It's too early in the morning for me to think about this, but it looks interesting.

But does it work if you go beyond the second generation? We are assuming that the alleles A and B are both rare in the population, so in the absence of linkage between the alleles, in the third generation (and beyond) the A allele will nearly always find itself in a disadvantageous Aabb genotype. Or are you assuming that the progeny of the A's mate among themselves?

Aplogies if I may have misunderstood your model, which is more than likely.

DavidB,

I put this up as-is because I really don't have time to think about it in depth. I would be very glad if you could do so!

But does it work if you go beyond the second generation? We are assuming that the alleles A and B are both rare in the population, so in the absence of linkage between the alleles, in the third generation (and beyond) the A allele will nearly always find itself in a disadvantageous Aabb genotype. Or are you assuming that the progeny of the A's mate among themselves?

I specifically want to look at the case when alleles A and B are both rare. I do not assume that the offspring mate among themselves. Note that a quarter of the offspring have the advantageous AB combination, which is the same as the fraction that have the disadvantageous Ab combination.

OK, forgive me for my hasty comment. Having looked at it a bit more, I think I see your point. With random mating in every generation the aAbB's continue to produce at least 25% aAbB offspring, gradually increasing from 25% as the proportion of A's and B's in the total population rises.

Incidentally, when you say in your post that aAbB has the same fitness as aabb, is that right? You are assuming that aAbB has a 10% fitness advantage, presumably measured in the number of offspring. Those offspring themselves collectively have average fitness, but there are still more of them!

Incidentally, when you say in your post that aAbB has the same fitness as aabb, is that right?

That's why I put in the parenthetical comment, "Those of you who want to quibble about the percentages can adjust them accordingly." - If you want to be exact you have to make the negative fitness contribution a little more than 10%.

Heck, my argument still holds even if the negative fitness impact to aAbb is 20%.

Well, as Haldane allegedly said, 'fitness is a bugger'.

All I'm saying is that aAbB actually has more offspring than aabb, and it is this that drives the increase in the frequency of the A and B alleles. I think.