One question, two contrasting answers. Two studies examining the remains of polar bears that lived hundreds of thousands of years ago have found evidence of mating and substantial genetic mixing between ancient polar bear and brown bear populations. Interestingly, they disagree on the direction in which the gene flow occurred.
The first study, published in Nature Ecology & Evolution and co-authored by Canadian polar bear expert Ian Stirling, looked at DNA taken from the tooth of a polar bear that lived over 100,000 years ago on the Beaufort Sea near Point McLeod in Arctic Alaska. Their analysis showed massive amounts of prehistoric mixing between the two species took place during this time, around 400,000 years after the two species diverged, when climate change caused their ranges to overlap. Evidence of this today, according to their research, can be found in all brown bear genomes, but not in those of living polar bears. They also found the direction of gene flow to have been one way — from polar bear to grizzly.
The second study, published in Proceedings of the National Academy of Sciences, also examined DNA taken from an ancient polar bear tooth — this one taken from the skull of an approximately 100,000-year-old polar bear found in Norway’s Svalbard archipelago. The researchers, however, concluded that gene flow between the two species was far more complex and intertwined than the Nature study suggests, similar to that seen in human evolutionary history. And, strikingly, that gene flow ran primarily from brown bears to polar bears.
It’s well known that these two sister species of bear have historically mated. As climate change once more causes their ranges to overlap, their future may become as intertwined as their past. Although the new findings are somewhat contradictory, scientists will now be in a better position to understand the history between the polar bear and the brown bear as well as anticipate what might happen in the future.
A different bear altogether