I’ll finish by summing up the points we've covered.
The first part of this talk was about Darwin's views on classification, including the point that evolution, or descent with modification, is an explanation for the observed hierarchy of natural groups.
The next part was about the meaning of relationship - does it just mean similarity, or does it mean what Darwin called ‘propinquity of descent’? We settle for propinquity of descent, or closer common ancestry.
Then we got to the problem of how you infer relationships of common ancestry, with the idea of shared derived characters, synapomorphies, as the key, and two methods of identifying shared derived characters, by ontogeny or development, and by outgroup comparison. We used higher primates as an example, and touched on how homology is deduced from congruence of characters, and how a cladogram is converted into a classification, with the distinction between monophyletic and paraphyletic groups.
And finally there were a few thoughts on the use of cladograms in biogeography.
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