Loudmouth writes:
Gene isolation is gene isolation, no matter the reason. It is not a question of "if they can" but "do they". There are many reasons that separate species may not interbreed, one of which is DNA incompatibilities. Others may include sexual selection, physical incompatibility, egg/sperm recognition, and hybrid sterility.
I was thinking that geographical isolation has nothing to do with it. You got a bunch of deer over here in the Northwest and another bunch of the same deer in the Northeast, and they never get together. Nonetheless they are the same species. (later, of course, much later, this situation might change--they evolve differently).
As regards physical compatibility, I suppose you mean something like big dog/little dog. But they are the same species.
There seem to be some exceptions. Hard to pin it down. I don't understand why DNA incompatibility would not be the deciding factor. Surely there are no cases where you have creatures that don't look alike having DNA compatibilities.