If the descendants of species A mutate in a significant way, and are no longer able to reproduce by mating with members of species A, then we would generally consider them a unique species.
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If the descendants of species A mutate in a significant way, and are no longer able to reproduce by mating with members of species A, then we would generally consider them a unique species. For this discussion lets call that species B.
If later descendants of species A mutate in a similar way, but with different genes being responsible then we would generally consider that to also be a new species.
But, what if later descendants of species A mutate in the same way that species B did? With the same genes and same mutations to those genes as species B?
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@m Don't think of a species as an entity, but as a set of specimens. In species with obligate sex of at least two participants, speciation can't happen via a single mutation creating a reproductive barrier. Speciation in such cases must necessarily happen (somewhat) gradually, with parts of what used to be one population drifting apart, until they become two populations. (This doesn't refute punctuated equilibrium; in the hypothetical equilibrium-punctuation case, the gradual drifting-apart would just happen relatively fast, but still, in multiple steps, and both subpopulations would eventually stabilise.)
