Australopithecus and orangutans

[Michael Bayly]

John Grehan wrote:
>I have commented earlier (if memory serves me correctly)
>that molecular 'synapomorphies' are not determined prior to analysis, but
>are inferred after parsimony or maximum likelihood methods that impute a
>derived status to characters. To me that's just applying tree construction
>techniques to characters that are an unknown combination of plesiomorphic
>and apomorphic states and the shortest resulting tree infers those
characters to be the synapomorphies.

In practice this is exactly what is done in most analyses of morphological
data.  Unless you have some "special" a priori knowledge of character
evolution and code your characters in a way that reflects this (effectively
constraining the results of analyses), determining the
apomorphy/plesiomorphy of states is a result of analysis.  (I did think all
this was well covered in posts some time back.)

>The difference is that the morphological synapomorphies can each be treated
>on their own merits whereas the molecular characters are an unknown mix of
>apomorphic and plesiomorphic states. In Ken's case he is positing one set
>of synapomorphies (human-chimp) over another (human-orangutan).

The morphological character states you talk of cannot all be
"synapomorphies".  One hypothesis of relationships is (presumably) correct,
and the other incorrect.  Characters supporting the correct hypothesis are
synapomorphies, others are paralellisms/convergences.  The way to determine
which characters are which (unless for some reason you "know" the answer
ahead of time) is to test for congruence with other characters
(morphological or molecular) in an analysis.

Mike


----------------
Dr Michael Bayly
Research Scientist in Plant Biosystematics
Museum of New Zealand Te Papa Tongarewa
PO Box 467
Wellington, New Zealand
Ph: +644 381 7262
Fax: +644 381 7070



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