There has been a long standing debate in evolution of the possibility of sympatric speciation, that is, speciation between two coterminous populations. Well, here is evidence from some fish of it happening, at least in the first stages, so that genetic differentiation is minimal to non-existent. I don't know how common sympatry is, but I've been to other talks pointing to similar phenomena in other taxa, so I think there has been a detection bias toward allopatry. But anyway, species concepts are a bugger. The main point is that population differentiation need not always be preconditioned on geographic separation.