[ArrogantBustard]

Polar bears and grizzlies have been successfully paired in zoos before - Stirling could not speculate why - and their offspring are fertile.

Does that mean that they're not different species, but rather different "races" of the same species?

[VadeRetro]

Yes and no. We're not going to revise the taxonomy over this, but by they probably are subspecies rather than species by the standard of cross-fertility. There's still some question whether the offspring are themselves fertile or do we have a horse-donkey-mule situation.

[Pearls Before Swine]

Does that mean that they're not different species, but rather different "races" of the same species?

Yep. I think that's the definition of species--that the members can breed fertile offspring. The two bears must be different breeds... Like chihuahuas and Saint Bernards... but not like horses and donkeys... which give you sterile mule offspring, and are different species.

So, the globabl warming tragedy of the polar bear is not a species problem after all. There will still be plenty of bears to go around.

[MeanWestTexan]

"Does that mean that they're not different species, but rather different "races" of the same species?"

It is certainly a very strong indication, but not conclusive.

For example, while EXTREMELY rare, donkey and horses and zebras can have fertile offspring (mules, et al are overwhelmingly infertile) --- and each species has pretty distict chromosomes.