New DNA Study Traces Polar Bear Evolution

The scientist have conducted an in depth analysis of the polar bear genomes, in order to trace the species evolution claiming that climate change and genetic exchange with brown bears helped create the polar bear.

The white coated massive polar bear that existed millions of years ago in ephemeral environment, have seen their numbers waxed and waned due to the climate change.

The international study that was led by the Pennsylvania State University and the University at Buffalo, traced the evidence of polar bear population oscillate due to climatic events over the million years. While the numbers increased during periods of cooling and shrinking in warmer times. An interesting fact the study presents is that polar bear evolved into a distinct species as many as four million years ago. They claim the animals may have interbred with brown bears until much more recently.

"These intimate relations may be tied to changes in Earth's climate, with the retreat of glaciers bringing the two species into greater contact as their ranges overlapped," said Charlotte Lindqvist, the study's senior author and an assistant professor of biology at UB.

"Maybe we're seeing a hint that in really warm times, polar bears changed their life style and came into contact, and indeed interbred, with brown bears," said Stephan Schuster, co-lead author, a professor of biochemistry and molecular biology at Penn State, and a research scientist at Nanyang Technological University in Singapore.

The study that was published in the Proceedings of the National Academy of Science on July 23, is the most extensive analysis to date of polar bear DNA.

In order to conduct this study, the researchers used, first rate set of data, including the coverage for entire geneomes of a polar bear, thee brown bears and a black bear, plus lower coverage of 23 additional polar bears including 120,000 year old individual and few vertebrate.

Based on this the scientists discovered that polar bears are actually an older species than previously thought.

"We showed, based on a consideration of the entire DNA sequence, that earlier inferences were entirely misleading," said study co-lead author Webb Miller, a Penn State professor of biology and computer science and engineering. "Rather than polar bears splitting from brown bears a few hundred thousand years ago, we estimate that the split occurred 4-5 million years ago."

"This means polar bears definitely persisted through warming periods during Earth's history," Lindqvist said. She cautions, however, that the species' endurance over several million years doesn't guarantee its future survival. To model historical populations of the polar bear, the scientists used computer simulations to analyze a deeply sequenced polar bear genome. "This is the first time we can see, from their genes, how the population history of polar bears tracked Earth's climate history," Lindqvist said. "We see an increase in polar bears at the end of the Early Pleistocene as the Earth became much colder, and a continuous decline in the size of the population during warmer times. We also found, perhaps unsurprisingly, that polar bears occur in much smaller numbers today than during prehistory," Lindqvist continued. "They have indeed lost a lot of their past genetic diversity, and because of this, they are very likely more sensitive to climate change threats today."

There was new analysis done, that showed more genetic similarities than previously known between polar bears and ABC brown bears, an isolated group from southeastern Alaska -- suggesting that these animals have exchanged genes since becoming separate species.

 "The ABC brown bears' mitochondrial sequences are much more like polar bears' than like other brown bears'," Miller said. "This made us wonder what other parts of their genomes is 'polar-bear-like'. We mapped such regions, which constitute 5 to 10 percent of their total DNA, onto the genomes of two ABC brown bears. As such, brown/polar bear hybridization, which has been observed recently in Arctic Canada, has undoubtedly contributed to shaping the modern polar bear's evolutionary story."

Evidence for that shows up in DNA strands common to both polar bears and other bears, suggesting the absence of their preferred habitat forced polar bears onto land, which led to hybridization.