Darwin and the Family Tree of Animals Note: These links do not work. Use the links within the outline to access the images in the popup windows. This text is the same as the scrolling text in the popup windows.. Introduction (Page 1) I. History of ideas on evolution (Page 2) Jean Lamarck: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/darwin/darwin_popups/lamarck.html This sketch is of Jean Lamarck, a proponent of inheritance of acquired characteristics. Charles Darwin: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/darwin/darwin_popups/darwin.html This is a picture of Charles Darwin in later life. Alfred Wallace: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/darwin/darwin_popups/wallace.html This is a photograph of Alfred Wallace in later life. He was a much younger man when he published his joint paper with Darwin proposing the theory of evolution. II. Descent with modification (Page 3) V. How does natural selection work? (Page 4) Character: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/darwin/darwin_popups/character.html This diagram shows how natural selection alters the characteristics of populations. Illustrated is a species of snail in which individuals vary in the color of their shells from very light to quite dark. In the original population the graph shows the proportions of individuals with different colors o shells. Note that the greatest numbers of snails in the population (in the upper part of the figure) are of an intermediate color and that snails with either lighter or darker shells become progressively rarer until very dark ones and very light ones are extremely scarce. These snails are preyed upon by birds. The birds can see snails that differ from the color of the leaf litter on which hey live better than they can see snails that match their background more closely. In the case of he original population, the litter on which the snails live is the same color as the medium-colored snails. Consequently, either lighter or darker snails stand out and are seen more easily by birds and they suffer greater predation from birds than do the more protectively colored snails. That is, he environment (in this case birds) is selecting for medium-color and against light color and dark
color. Accordingly, medium-colored snails escape birds more often and the next generation has a greater proportion of snails with genes for medium color. This is why the original population is composed mainly of medium-colored snails. Now lets take a situation where either the habitat occupied by the snails becomes darker, or they
expand into a new habitat where the litter is darker. Now, the medium-colored snails are more conspicuous than they were previously and the darker snails are better camouflaged. Very light snails are the most conspicuous and easily detected by birds. Under this new environment, predation on darker snails will be reduced whereas that on light and medium-colored ones will ncrease, and the characteristics of the population will change. In the next generation, seen here n the bottom figure, more dark snails have survived and they represent a greater proportion of the population than formerly, as seen by a shift of the curve toward darker snails. The curve for the old generation is represented by a dashed line and the curve for the new one is colored brown. The white arrow indicates selection against the lighter snails. If the habitat had become lighter or the snails expanded their range into one with a lighter background, the results would have been he opposite and the curve for the new generation would have been to the left of curve for the original population. This is not a fanciful illustration but is based on many years of study of real snails in Europe. Speciation: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/darwin/darwin_popups/speciation.html This diagram shows how speciation occurs. In the bottom figure you can see a single ancestral species. Now, suppose this species occupies a wide geographic range and that in different parts of its range it experiences different environmental conditions. In this example, let s suppose that parts of its range are relatively hot and other parts relatively cold. The graph indicates the percentage of individuals with different temperature tolerances. In some parts of the range there will be selection for higher temperature tolerances. In other parts of the range selection will be fo olerance to cold rather than to heat, as indicated by the two arrows. Thus, this species will begin o develop different characteristics in different parts of its range, particularly if there is some barrier that separates populations from each other so that they do not have opportunity to nterbreed and exchange genes. Over time, they will diverge independently, each in response to he selective pressures operating in their own portion of the range. In due course, they will become quite different and become separate species. Of course, it is not only one characteristic, ike temperature tolerances, that will be affected because geographic areas seldom differ in only one environmental feature, but a number of them. Eventually, so many characteristics between hese geographically separate populations will diverge to the extent that they will no longer be capable of interbreeding even if given the opportunity. Recall that the definition of a species is based on potential for interbreeding. Thus, by that definition the two populations at the top of the figure are probably so different that they no longer could interbreed and they are separate species- -in short, there are now two species instead of only one. Just where in this series of events is speciation complete? There is no one discrete point. It is a gradual process. There are some populations that have distinctively different features but which can still freely interbreed. These are considered to be the same species but often are designated as races or subspecies. Such might be the case with the population represented by the second graph from the bottom. Other populations may be able to interbreed but with limited reproductive success, many of the offspring dying early in development. Still other populations have accumulated greater differences that interfere with successful interbreeding. They may hybridize and produce offspring, but those offspring might be sterile. Good examples are horses and
