Polar bear genome may shed light on life in low-ice Arctic

Shapiro natural ecology The research also looked at what might have happened to other polar bear genomes during periods of low ice—in this case, about 120,000 or 125,000 years ago, according to Shapiro, when ice levels in the Arctic were similar to today. But here, she’s looking at the relationship between polar bears and brown bears.

Her team used Bruno’s genome and the genomes of currently living polar, brown and black bears to construct a phylogenetic tree — sort of like an evolutionary map showing how bears diverged from a common ancestor over time. (Shapiro was able to use one of Ryder’s southeastern Greenland polar bear genomes in her analysis, despite the huge time gap between its life and Bruno’s. She said the sample library was “missing 100,000 years of evolution.”)

From this and other analyses, the scientists have some evidence that brown bears and polar bears mixed to produce hybrid offspring about 20,000 years before Bruno was born. Scientists speculate that polar bears may have come ashore during this warm period. The carcasses of the marine mammals they hunt may attract brown bears — and thus provide mating opportunities. As a potential result of this ancient interbreeding, 10 percent of the genome of modern brown bears came from polar bear ancestors, Shapiro said.

Given the limited fossil record and evolutionary complexity, figuring out how and when polar and brown bears mixed, further specialized, or diverged is a daunting task. “Evolution is a messy process,” said Andrew Derocher, a polar bear researcher at the University of Alberta who was not associated with the studies. He likened the process of evolutionary speciation to “a large string of vines climbing up the roots of a tree,” crisscrossed and tangled. “Eventually, some of these vines may have their own trajectory, which is our species,” he said. “But in the process, they can cross, they can reconnect and merge, and of course it’s impossible to separate it because they’re so interconnected.”

Still, the two studies are connected, Ryder said, “in a sense: Where and how did polar bears exist when sea ice was low?” The study may shed some light on the past Some insights into how bears – and today’s southeastern Greenland bears – survive in warm climates with little ice.

But scientists say how genetic changes manifest in physical form, and how those changes have helped bears survive past warming events, remain open questions.These findings should not make us feel question of Arctic warming has been resolved, or today’s bears could easily adapt to rapidly shrinking sea ice levels. “It seems that global warming is happening too fast,” Lindquist said. She wondered if polar bears “could keep up”.

After all, polar bears depend on seals as a food source — and those seals depend on sea ice. “Certain parts of the Arctic used to be excellent seal habitat and excellent polar bear habitat,” Derocher said. “But there is no sea ice there anymore. As a result, there are almost no bears. There are very few seals and the ecosystem has basically collapsed.”

So, what might actually help? “Global action to combat climate change,” Laidre said. “That’s it.”

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