2.12 Translating a cladogram into a classification
Activity 11
Timing: 0 hours 10 minutes
This clip begins with a diagram by Ernst Haeckel published in 1866 (Figure 11). This is an illustration of how little ideas on the relationships of higher primates have changed in over a century.
The clip’s main focus is the information presented in the second diagram (Figure 12). The diagram is taken from an article by Dr. Patterson, which featured in New Scientist in the early 1980s. It shows three cladograms. The first of these (a) matches the pattern shown in the Andrews and Martin tree diagram (Figure 7), which you will be reminded of below the clip. An explanation of each of the cladograms show how they can be translated into a classification.
Figure 11 Haeckel's evolutionary tree, published in 1866. Engeco is the chimpanzee, and Hylobates, the gibbons
Figure 12 Classifications and implied cladograms of huminoids. The ancestry of humans and apes, as currently understood, is shown in (a), with the classificaion that fits this analysis. Two other classifications in use are shown in (b) and (c), with the cladistic relationships they imply. Redrawn from Patterson, C. (1982) Cladistics and classification, New Scientist, 94, 303–206, Figure 6
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Transcript: Audio clip 12
Dr. Colin Patterson
Finally, let me go on with this example of humans and apes to say something about translating a cladogram into a classification, and about the use of cladograms in biogeography.
The next little diagram is from Ernst Haeckel's evolutionary tree of mammals, published in 1866. I stuck this one in to show that ideas on the relationships of higher primates haven't changed much in over a century. The next diagram, is from an article of mine in New Scientist in the early 1980s when the question of the relationship between us and chimpanzees wasn't as clearly resolved as it is today. The first of the three cladograms shows the position as I saw it then, with us, chimps and gorillas put as a trichotomy or trifurcation.
Above this first cladogram, there's a classification that's consistent with it, because it names all the monophyletic groups in the diagram. We have a superfamily Hominoidea for the whole lot, a family Hylobatidae for the gibbons, and a family Hominidae for the rest. Within the hominids, there are two subfamilies - Ponginae containing Pongo, the orang-utan, and Homininae containing us, chimps and gorillas. As we've just seen, relationships within Homininae seem to be resolved now. And you might like to think how one would express the relationship between humans and chimps in this classification. One obvious solution would be to make two tribes - one for gorilla and one for us and chimps.
At the top of the other two diagrams, I've set out two widely used classifications in the form of brackets. And underneath the brackets is the cladogram or pattern of relationships that each classification implies. The middle classification, (b), has the hominoids divided into three families, Hylobatidae for gibbons, Pongidae for the three great apes, and Hominidae for us. This classification's still in use, for example, in a book published in 1990, R. D. Martin's 'Primate Origins and Evolution'.
Now Bob Martin doesn't disagree with the pattern of ape relationships shown in the first diagram, but he thinks it's more useful to classify the three great apes together. In doing so, he's produced what's called a paraphyletic group.
Paraphyletic groups can be defined in two ways, either in terms of a tree, a cladogram - where there are groups that include some but not all of the descendants of a common ancestor - or paraphyly can be defined in terms of characters - where it means a group sharing only primitive characters.
The easiest way of recognising a paraphyletic group is that it can serve as an ancestor. For example, if someone says, "humans evolved from apes," all they’re saying is that apes are paraphyletic, defined only by lacking the characters of the descendants.
In just the same way, if you say that vertebrates evolved from invertebrates, or tetrapods evolved from fishes, all you're saying is that invertebrates are a paraphyletic group lacking the characters of vertebrates. And fishes are vertebrates lacking the characters of tetrapods. So in general, paraphyletic groups convey no information, and they're best avoided.
The last of these three diagrams, (c), is a more traditional classification, the one you find in Simpson's classification of mammals, for example. There are just two families of hominoids - one for us, and one containing the gibbons in one subfamily, and the great apes in another. This conveys a pattern that's even further from the truth. And, once again, it does it by using a paraphyletic grouping for the apes. In this classification the pongids are all hominoids that lack the features of humans. It should be obvious that I favour classification (a), because it's the only one that expresses our ideas about hominoid relationships.
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Figure 7 Cladogram for the Hominoidea, from Andrews, P. and Martin, L. (1987) Cladistic analysisof extant and fossil hominoids, J. Human Evol., 16, 101–118, Figure 3 (redrawn)