Faith writes:
There is no reason to think that the observed changes in phenotype as a result of population split have any other basis than the expression of already- present alleles that are no longer in competition with others from the previous population.
At the risk of turning blue in the face at this point, I'm going to give this one more shot.
Your contention implicitly claims that the divergence of morphology observed to accompany speciation (or follow it) is solely a result of genetic drift between the two populations resulting in the manifestation of different sets of existing alleles. However, you have previously conceded that mutations can and do occur. Thus mutations would occur in these populations. The only difference is that, with speciation separating the populations, the 'fate' of any given mutation in terms of its final gene frequency (fixation/loss/something intermediate) is completely independent in the two populations. Some mutations could become fixed in one population but lost in the other - either by chance OR by selection.
This is precisely why speciation is such a key event - because it isolates gene pools from sharing any subsequent changes in gene frequencies or genotypic norms - HOWEVER these changes may arise. Species, by the biological definition, have unique evolutionary trajectories and fates, thus they are the major units of macro-evolution. There is no 'barrier to macroevolution' above the level of species, simple because each species is henceforth independent (genetically) from all others (barring the exceptional cases of lateral gene transfer). Macroevolution procedes over geological time with the divergence of multiple higher order taxa (Genus, Family, etc.) simply because no taxa above the level of species have anything linking them together genetically - they *must* diverge simply because there is no longer any mechanism that can keep them the same.
Edited by EZscience, : No reason given.
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