THE claim, made two years ago, that the polar bears alive today had Irish brown bear ancestors has been turned on its head.
According to Beth Shapiro, of the University of California at Santa Cruz: “We were wrong about the directionality of the gene flow between polar bears and Irish brown bears.”
Shapiro is co-author of a paper appearing in the current edition of PLOS Genetics. The brown bear was in Ireland when our hunter-gatherer forebears arrived 9,000 years ago. It became extinct four to six millennia later. Dr Ceiridwen Edwards, working at Trinity College Dublin, examined mitochondrial DNA from the teeth and bones of 17 bears found in eight Irish caves.
The animals had lived between 10,000 and 38,000 years ago. Edwards found a genetic sequence in their mitochondria which is also present in polar bears. Mitochondria are self-contained units which provide power within cells.
They have their own DNA which is inherited via the maternal line with no input from the male side. Irish bears and polar ones, the sequence showed, had common female ancestors. Interbreeding between the two species seemed to have led to the complete replacement of the polar bear mitochondria.
A paper by Edwards and co-authors, one of whom was Shapiro, appeared in Current Biology in 2011. It argued that polar and brown bears had been interbreeding for about 100,000 years and that the polar bears alive today are descended from browns living in, or near, Ireland during the last ice age.
The Santa Cruz team have come to a different conclusion. They looked at DNA from polar, black and brown bears, including one from Alaska’s ‘ABC islands’ (Admiralty, Baranof and Chicanof). Brown bear DNA, they found, is present in some polar bears but the wider polar bear population hasn’t brown bear mitochondria.
Clearly, not all polar bears have brown ancestors. The species interbred in the past only at locations such as the ABC islands where populations had become isolated.
The team also examined X chromosomes. About 6% of the ABC island brown bear X chromosomes, they found, had come recently from polar bears.
“This population of brown bears stood out as being really weird genetically, but we can explain it,” said Shapiro.
“Instead of the convoluted history some have proposed, it’s a very simple story.”
Young male brown bears wander far and wide from their place of birth. Females are more sedentary. With the ice melting, long ago, polar bears became stranded at locations such as the ABC islands. Then some brown males arrived on the scene. With few potential partners of their own kind available, they bred with polar bears instead. The species are closely related and produce fertile hybrid offspring. In time, the polar bears of the ABC Islands took on more and more brown bear characteristics.
We don’t have to delve into the remote past to find evidence for this theory; hybridisation is occurring today. The effects of global warming are more profound in the Arctic than in more temperate regions such as ours. The rise of temperature in Alaska has been dramatic.
Land, formerly covered in ice, is being exposed, leading the local brown bears, known as grizzlies, to venture further north. Meanwhile, polar bears, their traditional ice habitat reduced, are forced to spend more time on land. Some are becoming stranded.
Two ‘grizzlars’, bears with a polar bear mother and a grizzly bear father, were shot recently in Alaska.
Conditions in Ireland at the end of the last ice age resembled those of Alaska today. Polar bears may have been stranded here and brown bears could have mated with them, accounting for the presence of their DNA sequences in some polar bears today. Our Irish bears were, in fact, brown-polar hybrids.
With continuing global warming, will polar and brown bears merge to become a single species once again, just as they were 600,000 years ago?
* James Cahill et al. Genomic Evidence for Island Population Conversion Resolves Conflicting Theories of Polar Bear Evolution. PLOS Genetics. March 2013.
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