An analysis of newly sequenced polar bear genomes is providing important clues about the species' evolution, suggesting that climate change and genetic exchange with brown bears helped create the polar bear as we know it today. The international study, led by the Pennsylvania State University and the University at Buffalo, found evidence that the size of the polar bear population fluctuated with key climatic events over the past million years, growing during periods of cooling and shrinking in warmer times.
The research also suggests that while polar bears evolved into a distinct species as many as 4-5 million years ago, the animals may have interbred with brown bears until much more recently. These intimate relations may be tied to changes in the Earth's climate, with the retreat of glaciers bringing the two species into greater contact as their ranges overlapped.
As shown in this image, the nuclear genomes of bears (black outline), suggest that polar bears and brown bears diverged from one another 4 to 5 million years ago, and that occasional exchange of genes between the two species (shaded gray areas) followed. Results from maternally inherited mitochondrial DNA (dotted line) indicates extinction (marked with an "X") and replacement of polar bear mitochondrial DNA around 160,000 years ago due to interbreeding between the two species.