If you ask what separates two species, a lot of people would tell you that an inability to produce fertile offspring is the key test. But the polar bear provides a great example of how biology refuses to be pinned down by these simple binary distinctions. Despite a number of very obvious features that distinguish the polar bear from other bears, it readily produces fertile offspring with brown bears. And fossil evidence suggests that the two species separated about half a million years ago.

Researchers have now sequenced 80 separate polar bear genomes as well as a number of brown bears for comparison, and the results show that appearances only tell a very small part of the story. Polar bears have taken up a blubber-rich diet that may leave them with up to half their body weight made up by fat. And as it turns out, most of the polar bear's genes that have undergone rapid evolution seem to be involved in keeping its cholesterol under control and its heart from exploding under the strain.

Although fossil evidence has suggested that polar and brown bears shared a common ancestor relatively recently, there have been a few indications that the two may have been separated for over a million years. The new genome pretty much rules the older date out. The DNA indicates that the split occurred between 300,000 and 470,000 years ago. For a variety of reasons, the authors think the actual date is in the lower end of this range.

That, as it turns out, roughly lines up with the longest interglacial period of the last million years, a 50,000-year period in which forests established themselves in southern Greenland. The authors suggest that this will have allowed brown bears to settle many areas within the Arctic and left isolated populations when the glaciers began to grow again. These remnant populations would have been under intense pressure to adapt to the icy environment they found themselves in.

What ended up being a polar bear obviously has a lot of physical adaptations: white fur, broad paws for swimming, a distinctive head shape, etc. The authors came up with a couple of genes that are likely involved in the pale coat color, but they didn't look into anything else in any detail.

In part, that's because when they looked for genes that have changed rapidly, they came up with so many other things. One of the most obvious is a gene involved in cholesterol metabolism. Despite all the time since their split from a common ancestor, panda bears and brown bears have no differences in this gene. In contrast, polar bears have picked up nine different changes in this gene in the 400,000 years or so that they've been breeding separately.

Whatever it's doing, the modified protein isn't keeping the bear's cholesterol down; the authors note that "Cholesterol levels in blood plasma of polar bears are extreme." Instead, the bear's evolution seems to have reworked the heart to survive these extreme cholesterol levels. Nine of the 16 genes that are changing the most in response to selective pressure are involved in cardiovascular development or maintenance. A few of the rest are involved in forming adipose tissue—presumably to get the bear up to the 50 percent fat figure noted above.

Based on a typical generation time of a bit over 11 years, all of these changes have taken place in less than 20,000 generations—the blink of an eye in evolutionary terms, especially for a large mammal. So, although we tend to think of species as distinctive and evolution as a slow process, the polar bear clearly tells us that generalizations like these only count for so much.

CELL, 2012. DOI: 10.1016/j.cell.2014.03.054 (About DOIs).