Taxonomy A Pattern to the Diversity of Life Unless otherwise noted the artwork and photographs in this slide show are original and by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.
There is a common word I m thinking of and when you read it below I want you to get a visual image of the thing it names in your mind. The word is cat.
Is this what you had in mind?
The ambiguity in the word cat, unmodified by an adjective like house or big, reflects the observation that led to the establishment of a formal classification for organisms. Some animals look rather alike and rather dissimilar to other groups: cats include both housecats and big cats, apes include gorillas, chimps, orangutans, and gibbons, and so on. The process of classifying organisms, or identifying them once they have been classified, is called, variously, taxonomy, systematic taxonomy, or simply systematics. A group of similar (or related ) organisms may have a formal name (which we ll get to next) or, if we want to be somewhat unspecific about the nature of the group, we can call it a taxon. The plural is taxa. It just translates something like group. Taxonomy is therefore simply the naming of groups.
There is a formal hierarchy of names applied to all organisms as outlined below. Because an outline is also hierarchical, the outline method of illustrating the taxonomic hierarchy is perfectly appropriate: taxonomy of a group or organisms maps very well onto an outline, in other words. The list of names, beginning with the most inclusive category, is: KINGDOM PHYLUM (DIVISION for botanists) (plural is phyla ) CLASS ORDER FAMILY GENUS (plural is genera ) SPECIES (plural is the same)
For a housecat the proper names for each category are: KINGDOM: Animalia PHYLUM: Chordata CLASS: Mammalia ORDER: Carnivora FAMILY: Felidae GENUS: Felis SPECIES: Felis catus Note that the specific name that identifies a house cat has two parts it includes the generic name as well as the specific. The second part can repeat in different genera but the combination of the two is unique not other species has the name Felis catus. This is a binomen and the rpocedure for naming species is called binomial nomenclature.
Let s compare the classification of housecats and tigers: KINGDOM: Animalia PHYLUM: Chordata CLASS: Mammalia ORDER: Carnivora FAMILY: Felidae GENUS: Felis SPECIES: Felis catus KINGDOM: Animalia PHYLUM: Chordata CLASS: Mammalia ORDER: Carnivora FAMILY: Felidae GENUS: Panthera SPECIES: Panthera tigris The classification of the two is the same down to the level of Family they belong to what is informally called the cat family, but the big cats are in different genera (including Panthera) than the housecat (Felis). Other close relatives of tigers are lions (Panthera leo), leopards (P. pardus after its fiirst use a generic name can be abbreviated) and jaguars (P. onca).
Let s compare the classification of housecats and dogs: KINGDOM: Animalia PHYLUM: Chordata CLASS: Mammalia ORDER: Carnivora FAMILY: Felidae GENUS: Felis SPECIES: Felis catus KINGDOM: Animalia PHYLUM: Chordata CLASS: Mammalia ORDER: Carnivora FAMILY: Canidae GENUS: Canis SPECIES: Canis familiaris The classification of the two is the same down to the level of Order they belong to an order that is largely carnivorous, but the name doesn t refer to their diet really (many members eat mostly plants), but to the tooth structure they share that makes eating meat easy. Other members of the dog family are wolves and coyotes (Canis) and foxes (Vulpes).
I hope you re getting the rhythm of this. As we work out way up the list we are comparing groups with fewer and fewer similarities. Dogs and cats don t look so much like each other unless you compare their toot structure. That is similar, but different from what is found in other Orders, like humans. KINGDOM: Animalia PHYLUM: Chordata CLASS: Mammalia ORDER: Carnivora FAMILY: Felidae GENUS: Felis SPECIES: Felis catus KINGDOM: Animalia PHYLUM: Chordata CLASS: Mammalia ORDER: Primates FAMILY: Hominidae GENUS: Homo SPECIES: Homo sapiens The classification of the two is the same down to the level of Class they belong to an class that has hair, warm blood, live birth, and so on. Other living members of the Primates are apes, monkeys, tarsiers, and lemurs. There are numerous fossil hominid species, but only one living one.
Now, cats and bullfrogs: KINGDOM: Animalia PHYLUM: Chordata CLASS: Mammalia ORDER: Carnivora FAMILY: Felidae GENUS: Felis SPECIES: Felis catus KINGDOM: Animalia PHYLUM: Chordata CLASS: Amphibia ORDER: Anura FAMILY: Ranidae GENUS: Rana SPECIES: Rana catsbiana The classification of the two is the same down to the level of Phylum both species have a dorsal nerve chord and some other features common to members of the order: four limbs, for example. Unlike cats, bullfrogs are cold-blooded, lay eggs (without shells), lack hair, and so on.
