[Skidoo]

I'm a skeptic who's referring to, without necessarily agreeing upon, a hypothetical process, skeptically.

What process? Whichever you propose in explanation of a supposed speciation from A to B. Call it mutation and selection, if you like.

Another try at rephrasing:

One mechanism that stops the hypothetical process of speciation from occurring in the real world is an increasing infertility among the members of the genetically drifting population that accompanies their individual divergences from the original stock. That is, the same reproductive incompatibility that differentiates progenitor species A from descendant species B would also occur among the members of the latter population.