A new study shows genomic evidence that polar bears are much older than we thought and have been interbreeding with brown bears, likely in response to climate change

New species can arise in a variety of ways. One well know mechanism involved the isolation of two distinct populations from one species with plenty of time passed from the split, and you end up with two separate species. This may have been the case with polar bears, which likely split from brown bears long ago. Until now the timing of this event had not been clear cut.
Now, a new study published on July 23 in the Proceedings of the National Academies of Sciences, provides new insights into the polar bear's evolutionary history. Researchers from Pennsylvania State University and the University of Buffalo, USA, compared the genome of polar bears to that of black bears, and estimated that polar bears arrived as a distinct species between 4 and 5 million years ago, making polar bears much older than previously thought. Their analyses also show evidence of repeated interbreeding with brown bears in their past.
Researchers estimate that polar bear populations have been affected by climate change over millions of years. During warmer times, their populations dimmed, surviving in small patches of colder refuges and when their preferred climate returned, these populations expanded. More recently, it seems, polar bears have gone through a dramatic decline, as evidenced from their low genetic diversity.
"We cannot look into the future and tell the fate of polar bears," said Dr. Charlotte Lindqvist, "But we hope that getting better insight to their past will inform us in predicting how they might respond to future changes." said co-author Dr. Lindqvist.

And with warmer temperatures, interbreeding between polar and brown bears may become more common. For example, genomic analyses on brown bears from southeastern Alaska show that 5-11% of their genome comes from polar bears. Likewise, polar bears show up with as much as 20% of brown bear genome.
So what is the big deal? In a worst-case scenario, Earth's climate will become warmer and warmer, and the two seperate species will continue to mate. As a result we will no longer have polar or brown bears as we know them, but some new kind of hybrid species. In the end, Earth will be losing some of its rich diversity.

Original Article: Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change