THERE'S NO SUCH THING AS SPECIES:

In 2006, a hunter shot what he thought was a polar bear in the Northwest Territories of Canada. Closer examination, however, revealed brown patches on its white fur, uncharacteristically long claws and a slightly hunched back. The creature was in fact a hybrid, its mother a polar bear, its father a grizzly. Although this cross was known to be possible -- the two species had mated in captivity before -- this was the first documented case found in the wild. Since then, it has become clear that this was not an isolated incident. Conservationists and others worry that if climate change continues to drive grizzly bears into polar bear territory, such interbreeding will become more common and will devastate the polar bear population. Some have even proposed killing the hybrids in an effort to conserve the species.

But grizzlies and polar bears, as it turns out, have been mating since the species diverged hundreds of thousands of years ago. Polar bear genomes have retained mitochondrial DNA from ancient grizzly bears, and grizzlies have inherited genes from hybridizing with polar bears. "People worry that if they interbreed, polar bears will lose their beautiful white coats," said Michael Arnold, an evolutionary biologist at the University of Georgia. "But the truth is these organisms have not been looking entirely like themselves for a long time now."

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