donkeys. They are considered to be separate species and are quite different in appearance. Yet hey readily hybridize and produce offspring that survive to adulthood. The hybrid is called a mule. It is sterile, being unable to successfully interbreed with other mules or with horses or donkeys. Still further divergence can lead to species that only rarely produce viable offspring. For example, tigers and lions are usually sterile when mated to each other, but occasionally in zoos they will produce a rare hybrid. This perhaps is represented by the second figure from the op. The final result in speciation is when two entities never successfully produce offspring under any conditions (the top figure). In nature there exists the whole continuum of degrees of reproductive isolation, and speciation is a gradual process with no sharp boundaries between different stages. This is just what one would expect from Darwin s concept of evolution. Subspecies: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/darwin/darwin_popups/subspecies.html This examples shows Potter s wasps from southeastern Asia that have become isolated on different islands and are beginning to show differences in the color pattern on their bodies. Giraffe's Long Neck: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/darwin/darwin_popups/long_neck.html This top of this diagram illustrates Lamarck s ideas of evolutionary change. He attributed the ong neck of the giraffe to the continual stretching of it by the animal as it attempted to reach foliage high in trees. He felt that the stretched muscles was then transmitted to the offspring who hen had longer necks. The bottom of the diagram illustrates Darwin s concept of natural selection. He felt that the giraffe had a long neck because among the various animals in a population varying in neck size, hose with the longer necks were able to reach more foliage, get more to eat and thus survived better and left more offspring. The result was that the next generation had a greater proportion of ong-necked individuals than did the previous one, and so on with each generation. Genetics had not yet become a science in Lamarck s and Darwin s day and neither knew how raits were inherited. It was a weakness of Darwin s theory as much as it was for Lamarck s. Later research and the discovery of the laws of inheritance proved Darwin to be right and Lamarck wrong. Without that information, however, it would have been hard to choose which was correct. In the example of the neck of the giraffe, it turns out that both were wrong. Observations have shown that more often than not, giraffes do not stretch up to eat leaves, bur rather lower their heads and bend their necks to browse on foliage below the level of their heads. There is selection for long necks, but not because of the height of the foliage on which they feed. It is related to combat between males. Males fight over females by swinging their heads and bashing their opponents. The animal with the longest next has the greatest sweep of the stroke and can deliver he hardest blow. The longer-necked winner of fights has access to the female and hence leaves offspring while his opponent is denied the opportunity to mate.
V. How does natural selection work? (Page 5) Dogs: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/darwin/darwin_popups/dogs.html Domestic dogs descended from wolves. It isn t certain exactly when the process of domestication began. Some estimates are that it has been in progress only about 12,000 years; other estimates put the time as long ago as 100,000 years. As judged by the speed with which changes occur in he fossil record by natural means, even the longest estimate is still a very short time for such large differences to have accumulated. This illustrates that intense selection can accelerate the evolutionary process. The chart shows the family tree of the breeds of dogs. It is incomplete but does show the degree of relationship among a number of breeds. Like any phylogenetic tree it leads lineages back to common ancestors. In a short time, breeds as different as Chihuahuas, Great Danes, Greyhounds Rottweilers, Pekinese, Dachshunds, Poodles, etc. have been produced. Can anyone doubt the effectiveness of selection in altering the characteristics of an animal? All domestic dogs are still considered to be members of the same species as most breeds can freely interbreed. The dingo, a now wild dog brought to Australia by the early aborigines, is considered a separate subspecies. Dogs can hybridize with other species in their genus, such as wolves. Differences among some domestic breeds, however, must be approaching the species level. It is hard to imagine a female Chihuahua, for example, successfully carrying to term a fetus sired by a St. Bernard! The mechanical problems of mating would seem to impose reproductive barriers between the larger and smaller breeds.