Let me first say that I am not a biologist. Still I think I know the
answer, a simple answer - albeit not a "proof" (which is a notion more in
the realm of mathematics than science).
There is a concept in biology call "differentiation". It is a concept
that is just as congenial with microevolution as macroevolution. So one
does not need to embrace macroevolution to accept its validity.
The idea is quite simple. Species that are very close to each other
compete in a given local environment for the same resources. This being
disadvantageous, natural selection (of the microevolutionary sort, that
almost all Christians accept) has the effect of forcing them further apart.
One might ask why it is that a single species does not "differentiate".
Well, I suppose they do to a slight extent by showing different
phenotypes. But the process of breeding within species causes them to pull
toward each other. So they don't get very far apart.
I trust the biologists in this forum will correct any of my misstatements.
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There was an Australian applied mathematician, definitely an evolutionist,
by the name of May, or something close to that, who, perhaps 20 years ago,
argued, as I recall, that one needed to invoke probabilistic models to
properly model the process of differentiation. He was a popular speaker
in statistical circles, and I believe within the biological community. At
least he did produce serious models of differentiation. It might be that
his perceived need for nondeterministic models was linked more directly to
modeling evolution itself. He always had lots of impressive slides, spoke
eloquently, and was undoubtedly very bright.
Gordon Simons