Monday, March 5, 2007

EVOLUTION: Speculation

The raccoon and bear families are believed to have diverged about 30 million years ago. The spectacled bear split from other bears around 13 million years ago. The six distinct ursine species originated some 6 million years ago. According to both fossil and DNA evidence, the polar bear diverged from the brown bear roughly 200 thousand years ago; fossils show that between 10 and 20 thousand years ago the polar bear's molar teeth changed significantly from those of the brown bear.

Polar bears have, however, bred with brown bears to produce fertile grizzly–polar bear hybrids, suggesting that the two are close relatives. But neither species can survive long in the other's niche, and with distinctly different morphology, metabolism, social and feeding behaviors, and other phenotypic characters, the two bears are generally classified as separate species.

In a widely cited paper published in 1996, a comparison of the DNA of various brown bear populations showed that the brown bears of Alaska's ABC islands shared a more recent common ancestor with polar bears than with any other brown bear population in the world.[11] Also to see how the bear species once split yet are still connected, polar bears still have HIT (hibernation induction trigger) in their blood, but do not now utilize this to hibernate as the brown bear does. They may occasionally enter a dormant state referred to as "denning" (pregnant females in particular), though their body temperature does not decrease during this period as it would for a typical mammal in hibernation.


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