4. The standard and Em karyotypes of three species groups; grammicus, megalepidurus, and asper. S. mucronatus of the torquatus group is also illustrated to allow convenient comparison. The Em karyotypes differ from standard only by the relative enlargement of pair 9, which is indicated by the black border. A. S. mucronatus (F6202). B-D are all grammicus group: B S. grammicus (F5691), C. S. shannorum (F5065), D. S. heterolepis (5829). E and F are megalepidurus group: E. S. pictus (Y25591), F. S. megalepidurus (F6200). Asper group: G. S. asper (F5809). The central bar represents 10 micrometers.

196 177 Figure 5. Major karyotypes of the grammicus complex. Karyotypic modifications which differ from the primitive or standard condition are enclosed within the black borders. If the condition is fixed the border is solid, and if polymorphic the border is dashed. A. standard (S), B. polymorphic-1 (P1), C. fission-6 (F6), D. fission-5 (F5), E. fissions 5+6 (F5+6), F. multiple fissions (FMl) [Note that the FIS-3 mutation is present in the population at a low frequency, although this individual is homozygous metacentric for pair 3], G. multiple fissions (FM2) [Note that there is an "extra" pair of microchromosomes in this pattern, possibly resulting from a fissioning of micro-chromosome pair 14]. The bar between pairs 3 and 4 in column D represents 10 micrometers.

197 178 Figure 6. Comparison of standard grammicus and Agama [Stellio] caucasica. A. Sceloporus grammicus, B. Agama caucasica. The bars at the bottoms of each column represent 10 micron scales for each karyotype.

198 179 Figure 7a. Distribution of grammicus karyotypes in Mexico.

199 180 Figure 7b. Distribution of grammicus karyotypes in the Valley of Mexico. A. Karyotype in the Valley of Mexico. B. Karyotypes from the Rio Frio study area.

200 181 Figure 8. Karyotypic relationships of the clarkii group species. A. The standard karyotype (S. mucronatus, F6300), B. the clarkii karyotype (F5101), C. the melanorhinus karyotype, heterozygous for the Em mutation (Y44250), D. the asper karyotype (F5809). The chromosomes involved in the X 1 X 2 Y sex trivalent of melanorhinus are indicated by a black border. The arrows indicate the probable origins of these chromosomes from an ancestral karyotype like that of clarkii. The Em chromosomes of melanorhinus and asper are also indicated by a black border. The bar in the center of the figure represents 10 microns.

201 182 Figure 9. Male diakinesis arrays. Note the sex chromosomal dimorphism in all species. Chiasma positions for the sex chromosomes are indicated by small bars. Also note the heteromorphic pairing of the chromosomes in the 9th bivalent of melanorhinus. A. S. torquatus (F5165), B. S. grammicus (F5624), C. S. clarkii (SIU 95), D. S. melanorhinus (Y44250), E. S. megalepidurus (F6201), F. S. asper (F5809). The bar between pairs 2 and 3 represents 10 micrometers.

202 183 Figure 10. Microchromosomal arrays from male diakinesis. This is to show the XY dimorphism in S. clarkii: note the consistently unequal pairing of the 7th bivalents. A-G are all clarkii: A. (F5100), B. (SIU 104), C. (SIU 95), D. (SIU 95), E. (Y44246), F. (Y44246), G (Y44325); H. S. pictus (Y25568); I. S. cyanogenys (Y25488). The bar in row 9 of columns H and I represents 10 micrometers.

203 184 Figure 11. Reconstruction of the karyotypic evolution of the creviceusing Sceloporus and the clarkii group. Names of karyotyped species are underlined. Names in brackets are probable biological species not recognized as such by present taxonomy. Taxonomically recognized species thought to be connected by intergrading populations are indicated by a bracket linking their names. Arrows indicate the probable direction of karyotype evolution.

204 185 Figure 12. Relationships of the species groups of Sceloporus as proposed by Smith (193), as emended by more recent taxonomy.

205 186 Figure 13. Relationships and groupings of Sceloporus as proposed by Larsen (1972), based on multivariate analysis of external and skeletal morphology. Figure redrawn from Larsen (1972).

206 187 Figure 14. Geographic distributions of the 2n=34 orcutti group species. [Note added in Hall and Smith (1979) described the new species as S. hunsakeri.]

207 188 Figure 15. Geographic distributions of the 2n=40 clarkii group species.

