The evolutionary history of the polar bear includes a common ancestor with black and brown bears. Studies show that black and brown bears split first as separate species, and then polar bears separated from brown bears.
What currently occupies evolutionary biologists is the question of how long ago these lineage separations happened. With a limited fossil record, researchers have increasingly turned to DNA analysis for clues.
In a recent study, University of Alaska Fairbanks professor Matthew Cronin and colleagues at the University of California Davis and Montclair State University in New Jersey analyzed genetic variation in more than 300 polar bears from Alaska, as well as genetic elements not used in earlier studies.
The paper, published online in January in the Journal of Heredity, replicated other research and adds to the growing body of research on polar bear genetics. It concludes that polar bears diverged from brown bears as a separate species 1.2 million years ago, and black bears branched off 2.3 million years ago.
Other studies have marked the separation point for polar bears and brown bears anywhere from 600,000 to 4.5 million years ago. Such estimates have only emerged through recent advances in DNA technology. Until a few years ago, it was believed the separation happened much more recently. The oldest known polar bear fossil only dates back about 120,000 years.
Now, researchers are able to look at full genomes and billions, rather than thousands, of nucleotides, leading to estimates well beyond what is reflected in the fossil record. Using a "molecular clock" technique, scientists can determine when species diverged based on the number of mutations in DNA sequences, placing polar bears as an independent species much longer ago than previously thought.
Cronin, a professor of animal genetics with the UAF School of Natural Resources and Extension, noted that speciation is not a single event in time, and the "molecular clock" measures do not offer absolute certainty.
But if the measures are correct, it means bears have survived past warming cycles in the Earth's history, which Cronin said should have implications for policy decisions surrounding the listing of polar bears as an endangered species.
"It seems logical that if polar bears survived previous warm, ice-free periods, they could survive another," Cronin said in a news release from UAF about the study. "This is of course speculation, but so is predicting they will not survive, as the proponents of the Endangered Species Act listing of polar bears have done."
While he separated his research from policy, Cronin, who has been a vocal critic of the Endangered Species Act, said in an interview that he disagreed with the use of predictive models in scientific reviews for the law.
"I don't think you should base endangered species on predictions and models," Cronin said. "It should be focused on real-world problems."
Fellow scientists agree that polar bears have survived past warming and glaciation periods, but sharply diverged on whether past warming trends can be compared to the present one.
Steven C. Amstrup, scientist emeritus with the U.S. Geological Survey who wrote the report recommending polar bears be federally listed as threatened, and a longtime colleague of Cronin, said Cronin's arguments are "incautious" and "misleading."
In an email, Amstrup, now a senior scientist at Polar Bears International, noted that the current warming period is happening much faster than past cycles of glacial and interglacial periods, and includes human effects.
Another researcher, Charlotte Lindqvist, an evolutionary biologist at the University of Buffalo in New York, echoed that view. Lindqvist co-wrote a 2012 paper that analyzed mitochondrial DNA and pegged the divergence between brown bears and polar bears at 4.5 million years.
Lindqvist observed that modern polar bears have much lower genetic diversity than in the past, which suggests a genetic bottleneck may have been at work. That could mean that today's polar bears have much less of a buffer to withstand environmental changes, she said.
But she also said more research has to happen before any definitive conclusions can be made. Lingering questions include finding the genetic signature of what makes polar bears different as a species, and what the animal's earliest ancestors might have looked like, she said.
"The wonderful thing with all this genome-wide data we're producing these days is it really contains a treasure trove," Lindqvist said.
Reach Devin Kelly at firstname.lastname@example.org or 257-4314.
By DEVIN KELLY