Cats and sand dollars: KINGDOM: Animalia PHYLUM: Chordata CLASS: Mammalia ORDER: Carnivora FAMILY: Felidae GENUS: Felis SPECIES: Felis catus KINGDOM: Animalia PHYLUM: Echinodermata CLASS: Echinoidea ORDER: Clypeasteroida FAMILY: Protoscutellidae GENUS: Periarchus SPECIES: P. pileussinensis These two only share the basic features of animals: they are multicellular and they ingest food to capture energy. Sand dollars have no backbone, they have no legs, and so on. They are about (but not quite) as different from a cat as you can imagine, but they do have a few common features. This particular sand dollar is only known as fossils from rocks of late Eocene age (around the middle of the Cenozoic, numerically speaking.) I include it just to remind you that fossils are organisms just like living cats are in a taxonomist s mind.
Finally, cats and longleaf pines: KINGDOM: Animalia PHYLUM: Chordata CLASS: Mammalia ORDER: Carnivora FAMILY: Felidae GENUS: Felis SPECIES: Felis catus KINGDOM: Plantae DIVISION: Pinophyta (or Coniferae) CLASS: Pinopsida ORDER: Pinales FAMILY: Pinaceae GENUS: Pinus SPECIES: P. palustris Notice that these are only related in the sense that both are living organisms. They do share common features (reproduction by way of nucleic acids, for example) but only very inclusive taxa would include them both. (They both belong to the Superkingdom (or Domain) Eukaryota (or Eukarya) things with nucleated cells.)
One person in this photograph is not closely related to the others, all of whom are genetically closely related. Who is the odd one out and how do you know?
Another person in this photograph is closely related to both the oddball and the others Who is it and how do you know?
Even though the taxonomic systems was first established to recognize similarities in appearance ( morphology ) the fact that morphology is controlled by genes meant that the classification also reflected genetic relationships too. Unfortunately there is not a perfect match between the two and a purely morphology-based classification didn t always reflect relatedness. Taxonomists are now applying techniques to classification that attempt to get at the relationships that lead to similarities rather than simply the overall similarities themselves. Before the end of the term we ll come back to this idea, but for now start thinking about this: what does it mean to say that one species is related to another. For example, you ve presumably all heard that housecats are related to big cats. What does that mean?
What does it mean to be related in the more obvious sense?
Let s think about the underlying cause of the observation that led to this classification system. If we graph the heights of people in the United States against how many people are that height we would get a graph somewhat like the one shown here. Let me emphasize somewhat the exact pattern might be somewhat different but the general pattern would be there. Most people would be shorter than about 6 and the number of people in larger categories would drop off rapidly. If there are any people over 9 feet tall there are so few that they wouldn t even show up on a y-axis scale that would fit on a single page. There should be something that strikes you as a little odd about this graph something a little unexpected. Looking around the room you probably won t see anybody over 7, but you won t see anybody under 4 either. So what is the graph showing? Number of People 0 1 2 3 4 5 6 7 8 9 10 Height in feet
The graph is a lot more informative if we just graph adults together. Of course there are a lot of 1.5 tall people, but they are babies and there s not much sense in comparing them to adults. These two graphs are univariate graphs. They are focused on only one type of variation: how many people are of a particular size. Of course they utilize different size classes, but the point is the y-axis value. Number of People 0 1 2 3 4 5 6 7 8 9 10 Height in feet
Age in years If we measure people of all ages and plot their heights against their ages we produce a bivariate plot, and it is more interesting than a univariate plot. It tells us something about how people grow. The two variables, both of which we actually measure and have interest in, are age and height. A best-fit curve to the data is a mathematical description of how American people grow. Curves from other regions of the world might be slightly different. A curve for men would be a little different than a curve for women. There all sorts of things you can do with a curve like this. 8 6 Height (ft) 4 2 0 0 10 20 30 40 50
Age in years If we measure the growth of a single individual at different stages of their growth we get a plot of very similar shape. This should not be surprising since each person grows in a way similar to all other people. Another way to think of this is that each dot on the preceding graph is just a person caught at a single point corresponding to the ages on an individual s growth plot. 8 6 Height (ft) 4 2 0 0 10 20 30 40 50
Size Age If we do a size/age plot for non-mammal and non-bird species the flattening of growth with age would be either less pronounced or not evident at all. Most invertebrates (and most fossils are invertebrates) show little or no slowing of growth with age, so their graph would look something like this one. We can tell when they ve reached their most common adult size not by any flattening, but by the lower point density. They are mostly dying before reaching larger sizes
Size Age Plots like this point to something interesting and useful. Age and size correlate to each other. To put it in very obvious terms, as things get older they also get bigger. This is entirely boring observation if you don t think about how to use it, but in fact, you do sometimes think about it. Suppose you are a coroner and you are called in to examine a skeleton that may be from a murder victim. When you look at the skeleton the first thing you notice is that it s only 2 long. Immediately you conclude that it belonged to a small (that is, a young) child.