208 189 Figure 16. Geographic distributions of the Em karyotype species.

209 190 Figure 17. Geographic distributions of the standard karyotype forms of the grammicus species group.

210 191 Figure 18. Geographic distributions of the torquatus group species.

211 192 Figure 19. Early radiation of the large-sized, large-scaled Sceloporus. A semigeographic representation of their evolution in the deserts of western North America. The different background tones indicate the major classes of habitats exploited by the lineages at different stages in their evolution away from the deserts.

212 -a- MISCELLANEOUS MATERIAL Introduction The material presented in this review represents an abstract and summary of several studies which I have in various stages of preparation, combined with appropriate comparative material from the published works of others. The database for the karyology are chromosome preparations made in this lab by myself, T.P. Webster, B.B. Stamm, H. Londe, A, Russell, and D.L. Paull. Additional data not cited in Gorman (in press) is derived from the work of D.M.V.M. Peccinini (Thesis, Universidade de Sao Paulo) and B.D, Sage (pers. comm.--data on Liolaemus). These data included chromosome preparations from more than 1200 individual Sceloporus grammicus; from 300+ individuals from 43 of the 63 remaining, currently recognized species of Sceloporus; from approximately 100 individuals representing one to several species of each of the 8 other sceloporine genera, from 100+ individuals belonging to 9 other iguanid genera, and from 100+ individuals belonging to several other lizard families. To this is added the wealth of material represented by C.J. Cole's several publications cited below, and by the work summarized by Gorman (in press). [Notes added in 2003]. The following tables and illustrations were bound in a few distribution copies of my thesis, including the one formally presented for my degree. In the personal copy I retained (the only one now available to me), some of the pages are only of poor photocopy quality and after several overseas moves and weather damage to parts of my personal library, I am unable to locate originals. Thus, to provide legible images of some of the pages I have had to use high resolution grey scale scans in order to adjust brightness and contrast to improve readability.

213 -b- KARYOTYPE DISTRIBUTION IN THE SCELOPORINES This diagram summarizes karyotypic data available from all sources for the sceloporine branch of the Iguanidae, This indicates the comparatively derived position of the chromesomally variable Sceloporus with respect to the chromosomally conservative genera. There can be no reasonable doubt that a 12 metacentric macro chromosome, 22 microchromosome pattern is primitive in the sceloporines.

214 -c-

215 -d-

216 -e- * Numbers proceeding the colon are the 2n's of the sexes. In cases of Robertsonian polymorphism, the theoretically expected. range is given. The code between the colon and the "M" gives the types and numbers of the macrochromosomes. "I" Is telo- or acro-centric. "si" is subacrocentric. "sv" is sub-metacentric. "V" is metacentric or nearly so. Where both I and V or V and sv macros are present in a karyotype, the underlined numbers enclosed in parentheses between the number and the type designation (I or sv) Indicate which of the metacentric pairs are present or involved in the difference. Where a Robertsonian mutation is present as a polymorphism, the underlined number of the V chromosome involved is followed by a "p". Following the M is the number of microchromosomes, "m." "Em" designates the presence of an especially large microchromosome, intermediate in size between the macros and other micros. Except for this, no attempt is made to designate morphological details of the micros, although for many species the micros are differentiable. Where sex chromosomes are distinguishable these are indicated separately, either as micros or macros. Morphologlcally different and probably non-homologous sex chromosomes are so indicated: x = x 1 x' 1 = x' 1 x''; x 2 x' 2 X 2 y y' y'' y* Y**; Satellite shifts, small inversions, and some non-robertsonian differences are not indicated in the formulae. [H]: Hall, W.P. (in prep.) Comparative population cytogenetics, speciation, and evolution in the iguanid lizard genus Sceloporus. Thesis, Harvard University [L 1 ]: Lowe, C.H., J.W. Wright, and C.J. Cole, Chromosomes and karyotypes of sceloporine iguanid lizards in the North American Southwest. Mamm. Chrom. News. #22: [L 2 ]: Lowe, C.H., C.J. Cole, and J.L. Patton Karyotype evolution and speciation in lizards (genus Sceloporus) during evolution of the North American Desert. Syst. Zool. 16: [C 1 ]: Cole, C.J, C.H. Lowe, and J.W. Wright Sex chromosomes in lizards. Science 155; [C 2 ]: Cole, C.J. and C.H. Lowe The karyotype of a lizard (Sceloporus virgatus) and description of a spontaneous chromosomal aberration. J. Arizona Acad. Sci. 5: [C 3 ]: Cole, C.J Karyotypes and evolution of the spinosus group of lizards in the genus Sceloporus. Amer. Mus. Novitates #2431, 47p. [C 4 ]: Cole, C.J Kayyotypes of the five monotypic species groups of lizards in the genus Sceloporus. Amer. Mus. Novitates #2450, l7p.

