American Indians were on their way to be a separate subspecies, being reproductively isolated -- until those damnable Vikings wandered by...
The same is probably true for the inhabitants of Africa, south of the Sahara. And for the natives of Australia. And probably other groups. It was the development of ocean-going ships that ended everyone's genetic isolation. Then railroads, then planes. Nowadays, you can swap genes all over the planet during a two-week vacation.
I've read that Australian Aborigines are genetically different enough from other people that breeding with non- Aborigines is difficult, leading to a very high rate of miscarriage.
As we learn more about the human genome, I wonder what we'll discover about various genetically-isolated groups of humans.