Some Other Character Diameter (Age) We can, in fact, use size as a proxy for age. We may not be able to assign year-values to fossil sand dollars (actually, in some cases we can) but we can be reasonably sure that a 4 diameter one was older when it died than a 2 diameter one was. We use these kinds of graphs all the time, and find many useful insights about the growth of fossil species from them. This is important because, given that all our fossils stopped growing a long, long time ago, this is the only way we can learn about it.
We can graph and illustrate comparisons of two characters (bivariate or two dimensional) or even three characters (trivariate or three dimensional) on a two dimensional piece of paper. Though the 3-D graph is a bit more difficult to read (because we can t really see the z direction on the page) it is at least obvious what we are trying to do. But how would we illustrate a 4- dimensional graph? We can t really visualize things in more space than we actually have. Y Y X Z X
As it turns out there are mathematical ways of reducing the information in a multidimensional data set so that they can be summarized on a two or 3 axis graph. However, instead of linear groups of points that we can fit best-fit lines to what we get instead is clusters of points that are generally similar in the values of their many dimensions of variables. This is a simple example of what a plot of many characters of the Phylum Echinodermata (the one that includes sand dollars) might look like for early Paleozoic (Cambrian and Ordovician) fossil genera. It is very schematic and seriously simplified, but the pattern is reasonable. Cy Cr Ed Bl Ca Ec H
Notice the prevalence of clusters the clumpiness of the pattern. The genera can be lumped into families (dotted outlines), the families into orders (dashed outlines), and the orders into classes (solid outlines) whose names are abbreviated by each cluster. The entire chart, of course, represents the Phylum Echinodermata. On a plot of other types of animals this group would be in a clump somewhat removed from the other phyla included in the analysis. This clumpiness of shape in living organisms is intriguing. Where does it come from? Cr Ed Cy Bl Ec Ca H
The Taxonomic Hierarchy Maps onto the Geologic Time Scale
Let s start with the echinoderm plot from the early Paleozoic (and two slides back) and see what happens over time. Cr Ed Cy Bl Ec Ca H
By the late Paleozoic (Devonian and Carboniferous) Some classes were extinct (H, Ca, Cy) and the others had diversified in new ways. (The original dots are included here as open circles). The early Paleozoic genera were extinct, but some genera of the remaining classes were, obviously, still in existence. Notice also that the internal clustering is not necessarily the same as it had been earlier new families and orders had appeared as part of the diversification. Cr Ed Cy Bl Ec Ca H
By the middle Mesozoic (and Cenozoic, with only a little change evident at this scale) the pattern was again noticeably different. Two additional classes (Bl and Ed) were extinct. Early Paleozoic dots are now light open circles and late Paleozoic ones light filled circles. Of the remaining classes one had lost diversity dramatically and the other had gained is just as dramatically. What you should get out of this is a couple of big ideas: First, taxa come and go at all levels, but it is at the lower levels (species, genus, etc) that the effect is most obvious. Second, parts of the time scale that are close to each other (Cambrian and Ordovician or Devonian and Carboniferous, for example) are likely to contain close relatives. Parts that are farther apart probably will contain only very distant relatives. Cr Bl Ec
It hasn t happened in the echinoderms since the early Paleozoic (as far as we know if it did it was in a group with poor or no fossilization potential), but in other groups a new subgroup occasionally appears in the fossil record. Sometimes it is obvious who the ancestors are, sometimes it is not so obvious. This is much more common with lower taxa (families, genera, and species, for example.)
Let s look at this another way. A certain contiguous body of rock will contain every individual of a particular fossil species (S). A thicker, but still contiguous, body will contain all individuals of all the species that nest within or belong to the same genus as S (G). A thicker, but still contiguous body of rock will contain all fossils of that genus and its close (family level) relations, and so on S G F (etc.)
Of course the body of rock containing fossils of each taxon doesn t have to be in the middle of the pattern. It can be at the bottom (as shown here) or the top, or anywhere, as long as it is within the range of the higher taxon. F (etc.) S G Very rarely we find a fossil or living species that is out of its previously known temporal range, and we extend the range. Living coelocanth fishes, for example, were found in the Indian Ocean in the last century. These living fossils extended the range from the Mesozoic to the Recent. Such events are rare and becoming rarer as we explore more and more of the world, but they do happen.