217 -f- [C 5 ]: Cole, C.J Karyotypes and relationships of the pyrocephalus group of lizards in the genus Sceloporus. Herpetologica 27:1-8. [A]: Axtell, C.A. (pers. comm.) [J]; Jackson, L, and D. Hunsaker Chromosome morphology of sceloporine lizards (Sceloporus occidentalis and S. graciosus). Experientia 26: 198.

218 -g- PHYLOGENY OF SCELOPORUS I present the following two illustrations representing my reconstruction of the evolutionary relationships of the species In the small-sized, small-scaled, species of Sceloporus without a detailed discussion of my reasoning. However, much of my justification is discussed, at least peripherally. In the main manuscript of this thesis. The phylogenles are followed by maps showing the approximate ranges of species groups not Illustrated earlier in this thesis. [Note added in 2003: Figures 1 and 2 are superseded by diagrams in the 1979 manuscript, Cascading chromosomal speciation and the paradoxical role of contact hybridization as a barrier to gene flow.] Figure 1. Phylogenetic relationships of the small-sized, small-scaled species.

219 -h- Figure 2. Karyotype evolution of the large-sized, large-scaled Sceloporus

220 -i- Figure 3. Distribution of the 2n=30 groups.

221 -j- Figure 4. Distribution of the 2n=26, inverted 1 magister group.

222 -k- Figure 5. Distribution of the 2n=22 species groups.

223 -l- THE IGUANID NASAL APPARATUS As discussed in the major manuscript of this thesis, Sceloporus appear to be primitively adapted to sandy and xeric habitats [with a] nasal morphology involving an extreme lengthening of the anterior limb of the nasal passage, and the behavioral trait of "shimmy burial." The following set of Illustrations shows the range of variation in nasal morphology found within the Iguanidae. [Note added in 2003: The relevant references are: Stebbins, R. C Adaptations in the nasal, passages for sand burrowing in the saurian genus Uma. Amer. Nat. 77: Stebbins, R. C. 1944, Some aspects of the ecology of the iguanid genus Uma. Ecol. Monogr. 14: Stebbins, R. C Nasal structure in lizards with reference to olfaction and conditioning of the inspired air. Amer. J. Anat. 83: Stebbins performed a variety of experiments with Uma demonstrating how the anatomy of the sceloporine type nasal apparatus allowed the lizards to respire while buried in fine sand, and how the dorso-ventral flattening and musculo-sceleto system typical of most sceloporines enabled the respiration to take place by the raising and lowering of the mid thoracic body wall without loose sand filling the space required for expansion of the lungs. Stebbins also noted the sequence from a simple Anolis-like nasal apparatus, and the anatomical differences between extreme desert forms represented by Dipsosaurus (which normally respires by compressing and expanding the dorso-lateral wall of the thorax and unable to breath when buried) versus the extreme desert forms of the sceloporines that are highly adapted to respire when buried in fine sand.]

224 -m- Figure 1a. The iguanid nasal apparatus as illustrated by Stebbins (1948).

225 -n- Figure 1b. The iguanid nasal apparatus as illustrated by Stebbins (1948). Fig. 8 Crotaphytus wislisenii. Lateral and dorsal views of dissected. nasal passage, and roof of mouth (X 3.2). Transverse section at the level of the broken line, A, in the upper right hand drawing (X 5.3). [from Stebbins 1948 a.c. area of attachment of the concha to the lateral wall of the nasal chamber a.l.n.p. anterior limb of the nasal passage l.w.n.c. lateral wall of the nasal cavity m.s.c. median surface of the concha

