The species distinction is often made when two populations of otherwise compatible creatures do not reproduce due to time, location, or physiological factors. I

The species distinction is often made when two populations of otherwise compatible creatures do not reproduce due to time, location, or physiological factors. In this case, geography has historically kept Grizzlies and Polar Bears separate, so their physical features have diverged a great deal despite remaining sexually compatible. In other cases, different mating seasons or physiological incompatibility might prevent breeding, even though the gametes are compatible. Coyotes and wolves and dogs can breed, for instance, but they usually don't and so they're considered separate species. The mathematician in me hates this abuse of what should be an absolute and well-defined term, and the cynic wonders about eugenicists making a species distinction between the various human lineages.