A Genetic Makeover: From Polar Bear to Brown Bear

Despite their blatant physical differences in fur color and size, polar bears and brown bears have a closely knit genetic makeup, so close that the two produce fertile offspring. Naturally, hybrids are expected to look half and half. But ecologist Beth Shapiro, of the University of California, Santa Cruz, and a team of researchers have new evidence that suggests hybridizing these bears is washing the “polar” right out of them.

Shapiro and her team’s conclusions and new species-evolution hypothesis were published on March 14 in PLOS Genetics.

Until recently, scientists believed all polar bears may have arisen from a smaller population of brown bears. But this new research shows polar bears are clean of brown bear genes, and over generations, can transform a polar bear population to brown bears.

“What our data suggested was a gradual addition of these male brown bears into the polar bear population,” says graduate student James Cahill, of UC Santa Cruz, the study’s first author.

During the last ice age a group of islands called Alaska’s Admiralty, Baranof and Chicagof Islands, or ABC Islands, was covered in glaciers – ideal for polar bears. But when the ice age ended about 10,000 years ago and glaciers receded, the polar bears were stranded there.

Results stemmed from a DNA analysis of 10 bears: seven polar bears, two brown bears and one black bear. Scientists compared different combinations of polar bear, brown bear and black bear DNA against each other, and took note of differing genes.  

Physically, mainland brown bears and ABC brown bears are identical. But genetically, the ABC brown bears harbor a peculiar secret. A larger portion, 6.5 percent, of their maternally inherited X chromosome is polar bear DNA, whereas the rest of the genome is only 1 percent polar bear DNA. This suggests ABC bears have a greater genetic overlap with female polar bears than male polar bears.

Male brown bears from the mainland swam to the islands in an instinctual pursuit of new territory. These wandering brown bears mated with female polar bears. Over time, the polar bear population steadily dwindled. They transformed into brown bears without so much as a shred of visible evidence to their polar bear past.

“Toward the end of the population conversion, these are basically brown bears that just have echoes of what was once a polar bear population,” says Cahill.

Since the initially sequenced 10 bears, Cahill and other researchers have sequenced five more brown bear genomes – including two from the ABC islands with even more evidence of a polar bear lineage. Cahill will be presenting the research on the new brown bears at The Biology of Genomes conference later this month at Cold Spring Harbor Laboratory.

The team mainly examined the genetic relationships among polar bears and the ABC brown bears – but not the bigger questions of when and how polar bears first arose. The bottom line of explaining the population conversion is clear, Cahill said: “To show the tenuousness of what it is to be a polar bear.”

As temperatures rise, so too does our anxiety about this keystone Arctic predator’s resilience as a species. Shrinking Artic ice pushes polar bear habitats and brown bear habitats uncomfortably close, sharply increasing the likelihood of a new wave of population conversion.