2.11 Consequences of human / chimp pairing
Activity 10
Timing: 0 hours 10 minutes
This clip refers back to the table of molecular characters, which is shown again here (Figure 9). It explains the consequences of human / chimp pairing in terms of homologous and non-homologous characters.
Figure 9 Nucleotides at selectived positions (left column) in sequences of non-codingg DNA in the region of the beta haemoglobin family of genes in various higher primates. Asterisks denote gaps in the sequences of the species concerned. Based on Williams, S. A. and Goodman, M. (1989) A statistical test that supports a human/chimpanzee clade based on non-coding sequence data, Mol. Biol. Evol., 6, 325-330
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Dr. Colin Patterson
Now accepting this human / chimp pairing has various consequences. One is that it means that characters uniquely shared by humans and chimps - like the twelve nucleotides in the table, or the fusion of premaxilla and maxilla in the adult - are homologous. But characters uniquely shared by chimps and gorillas, or by humans and gorillas, aren't necessarily non-homologous.
Look at character 9 in the table, position 5156, where chimp and gorilla share Guanine. We can explain that in two ways. Either the Guanine was independently acquired in chimp and gorilla by two separate mutations from the ancestral Adenine, or Guanine was acquired by a mutation in the common ancestry of human, chimp and gorilla, and then humans reverted to adenine by a second mutation. Each explanation requires just two mutations, so they're equally parsimonious. But the explanation by independent mutations in chimps and gorillas says that the two Guanines are non-homologous – they’re convergent or chance similarity.
The other explanation, a mutation in the common ancestry of human, chimp and gorilla, says that the two Guanines are homologous, but are primitive for apes, just as Adenine is primitive at this site for the whole group in the table. This same sort of argument works for morphology. I said, a few minutes ago, that chimps and gorillas share various features of the hand and arm, associated with knuckle-walking. Now, given that chimps are most closely related to us, we can explain that in two ways. Either chimps and gorillas independently acquired knuckle-walking, or it developed in the common ancestry of humans and African apes, and was then lost in the human line. In the first case, the features are non-homologous in chimps and gorillas, but were acquired by parallel evolution. And, in the second case they are homologous. In this instance, we might guess that knuckle-walking is too complex to develop twice in exactly the same way, so it's probably primitive for the group, and is lost in us. Or, we could guess that knuckle-walking is obviously adaptive, and might well develop by natural selection, independently in two closely related lines.
I think all you need remember from this is that homology is a conclusion we infer from a tree or a cladogram. It's not something we can establish directly. And then that there are two kinds of homologies, derived ones and primitive ones. But every homology has to fit on the tree somewhere, as a derived character, a synapomorphy of a group.
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