[Skidoo]

[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
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.

[/ QUOTE ]

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

[/ QUOTE ]

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

[/ QUOTE ]

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?

[/ QUOTE ]

[/ QUOTE ]

The issue I'm raising only deals with the relationship between A1 and A2 indirectly, but since it keeps coming up, I'll address them. In order for A1 and A2 to be mutually infertile (the sense of "speciation" used here), either or both of them must become infertile with A. The logic of that is plain enough, I hope. In any case, group A still has to produce a descendant group with which it is infertile. That is why it is only necessary for me to address A to A1, while ignoring A1 to A2, to make my point.

[ QUOTE ]
That and this:

[ QUOTE ]
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.

[/ QUOTE ]

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.

[/ QUOTE ]

My use of the term "genetic drift" was actually not quite correct in that it's too narrow for what I'm trying to get at. Substitute random mutation, often effecting fertility. Maybe I was a bit hasty, but it should be easy to clarify.

The principle at play here is akin to isotropic expansion. If A1 diverges in it's reproductive compatibility from A, what would stop the same tendency from operated within A1? If you say the propagation of such mutations within the isolated group will preserve its homogeneity, that's fine. However, if a mutation carries with it a tendency to reduce fertility without otherwise improving adaptation (and there's little reason to assume otherwise), merely duplicating the basis of that tendency in a potential reproductive partner does not necessarily correct the problem. In other words, two wrongs don't make a right. A mutation tending to produce infertility or, generally, reproductive incompatibility is only in the most extremely improbable case going to be compensated for by its duplication in the opposite member. Thus, the same process pulling A1 from A also pulls apart A1 and prevents true speciation.