Credit: Veer Images
Dublin: Traces of an ancient polar bear-brown bear hybrid have revealed that all living polar bears are related to single female brown bear ancestor living near Ireland between 38,000 and 10,000 years ago.
Irish brown bears are now extinct, but the mitochondrial DNA from one of them lives on in polar bears. The study suggests interbreeding between brown bears and polar bears may have occurred periodically, especially when the habitat of the two bear species overlap during times of climate change, for tens of thousands of years.
"Environmental conditions appear to have played, and continue to play, a big part in the evolutionary history of polar bears," said Ceiridwen Edwards, lead author of the paper in the most recent issue of Current Biology and expert in ancient DNA at Trinity College Dublin (now at Oxford University).
"Today's warming climate is again bringing polar bears into contact with brown bears, in places like Canada and Alaska, and we might expect to see more examples of hybridisation occurring naturally in the wild."
Bear necessities
It had been believed that the brown bears most closely related to polar bears lived on islands off Alaska. The study now shows that this relationship is probably the result of another hybridisation event, with polar bears mating with brown bears on the Alaskan islands.
Author Beth Shapiro of Pennsylvania State University said that this interbreeding happened after the polar bears had already captured the mitochondrial DNA from the Irish brown bears.
Edwards analysed mitochondrial sequences from 17 ancient brown bears using DNA extracted from teeth and skeletons found at eight cave sites across Ireland. Mitochondria are parts of cells with their own DNA; this DNA is passed from mother to cub. The constant and cool temperatures in caves allow genetic material to survive for tens of thousands of years.
Hybrid bearsLonger evolutionary history
Hybridisation is a very important way in which new species can arise, noted Andrew Kitchener, curator of vertebrates at the National Museum of Scotland, Edinburgh.
He said he had previously proposed that polar bears have a much longer evolutionary history than the mitochondrial DNA suggests and that this is a consequence of hybridisation with brown bears during warm interglacials when polar bears have nowhere to go.
"Once the climate becomes much colder, there is rapid natural selection for the polar bear phenotype, but the genetic legacy of the brown bear persists," said Kitchener. He said bear hybrids could be one way for polar bears to survive man-made global warming.
Responding to climate change
Derek Yalden, author of The History of British Mammals, described the study as impressive and fascinating. "The combination of modern and ancient DNA, and the careful comparison of both mitochondrial and nuclear DNA, makes for an intriguing story. The sampling of modern bears looks very comprehensive," he said.
"It has been evident for some time that brown and polar bears are closely related genetically," he added. "This paper does a good job of revising the possible dates for speciation events. A degree of repeated interbreeding seems very plausible as a factor confusing the story."
The study found two distinct clades of brown bear in Europe. One of these lineages had survived the Ice Age in Europe, but the other bear lineage journeyed al the way from Alaska and Siberia, dispersing rapidly across the northern hemisphere around 60,000 years ago and ending up in eastern Europe.
The scientists say the approach employed in the study provides a way of going back in time and directly measuring the movement of species in response to past climate change.
Polar bears..
I'm sorry, but weren't we being told that man-caused global warming was going to be the end of polar bears, and dramatically warned that the interbreeding would see the species disappear? Looks like it's all happened before, as with everything in Earths climate history.
Far too many people think that the world is static and the animals we see around us now are the "end" of evolution and change to suit the environment.