Modern polar bears are partially descended from the now-extinct Irish brown bear, according to a recent study of the DNA of modern polar bears and of fossils from both ancient polar bears and ancient Irish bears. Modern polar bears are a threatened species as global warming has caused the sea ice to melt, hindering their access to the ocean to hunt seals. The new Current Biology study suggests that hybridization with brown bears does not “itself doom the polar bear,” though “threats from human activity could push the species over the edge,” as Graham Slater, an evolutionary biologist at UCLA who was not involved with the study, says in ScienceNOW.
Back in the last Ice Age, polar bears made their way to Ireland when the climate appears to have cooled. Meanwhile, Irish brown bears found their habitat disappearing and retreated to the coasts, where they encountered the polar bears. When the climate changed again, the bears retreated to their original habitats.
For the study, researchers analyzed genetic material from 242 bears in 14 different locations. Researchers included Daniel Bradley and Ceiridwen Edwards, geneticists at Trinity College Dublin, and Beth Shapiro, an evolutionary biologist at Pennsylvania State University, University Park:
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.
As a Guardian write-up of the study points out, polar bears and brown bears are very different behaviorally. Polar bears are the world’s largest carnivores” and, being “expert swimmers,” they prey on seals. Brown bears live in the forests, are climbers and have a “varied omnivorous diet.” They also differ in color, skull shape, body size, teeth, and other traits, yet have mated successfully together when they’ve come into contact over the past 100,000 years.
Due to climate changes today, modern polar bears are again coming into contact with brown bears, in Canada and Alaska. As Andrew Whiteley, a geneticist at the University of Massachusetts, Amherst, points out, it’s not clear how quickly hybridization can occur. Indeed, the effects of interbreeding might be more negative and even detrimental than in the past. As it faces the loss of its habitat, the polar bear’s fate remains uncertain.
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Photo by Alastair Rae