But then, having found each other again, the same behavior (breeding only among southern frogs) led to a speciation in only 8000 years?
What was different between the physical separation and the "choice" separation?
Isn't it much more likely that the southern population already have females which were incompatable with the other group? And when they started inbreeding again, those who couldn't get pregnant from the northern group would have babies which couldn't get pregnant from the northern group?
Meanwhile those who still COULD would randomly get inpregnated from their own group OR the northern group, and since the cross-breed children were inferior they tended to die out?
So over time simple natural selection tended to select toward those southern frogs who could only get pregnant by inbreeding?
How do they know what the ones from 8000 years ago did?
And it is rational that frogs would somehow notice their children were dying if they bred with the "wrong" frogs, and some of them would be smart enough to change their behavior and only breed with the "right" frogs?
That seems like a stretch even for true believers.
I understand it to go something like this:
A group of frogs got physically isolated. During that time, each subgroup developed subtle genetic differences independently of each other. However, because the two groups physically could not interbreed, there was no environmental pressure for the females to differentiate between the two groups. This means that while differences did develop, there was no reason for natural selection to encourage a trait that caused the groups to discriminate in regards to interbreeding.
Once the groups reunited, however, this changed. Since the hybrid spawn lacked viability, the females that displayed the trait of being more particular about mating only within their group stood a better chance of passing along their genes, including this 'picky gene'. This altered the environmental conditions for the males as well, by causing those that were readily discernible by the females to be preferred. This resulted in their genes, with the differentiating identifier, to be passed along more readily than those that weren't.
This all culminated in an acceleration of the natural selection process resulting in the sped up genetic development of a new species.
Had these two groups not come back into contact, this new environmental pressure would not have been a factor, as both the 'easy' and 'picky' females would still be producing viable offspring (since they would not have had the choice of a male that would result in nonviable offspring).
Wrong, this has all culminated in an acceleration of biologian's rabid voyeristic speculation about which frogs were where and screwing who for the past 8,000 unobserved years.