CENOZOIC MESOZOIC PALEOZOIC The information on the multivariate graphs we saw earlier can be translated into a spindle diagram that summarizes the rocks that contain (or don t contain) each of the groups included on the graphs. The total length of the spindle shows all the rocks containing the taxon the fat parts show where they are very common. If a spindle ends that means the taxon became extinct, or at least that we have never found a fossil in younger rocks. You should see the principle of fossil succession pretty clearly here the classes Cy, Ca, and H are found in the oldest/lowest rocks, Ed, Bl, and Cr are restricted to or most common in rocks just above those, and Ec dominates in ones above that. The same pattern holds if wee look at lower and lower taxa (closer to species) and at a more precise temporal level. Cy Ca H Ed Bl Cr Ec This pattern is characteristic of all taxa with a good fossil record.
Because the time scale is based directly on the principle of fossil succession and because the most closely related taxa occurred together in the same times, or in immediately adjacent times, each of the two ideas taxonomy and time scale map onto each other.
The Taxonomic Hierarchy Maps onto Geography
Remember that distributions of fossil organisms do not make sense on a modern map. Ocean barriers that should have blocked their movements evidently did not. Glossopteris (a seed fern or tongue fern ) Mesosaurus (a freshwater lizard-like reptile) Lystrosaurus (a terrestrial reptile) Cynognathus (a terrestrial reptile)
We fix this by realizing that the continents were not separated by oceans (barriers) when the fossils were living. All organisms (and their relatives of various degrees) were free to move around in a single large region. Any newly appearing groups (as we saw in the last section) would spread as far in this area as their ecological tolerances would allow. S. America Africa India Madagascar Antarctica Glossopteris Mesosaurus Lystrosaurus Cynognathus Australia
But one the continents separated, the freedom of movement was gone. Coyotes etc. Tigers etc. Alpacas etc. Giraffes etc. Kangaroos, etc. PENGUINS, etc. In consequence, new species, new genera, new families, etc appearing in any of the smaller regions were only to move around within that region. Gradually, as the old folks (Glossopteris, Mesosaurus, etc.) became extinct, and as new taxa appeared instead, the make-up of each small region changed until each presently has a very distinctive fauna (the animals of a region).
Eurasia has been a continuous landmass since the Paleozoic so the biota here is more homogenous Cacti etc. Lindens etc. Central America has only been continuous land during the late Cenozoic, so some mixing has occurred between the Americas. Bromeliads etc. African Mahogany etc. X Eucalyptus, etc. Australia has been isolated (from everything but Antarctica) since the Paleozoic so it has the oddest biota of all. and a distinctive flora as well (the plants of a region). (The collective term for both animals and plants is biota.)
Eurasia has been a continuous landmass since the Paleozoic so the biota here is more homogenous Central America has only been continuous land during the late Cenozoic, so some mixing has occurred between the Americas. Africa has had a little contact with Eurasia since the late Cenozoic and some interchange has been possible. X Australia has been isolated (from everything but Antarctica) since the Paleozoic so it has the oddest biota of all. The proportion of endemic species is related to how long a continent has been separated from the others (or how long it has seen a new connection). ( Endemic means restricted to a single area.)
Hands down the most distinctive biotas those with the greatest number of endemic species are islands. Any nature program you see about an island or group of islands will include a variant of this sentence: The Blah Blah Islands have umpteen dozen species that are found nowhere else on Earth. The prize might go to the volcanic Galapagos Islands off the coast of South America. Several islands have one or more giant tortoises living on them. Lowland islands generally have a single short-necked tortoise. Those with more recently active volcanoes may have a second, long-necked tortoise in the uplands, where the vegetation they feed on is taller bushes rather than grasses. The kicker is that each island has unique tortoises, easily distinguished from each other. The largest Island, Albemarle (remember Albemarle?), has two volcanic mountains and each one has a separate tortoise. Talk about taxonomy mapping onto geography There is one more relationship between island biogeography and taxonomy. The closest relatives of the island inhabitants are always on the closest continent.
It is interesting to realize that biogeography the distribution of organisms on Earth has a history. The world didn t come into being with plants and animals already in position. Instead, their distributions have become modified as geography itself has become modified, and as it has become easier or harder to move among regions. We can work out that history by paying attention to the taxonomy of organisms in the various regions, coupled with our understanding of fossils as indicators of geologic age. And, of course, the basis for that fossil succession is also intricately tied up with taxonomy. It s little wonder that geologists are so interested in classifying organisms.