Brown Bears from Alaska are Descendants of Ancient Polar Bears: Study

Brown bears found in southeastern Alaska evolved from ancient polar bears and are not the ancestors of the modern polar bears, says a new study.

At the heart of the controversy of the origin of polar bears are the brown bears found on the islands called Admiralty, Baranof and Chichagof (ABC) Islands in Alaska. Though these bears appear brown, they are genetically linked with polar bears and it was assumed that these brown bears might have been the ancestors of the modern polar bears. It is known that brown bears and polar bears can interbreed both in captivity and in the wild.

However, the new study shows through DNA analysis that the brown bears on the Alaskan islands did not give rise to modern day polar bears.

"The key to solving this mystery was to analyze DNA from the ABC Islands bears' nuclear genomes, and in particular their X-chromosomes. Focusing on the X gave us a surprising result," said Beth Shapiro, associate professor at UC Santa Cruz, lead author of the study.

For the study, researchers compared the X chromosome of the brown bears found on the ABC islands to the brown bears found on mainland Alaska. Study results showed that more than 6 percent of X chromosomes of the ABC island bears came from polar bears.  

The study team then used various simulations to determine how the ABC bears evolved. Their results showed that ABC brown bears actually came from polar bears that drifted to Alaska and mated with mainland brown bears.

"During a previous ice age, polar bears ranged much farther to the south than they do today, reaching the present-day ABC Islands and Alexander Archipelago. As the climate warmed and ice began to retreat, it is possible that some of the animals began to spend progressively longer on land with reduced access to ice. We see the same sort of thing happening today with polar bears in areas such as western Hudson Bay or the Russian coast, in response to continued climate warming and loss of ice," said Ian Stirling, an Adjunct Professor at the University of Alberta in Edmonton, Canada.

"One of the most exciting results from this study is that we now finally understand what happened between the polar and brown bears on the ABC islands. But, for the moment at least, we are back to not being certain where in the Arctic polar bears first diverged from brown bears," said Stirling in a news release.

The study is published in the journal PLOS Genetics.