Species: Polar and Grizzly bears

Traditionally, the definition of species has been that if two animals could produce viable, fertile, offspring, they were the same species.

TORONTO, Ontario (AP) -- A DNA test has confirmed what zoologists, big-game hunters and aboriginal trackers in the far northern reaches of Canada have imagined for years: the first documented case of a hybrid grizzly-polar bear in the wild.
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"It's something we've all known was theoretically possible because their habitats overlap a little bit and their breeding seasons overlap a little bit," said Ian Stirling, a polar bear biologist with the Canadian Wildlife Service in Edmonton. "It's the first time it's known to have happened in the wild."
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Stirling said polar bears and grizzlies have been successfully paired in zoos and that their offspring are fertile, but there has been no documented case in the wild.
CNN/AP

Well, by that definition, polar bears and grizzly bears are the same species! Now, DNA evidence did conclude that this hybrid had a polar father and grizzly mother, so there are significant enough DNA differences between the two species. Evolutionary biologists have definitely been moving towards a more genetic-based definition of species, and this seems like further evidence that we should do so.