What is a species? In high school biology class, I learned that the test line for we call "species" is traditionally drawn over whether or not an animal can breed with another animal and produce fertile offspring. (Thus a horse might breed with a donkey, but the fact that sterile mules result means that horses and donkeys are separate species.) That's still considered the definition by many scientists, although it seems to be in flux:

A species is a group of organisms that can interbreed in nature to produce a fertile offspring. Stated in another way, species are reproductively isolated groups of populations. Organisms classified in the same species have very similar gene pools.

Now I see that a wild bear recently shot has turned out to be a fertile -- and natural -- cross between a Polar Bear and a Grizzly Bear:

Territorial officials seized the creature after noticing its white fur was scattered with brown patches and that it had the long claws and humped back of a grizzly. Now a DNA test has confirmed that it is indeed a hybrid - possibly the first documented in the wild.

"We've known it's possible, but actually most of us never thought it would happen," said Ian Stirling, a polar bear biologist with the Canadian Wildlife Service in Edmonton.

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

How many specimens would they have to find before the new breed becomes an endangered subspecies?