What prevents evolution?

[Sephus]

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Speciation, for purposes of discussion, is where one internally fertile group develops into another.

There is no applicable instance of this phenomenon in the data. In fact, it's unlikely even from the perspective of logic. How can there be descent of any sort through a discontinuity in fertility?

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this is the last time i'm going to repeat the same idea. there doesn't have to be a discontinuity between one generation and its children.

let's say A is one generation within a population, B is the next, etc. suppose a member of A can mate successfully with a member of B 90% of the time and this rule generally holds over time, meaning B can mate successfully with C 90% of the time. it does NOT follow that A can mate successfully with C 90% of the time. the percentage could very well tend to zero as the gap gets larger depending on what's happening to the DNA over time, even though each generation has a very good success rate when mating with its contemporaries.

don't get hung up by taking it too literally, or suppose that A->C is actually 89.99%, and A->A(100,000) is 0%