If she could have talked, the mother of the modern polar bear may have had a brogue. Genetic studies of fossil and modern bears have revealed some hanky-panky 45,000 years ago, when polar bears interbred with now-extinct Irish brown bears. Although a decidedly different species, today's polar bears have Irish brown bear genetic material in their cells, indicating that the polar bear "Eve" was brown and that bear evolution was far from simple.
The finding also shows that interbreeding doesn't necessarily destroy a species. Hybridization with brown bears, a concern today because declining sea ice cover is forcing polar bears to extend their range and come into contact with brown bears, doesn't by itself doom the polar bear, says Graham Slater, an evolutionary biologist at the University of California, Los Angeles, who was not involved with the work. "A little bit of hybridization isn't necessarily a bad thing," he notes. However, he cautions that the threats from human activity could push the species over the edge.
Global warming has caused big problems for polar bears, which depend on sea ice for access to the ocean so they can hunt seals and other prey. As sea ice disappears, polar bears are being forced to hunt more on land, which brings them into conflict with humans and increases contact with brown bears. There have been several recent documented cases of hybrids caught in the wild, and interbreeding in zoos has resulted in fertile offspring. To understand the implications of these changes, Beth Shapiro, an evolutionary biologist at Pennsylvania State University, University Park, and her colleagues wanted to know how past climate change had affected the genetic diversity and distribution of bears.
The team gathered genetic material from 242 bears from 14 different locations. The material included 8000-year-old polar bear fossils as well as modern samples of polar bears. Daniel Bradley and Ceiridwen Edwards, geneticists at Trinity College Dublin, obtained samples from 23 ancient Irish bear fossils found in caves in Ireland as well. Past studies by other researchers had looked at brown bear DNA in comparison to polar bear DNA, but this was the first time anyone had been able to get DNA material from Irish brown bears.
Bradley, Edwards, Shapiro, and their colleagues sequenced part of a gene from the samples' mitochondria, cellular components responsible for generating a cell's energy. Billions of years ago, mitochondria were independent organisms, and they still retain some of their own genetic material, which passes from generation to generation in the egg but not the sperm. Because it's inherited just from the mother, mitochondrial DNA reveals the history of the female lineage of a species.
By grouping the sequenced DNA pieces according to how similar they were, the researchers were able to reconstruct the bears' past. They discovered that modern polar bear mitochondrial DNA was most similar to that of the extinct Irish brown bear. Thus modern polar bears come from Europe, not islands between Alaska and Siberia , as had been previously thought, Bradley, Edwards, Shapiro, and their colleagues report today in Current Biology. Extinct polar bears had different mitochondria.
"That it happened in Ireland, no one had ever thought of that before," Slater says. But Charlotte Lindqvist, an evolutionary biologist at the University at Buffalo in New York, cautions that this conclusion is based on the analysis of just one gene and needs to be backed up by studies of other genetic material. "I question the reliability and strength of their results and conclusions," she says.
The fossil record indicates that polar bears and brown bears were separate species by at least 110,000 years ago, living in separate places and evolving independently. That deep divergence helps explain the vast differences between the two species: not just color but also skull shape, body size, teeth, and other traits clearly distinguish the two. "We know they diverged and that after that divergence, there must have been some genetic mixing between the two species," Shapiro says.
At least twice, the two species came together, as they now seem to be coming together again, she explains. It seems that after the climate cooled during the last glacial period, disappearing habitat inland forced brown bears toward the coasts, where they encountered polar bears shifted there by British-Irish ice sheets. As the climate changed again, the bears again went their separate ways.
"The big question for conservation of polar bears is if hybridization occurs rapidly and in combination with other stressors, will that hybridization have more of a negative effect now than it did in the past," says Andrew Whiteley, a geneticist at the University of Massachusetts, Amherst. He thinks it could have a detrimental effect, and Lindqvist agrees, saying, "We shouldn't sit back and say there is nothing to worry about."