[Rduke55]

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Wonderful, but as a given member of A1 diverges in terms of reproductive compatibility from A, it simultaneously diverges from the other members of A1. Thus, A1 becomes internally infertile.

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how does a "member" diverge from the group in terms of reproductive compatibility?

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According to the same process causing all successive generations to tend to diverge: genetic drift.

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i assumed "member" referred to a single organism. it refers to a subgroup of A1? oh, it must refer to a "lineage." the reason it doesn't "drift" away from A1 is that it's not isolated from the rest of A1. A1 keeps on interbreeding.

if organisms from group A1 all live together and interbreed, if they "drift" they drift together, right? what's the pressure causing them to split into A1a and A1b?

the only reason A became A1 and A2 is that they became isolated from each other, right?

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That and this:

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you can't be serious. each population stays homogenous enough to continue reproducing. A1 breeds with A1 just fine. A2 breeds with A2 just fine. but they're isolated from each other, so there's nothing keeping them the same.

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are really the main relevant points here. I think Skidoo is not taking this into account. Genetic drift is the change of gene frequencies in a population.

Well put Sephus.