Polar Bears Have a Touch of the Irish
One cannot always judge a species by its cover. For example, grizzly and polar bears are in no danger of being mistaken for one another morphologically, and yet molecular studies have shown that the polar bear is actually nested within the brown* bear phylogenetic tree—meaning that polar bears are actually more recently diverged from some clades of brown bears than some browns are from one another (Waits et al. 2008, see figure from that paper below). Appearances can be deceiving.
Specifically, the polar bears comprise part of a clade along with browns from the ABC Islands and parts of Alaska, while brown bears from other parts of Alaska, Canada, and the lower 48 states of the U.S. were intermixed in 3 additional clades. Thus, the species we know as Ursus maritimus is really a clade within Ursus arctos, although it continues to be recognized as a species instead of another subspecies, and despite its nest position there has been little argument over its validity as a species (there are currently 16 accepted subspecies of U. arctos). All of this is very interesting, and it also makes one wonder about how much natural interbreeding occurs between such tight-knit clades.
There has been at least one documented report of a wild polar/grizzly bear hybrid (which was shot on sight, of course), and several have been produced in zoos, which brings up the question: if the polar bear is so closely related to extant brown bears, how much genetic exchange has there been over the millennia since their split? A new study gives us further information on these dynamics.
A team of researchers led by Beth Shapiro of Penn State University have a new paper in Current Biology (Edwards et al. 2011) reporting evidence that the female ancestor of all living polar bears—a sort of ursine Eve—was a brown bear living in modern Ireland at the height of the most recent ice age, sometime between 20,000 and 50,000 years ago.
Although this hybridization seems a novelty to us, it is in no way a new phenomenon. As the new study shows, genetic exchange between the two lineages has been occurring for tens of thousands of years. Research by Waits and colleagues had already shown that polar bears carried mitochondrial DNA inherited from female brown bear ancestors, but Shapiro’s group sought to determine just how early this genetic material had become fixed in the polar bear’s mitochondrial genome. This fixation would have occurred at a time when the population underwent a sharp decline that resulted in the elimination of all but one version of a gene in the surviving members of the species. For mitochondrial genes, this means that it narrows the gene pool down into descendants of a single common female ancestor. In the case of the polar bears, this female ancestor happened to be a member of another species, a brown bear from Ireland.
The timeframe during which Shapiro’s group suggests this mitochondrial fixation took place—the window between 20,000-50,000 years ago—is of great interest, as it encompasses the peak of the most recent major ice age, at a time when many large animals were able to redistribute themselves across continents via convenient ice/land bridges, only to be isolated once the glaciers had melted and rising sea levels had separated the continents once again.
In addition, climatic cycles in which temperatures waxed and waned over generations could have led to variety of biotic and abiotic environmental pressures, creating ebbs and flows in spatial overlap between the two types of bear, such as when melting sea ice forced polar bears onto more solid coastal ground, bringing them closer to their brown relatives. The authors suggest “interspecific hybridization not only may be more common than previously considered but may be a mechanism by which species deal with marginal habitats during periods of environmental deterioration.”
Another noteworthy aspect of this research is that the population of brown bears that contributed their mtDNA to the polar bear’s lineage has actually been extinct for over 9,000 years. So in some way, the mitochondrial genome of polar bears is like a living fossil, remnant DNA from a disappeared race.
This is fascinating, and the polar bear’s situation is also ominous. Today’s polar bears face extremely dire circumstances, with sea ice melting precipitously, thwarting the bears’ foraging and denning patterns in the process (see this month’s National Geographic feature on their plight). Researchers think that drifting sea ice in the high Arctic may allow polar bears to hang out for the rest of this century, but beyond that the outlook is not good.
It would be misguided to think that the species has never faced rising temperatures and thinning ice before in its evolutionary history—and it appears genetic input from the brown bear has definitely occurred and may have even helped the polar bears to weather some of those tough times. But in the modern world, the available options for species to adapt and adjust their behaviors and movement patterns are very limited, with human activities and settlements encroaching from every side. Brown bears have also been knocked out of many of their former habitats, and coastal activities and settlements make relations with both marine and terrestrial carnivores very tense. Populations that had already been reduced from anthropogenic habitat destruction and persecution will have a harder time hanging on through this latest episode of changing climate. It’s a shame that we are just untangling the species’ origins at what may be the twilight of its tenure as part of our biodiversity, but hopefully this and future work will help to refine our understanding and appreciation for a species that is one of the last great megafaunal mammals to hold over from the last ice age.
*Although here in America we tend to think of all brown bears as grizzlies, the term officially refers to only one of the 16 subspecies, U. arctos horriblis, which occurs in western Canada, Alaska, and the northwestern United States. Members of other subspecies across go by different names, such as the Tibetan blue bear, Eurasian brown bear, Kamchatka brown bear, and several others.
Edwards, C., Suchard, M., Lemey, P., Welch, J., Barnes, I., Fulton, T., Barnett, R., O’Connell, T., Coxon, P., Monaghan, N., Valdiosera, C., Lorenzen, E., Willerslev, E., Baryshnikov, G., Rambaut, A., Thomas, M., Bradley, D., & Shapiro, B. (2011). Ancient Hybridization and an Irish Origin for the Modern Polar Bear Matriline Current Biology DOI: 10.1016/j.cub.2011.05.058
Waits, L. P., S. L. Talbot, R. H. Ward, and G. F. Shields. 2008. Mitochondrial DNA phylogeography of the North American brown bear and implications for conservation. Conservtion Biology 12: 408-417.
Phylogeny: Waits et al. 2008
Hybrid photo – Canadian Wildlife Service/AP
Stranded polar bears – metro.co.uk story