What prevents evolution?

[Rduke55]

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The mutual infertility of the individuals within group B increases according to the same genetic drift that underlies the development of mutually fertile group A into (prospectively) mutually fertile group B.

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Before I agree with this, what do you mean by genetic drift?

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The usual random mutation referred to in this context.

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I just dont think random genetic drift would be sufficient to explain the amount of speciation in the time scale we have. I could be wrong though. Rduke? I think if we just allow for a random, Brownian-type walking motion through all the possible configurations, its going to take forever for group A and group B to drift far enough apart to be different species. I mean, it will definitely happen, and probably millions of times in the time span. But not nearly often enough.

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Drift can be HUGE in speciation. Depending on population size.