Evolution versus Speciation

One problem with many of the anti-evolution arguments is they don't think natural selection will produce new species. Dogs are given as a counter example, but they are technically compatible enough to be considered one species (depending on how restrictive you are with that definition.) I don't think that is a good counter example either, but I can't just leave it at that...


I believe it was at Sandwalk but may well have been Pharyngula that I originally saw the explanation of the speciation (separation of species in which the two are not able to reproduce together) mechanism not being gradual change in a single population by chance causing them to be incompatible with each other. And really, that would be pretty ridiculous, for a population to do so well that it can support a subgroup unable to mate with the rest. The mechanism described was much simpler: geographic separation.
And it is simple: Species A separates into Group A and Group B by some great distance or due to some natural phenomena (eg a river forms and becomes too large for Species A to cross it, or crosses an ocean to avoid some temporary resource shortage.) Group A continues to have minor developmental changes, and continues to survive. Group B also continues to have developmental changes. Because these changes have no great impact on the local survival rate (still compatible with each other, still doing well in the environment,) those passed on within Group A are completely random from Group B's changes. Eventually (over thousands of generations) Group A has made its way back across the ocean (in my example above) as has Group B.  They are again able to interact, but the multitude of unrelated changes in both populations cause them to be no longer biologically compatible with each other, even though they may be physically similar. Consider the still living example of chimpanzees and gorillas. Or the no longer living example of neanderthal versus the still living homo sapien.