Tuesday, July 24, 2012

How Old Is the Endangered Polar Bear?

Polar bears may have trod the planet for millions of years, according to a new genetic analysis. That suggests the white-coated, massive bears have weathered previous natural climate changes, and may predate the Arctic ice that is their preferred—and only—habitat today, which is why the species future remains uncertain presently. "There's no guarantee that they'll survive this time," says geneticist Webb Miller of Pennsylvania State University, an author of the study published July 23 in Proceedings of the National Academy of Sciences. After all, Miller notes, the species currently lacks much genetic diversity to help it adapt to changing conditions as well as facing unprecedented threats such as heavy metal pollution accumulating in the Arctic. By better understanding how old the species is, the scientists hope to better understand what might be done to allow the polar bear to cope with onrushing climate change and other existential challenges. By analyzing the genomes of 28 bears—polar bears, including a roughly 120,000-year-old specimen from Norway's Svalbard archipelago, as well as modern brown bears and black bears—the scientists in effect read back in time to a common ancestor at least four million years ago. That finding conflicts with a genetic analysis published in Science earlier this year that suggested the species was only 600,000 years old or so, which the team behind the new research suggests may result from misreading past interbreeding events with closely related brown bears. In fact, the key problem here may be that technically the polar bear may not be a species at all. "If one defines that two species separate as when they cannot produce viable offspring, then perhaps brown bears and polar bears aren't yet separate species," Miller admits.
What makes a polar bear a polar bear? There's the white coat and black skin as well as less visible differences like a thicker layer of subcutaneous fat layers and richer milk. These and other unique features of adaptation to the harsh conditions of Arctic life have convinced biologists that the polar bears represent a unique type of animal—a species of its own. After all, the brown bear that is its closest living relative lacks all of these adaptations, looks different, eats different food and would not fare well in the harsh conditions out on the Arctic ice. Yet, brown bears and polar bears, when they meet, can mate, as evidenced both by the genetic record and observations in the wild. Because polar bears have been spending more time off the ice in recent years, they appear to have begun to interbreed with adjacent brown bear populations, and some of these hybrids are into their second generations. If the basic definition of a species is a group of organisms capable of mating and producing fertile offspring only within their own group, the polar bear and brown bear fail to qualify. Such interbreeding between bear species makes genetic analysis that much more difficult. After all, if the species interbreed even once, that single event can appear to determine the point at which the two species diverged if a scientist happens to analyze only the portion of the genome influenced by that mating event. And the mixing of genetic material has been going on for a long time between polar and brown bears, making disentangling their genetic history that much harder.

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