DNA tests reveal the polar bear's adaptability
The world's 20,000 to 25,000 remaining polar bears are classified as vulnerable, with dwindling Arctic sea ice reducing their chances of hunting and breeding. Increasing greenhouse gas emissions are largely responsible for the deterioration of the polar bear's habitat, and the genetic diversity of the population has also suffered as numbers have dwindled. An international group of researchers led by biologist Charlotte Lindqvist of the University at Buffalo investigated how the polar bear arose and developed, using DNA from an ancient polar bear and a modern one, as well as from modern brown and black bears.
Contrary to past evidence from maternally inherited mitochondrial DNA, which had suggested a fairly recent origin for the polar bear, the new DNA tests showed that polar and brown bears diverged into distinct species 4 million to 5 million years ago. There has, however, been interbreeding, and hybrid bears have been observed in the wild.
The species split may have coincided with a boundary between geologic eras that saw changes in climate, including year-round Arctic sea ice. Cyclical climate variations that are tied to the Earth's orbit around the sun - a phenomenon known as the Milankovitch cycle - were mirrored in the polar bear population.
"During warming, polar bears contracted into small populations in areas that provided sea ice, and then with cooling the habitat expanded," Lindqvist said, but when the bears "expanded again from small pockets, they had lost genetic diversity."
Though brown bears were found to share a small amount of DNA with polar bears, the polar bear clearly looks different, and the researchers identified some of the genes responsible for its unique Arctic adaptations - fat metabolism and pigmentation, Lindqvist said.
"This is just a first look. It's an interesting area, trying to understand the genetic underpinnings of how species adapt to new environments," she said.
The study was published Monday in the Proceedings of the National Academy of Sciences.
The researchers' estimates of the historical polar bear population show that the bears have been in decline for the past 500,000 years or so, with warming periods bringing about greater loss in numbers. The population has been so spread out in the Arctic that even during cooling periods, when sea ice expands, the bears have trouble bouncing back, Lindqvist said. The current warming period, which started about 11,000 years ago at the end of the last ice age, has seen sharp falls in the populations of all bears studied.
Steven Amstrup, senior scientist at the conversation group Polar Bears International, is worried that the public will take these results to mean that polar bears are resilient to global warming.
"Regardless of surviving warming in the past, in 50 years we will be off the charts (in temperature) of anything polar bears have been experiencing in their evolutionary history," he said.
Amstrup is also skeptical of one of the suggestions of the new study, that polar and brown bears will increasingly interbreed. It's "reasonable that polar bears will spend time on land . . . as sea ice retreats, (but) this is (in late summer and fall) outside breeding season. I don't see . . . a huge flux of interbreeding happening. We are going to see polar bear loss due to starvation way before their genes are swamped out of existence."
The biggest challenge facing polar bears is that sea ice is melting earlier and freezing later, shrinking the feeding season. Amstrup's own research projects that the polar bear could become extinct by the end of the century, if the current warming trend is not curbed. "If we get our act together and lessen greenhouse gases and hold (carbon dioxide) levels to 450 (parts per million), we could save a substantial number of polar bears," Amstrup said.
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