Polar bears evolved adaptations specific to the Arctic in fewer than 20,500 generations, and diverged from brown bears much more recently than is sometimes claimed.

Rasmus Nielsen at the University of California, Berkeley, and his colleagues sequenced the genomes of 79 polar bears (Ursus maritimus) and 10 brown bears (Ursus arctos) and found that the two species diverged between 343,000 and 479,000 years ago.

Many of the genes under the greatest selection pressure in the polar bear are associated with the cardiovascular system. In particular, this bear seems to have evolved modifications in its vascular system that allow the animal to tolerate an extremely fatty diet made up mostly of blubbery seal meat.

Cell http://doi.org/sp3 (2014)

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