Molecular biology = micromorphology

[John Grehan]

> -----Original Message-----
> From: Derek Sikes [mailto:dsikes at ucalgary.ca]
> John,
> 
> You use the word 'necessarily' a lot. Can you define it for us?
> 
> Do you mean "100% correlated with"?

It seems that the molecular view of the chimpanzee relationship with
humans is that the predominant sequence similarities between humans and
chimpanzees are correlated with their close phylogenetic relationship so
that contradictory morphological evidence is dismissed. There appears to
be no empirical evidence to support this choice.
> 
> If so, then you may disappointed to learn that no type of evidence is
> 100% correlated with phylogeny (that is, 100% lacking in homoplasy).

I agree, but it's the idea that since most sequence analyses support the
chimp rather than orang, that it is the chimp that is related
irrespective of other evidence. 

> The analogy to adaptation by natural selection should be obvious -
> those characters involved in adaptation are generally more likely to
> be convergent between lineages, that is homoplasious, while errors of
> random mutation that are selectively neutral and identical between
> lineages are in general more likely to reveal true relationships.

This is a nice theory, but its just a theory.

> Because the shared presence of these identical states cannot be
> explained as due to positive selection resulting from adaptive
> advantage - they can only be explained as due to (1) pure coincidence
> or (2) shared ancestry.

And that's where things get interesting, with about 40 uniquely derived
states shared between humans and orangs and basically nothing uniquely
shared between humans and chimps. So what does this mean for sequence
analysis? Perhaps the sequences being studied do not reflect phylogeny
in this case, or perhaps it is the morphology that is pure coincidence
(parallelism, convergence), or really primitive retention (i.e. one
would have to invoke the features as ancestral to a human-chimp lineage
with chimps reverting to an even earlier state shared with gibbons and
monkeys).

Hope this helps.

John Grehan