An introduction to biological systematics
An introduction to biological systematics

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An introduction to biological systematics

2.7 Inferring relationships of common ancestry

Activity 6

0 hours 10 minutes

This clip addresses the question of how one might go about building a tree, or inferring relationships of common ancestry, by recognising evolutionary novelties, or shared derived characters, or synapomorphy. Dr. Patterson uses a diagram developed by Andrews and Martin to explain this, which he refers to as ‘number 5’. This is Figure 7. The clip ends with an explanation of Von Baer’s Law.

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)
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Dr. Colin Patterson
If we can agree that Hennig's is the only theoretically justifiable definition of relationship, and the one we should accept, how do we set about building a tree, or inferring relationships of common ancestry?
If you think about the problem, you'll realise that the ideal way of building the tree would be by recognising evolutionary novelties, the innovations that characterise different lineages or groups of species. In an ideal tree, each node would be marked by one or more novelties, characters unique to the group of species stemming from that node. I've put an example as number 5, one dealing with familiar animals, the apes, or hominoids. That tree has one peculiar feature, the way the chimpanzee is linked to two different places. But that's done to emphasise a particular problem, that we'll get to in a minute, the fact that there are two different sets of characters - the ones labelled 7a and 7b, and each suggests different relationships for the chimpanzee.
The question I want to tackle, at the moment, is how this tree was built up by recognising evolutionary novelties, or shared derived characters. Synapomorphy is the technical term for a shared derived character. Here the word ‘derived’ is used in the evolutionary sense of advanced, or specialised.
The authors of this tree of hominoids, Peter Andrews and Lawrence Martin, gave a list of characters for each of the numbered branches of the tree. As an example, let's take branch 5, the one distinguishing African apes and us from the orang-utan. They listed about 10 characters for that branch, but I'll just mention three of them. The first is fusion of the os centrale, the second is that the frontal sinus is developed, and the third relates to mutations in DNA. And I want to ask how you might decide that these features are innovations or synapomorphies.
Take the os centrale first. It's a bone in the wrist, one of the carpals. In orangs and gibbons, there are nine bones in the wrist - nine carpals - but in African apes and us there are only eight. Given that information, either state might be primitive or derived, so how do we decide that eight is derived?
In this case it's easy, because in the embryo of all these animals, there are nine carpals, but in us, and in African apes, two of them - the centrale and the scaphoid - fuse together. So we begin life with no carpals, then we develop nine carpals, and we end life with eight of them.
Now, in using this developmental sequence as evidence for evolutionary transformation, we aren't just appealing to the theory of recapitulation - the idea that ontogeny recapitulates phylogeny. We're using a much older theory, or law - one proposed by the embryologist von Baer, in the 182Os. Von Baer's Law says that development proceeds from the general to the particular. The most general characters appear first, and the most particular, or restricted, appear last. The idea here is that development recapitulates not phylogenetic history, but the systematic hierarchy, so that characters of the largest groups appear first, and characters of the most restricted groups appear last.
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