226 -oa.s. antiorbital space a.w.n.c. anterior wall of the nasal cavity m.w.n.c. median wall of the nasal cavity n.s. nasal septum c. concha n.v. nasal valve c.s.m. membrane connecting the np.c. concha with the nasal nasopharyngeal canal septum d.l.n.p. dorsal limb of the nasal passage d.s.c. dorsal surface of the concha d.w.n.c. dorsal wall of the nasal chamber o.v.n.c. opening of the vestibule into the primary nasal chamber p.f. palatine fold p.l.n.p. posterior limb of the nasal passage e.n. external naris p.n.c. principal nasal cavity f.n.c. p.s.c. posterior surface of the floor of the nasal cavity concha i.n. p.w.n.c. posterior wall of the nasal internal naris or choana cavity l.d. lateral diverticulum of the s.p.n.g. secretory pore of the vestibule lateral nasal gland l.n.g. lateral nasal gland v. vestibule l.p. lateral passageway between concha and lateral wall of the nasal chamber l.p.n.s. lateral projection from the base of the nasal septum v.l.n.p. ventral limb of the nasal passage v.s.c. ventral surface of the concha [Note added in The following illustrations are all my own, completed at the Museum of Comparative Zoology in , based on dissections of specimens from the herpetology collection. These camera lucida drawings were done using a dissecting microscope during an exploratory survey that I intended to complete as a detailed comparative study of the evolution of the iguanid nasal aparatus. The head was skinned, the nasal, frontal, prefronal maxillary and lacrimal bones were delicately removed using microdissection tools, and the drawings were built up progressively as layers of the soft anatomy of the nasal apparatus were progressively dissected away. The Stebbins illustrations illustrate the situation of the nasal apparatus with regard to superficial anatomy. I no longer have accession numbers for the specimens illustrated.

227 -p- Figure 2. Nasal Apparatus of Anolis sagrei. The nasal morphology illustrated here is typical of all Anolis examined and has been seen in no other iguanids. Note the short, straight anterior nasal passage and relatively flat and small central chamber. [Note: this illustration was not bound in the thesis with the following ones]

228 -q- Figure 3. Nasal Apparatus of Leiocephalus schreibersi. The nasal morphology illustrated here is typical of Iguana and similar to that of Tropidurus torquatus, and is probably characteristic of those iguanids which have never become adapted to xeric conditions. Here the anterior nasal passage leading to the central chamber is very short and slightly curved in dorsal views. This curve may be either accentuated to increase the length of the anterior passage or it may be lengthened by growing straight back. The Anolis of the humid tropics show also an extreme reduction in the size and complexity of the nasal chamber, possibly associated with their reputed deficiency in olfaction.

229 -r- Figure 4. Nasal apparatus of Liolaemus multiformus. Note the extremely close parallelism with Crotaphytus and Dipsosaurus, and that Liolaemus is the major iguanid radiation in the xeric areas of southern South America. In North America the ecologically intermediate Ctenosaura show an intermediate nasal morphology, clearly on the line leading to the great sinuosity of the lengthened nasal passage seen here. [Note from 2003: the following mss. text is associated with the original drawing above: The nasal morphology illustrated here is typical of Crotaphytus, Sauromalus, and Dipsosaurus. Ctenosaura is intermediate between this and the morphology illustrated for Leiocephalus. The three first mentioned genera are all adapted to the most xeric desert area. Ctenosaura extends into the margin of the desert areas. Many Liolaemus species are adapted to the deserts of Argentina and Peru.]

230 -s- Figure 5. Nasal Apparatus of Plica plica. An intermediate in the evolution of the sceloporine type nasal morphology. Tropidurus torquatus shows some tendencies in this direction, and Ophryoessoides are intermediate between this and the average sceloporine condition.

231 -t- Figure 6. Nasal Apparatus of Petrosaurus thalassinus. Petrosaurus thalassinus has the most primitive morphology found in the sceloporines. This species is restricted to the relictual mesic areas of the sourthern end of the Baja California penninsula. Compare this with Sceloporus clarkii, illustrated on the next page, which is identical to most other sceloporines, including P. mearnsi.

232 -u- Figure 7. Nasal Apparatus of Sceloporus clarki. This condition is typical of P. mearnsi, Uta, Urosaurus, and all Sceloporus examined, including the tropical, arboreal, and montane formosus. A similar nasal apparatus was also found in Polychrus, at least some species of which are reported to occur in xeric areas of northern South America. Callisaurus and Uma show slightly more extreme modifications along this line, and in Phrynosoma the nasal passage is tilted so that the anterior passage rises almost vertically, but it also is cleaarly only a modification along this same line. The tropidurines Plica plica, Ophryoessoides and Petrosaurus thalassinus represent a series of intermediates between the Leiocephalus and Sceloporus condition of the nasal apparatus.