[shuckmaster]

over many, many generations the accumulating small genetic changes can reach the point where "Generation N+1000" is different enough from the original "Generation N" that they've become a new species from that original population.

So, if I set my time machine back to one million bc, I wouldn't be able to mate with Raquel Welch?

I'm just kidding and really appreciate your comments.

[King Prout]

you'd be able to mate, just not produce viable offspring ;)

[RightWingNilla]

So, if I set my time machine back to one million bc, I wouldn't be able to mate with Raquel Welch?

You would have to settle for Raquel's Australopithecus counterpart.