[Michael_Michaelangelo]

However, eventually you'll find that individuals in X1 and X2 can no longer interbreed

One more question. How do we know they can't interbreed unless we artificially inseminate them and see what happens? (there's a fun job...NOT). They may just be picky, or it could be a passe' thing to do at that point in time/history.

[balrog666]

One more question. How do we know they can't interbreed unless we artificially inseminate them and see what happens? (there's a fun job...NOT). They may just be picky, or it could be a passe' thing to do at that point in time/history.

Oh, they try all on their own. For a real life example, check the fertility rates of Englishmen/Australian Aborigine marriages.