From “Polar Bear Evolution Tracked Climate Change” (Science Daily, July 23, 2012), we learn,
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.
It happens today. See below.
These intimate relations may be tied to changes in Earth’s climate, with the retreat of glaciers bringing the two species into greater contact as their ranges overlapped, said Charlotte Lindqvist, the study’s senior author and an assistant professor of biology at UB.
Raises the question how separate these species ever were or could be.