Polar bears and brown bears are two different species. However, this did not prevent them from intermarrying with each other throughout history. New research, including a study of DNA from ancient polar bear teeth, has shed more light on the evolution of this species, revealing a level of complexity comparable to that of human evolution.
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“Species formation and conservation can be a complex process,” says Charlotte Lindqvist, associate professor of biological sciences at the University at Buffalo College of Arts and Sciences and study leader. “What happened with polar bears and grizzlies is a clear analogy to what is being learned about human evolution: that the division of species may be incomplete.”
“As more and more genomes are recovered from ancient human groups, including Neanderthals and Denisovans, we see ancient groups interbreeding with the ancestors of modern humans, and this has led to genetic admixture. Polar bears and grizzly bears are another order in which this is seen to happen,” continues Lindqvist, The expert in genetics bears.
The study was published in the journal Proceedings of the National Academy of Sciences It was the result of an international collaboration between the University at Buffalo, Texas Tech University, the National Laboratory for Biodiversity Genomics, in Mexico, and the Finnish University of Oulu and other institutions, such as museums and study centers, in the USA, Finland, Singapore and Denmark.
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The concept of Arctic-adapted polar bears that exchange genetic material with brown bears, which adapt to life at low latitudes, is an important topic for studying the effects of climate change on endangered species.
“With global warming and declining Arctic sea ice, polar and brown bears can be found more frequently in places where their ranges overlap. This makes their common evolutionary history a particularly attractive subject for study,” Lindqvist explains.
Splitting species can be a complex process
It was previously thought that modern humans and Neanderthals simply split into separate species after evolving from a common ancestor, she says.
Subsequent studies of Neanderthals and modern DNA showed that modern humans and Neanderthals interbred and exchanged genes with each other at some point in their common evolutionary history. In other words, intermingling can be complex, and not necessarily a one-way street, Lindqvist continues.
The new bear study tells a similar story: The analysis found evidence of genetic exchange in both the polar bear and brown bear genomes. Polar, in particular, has strong evidence of DNA flux or a “genetic signature” from brown. According to Lindqvist, previous research suggested only the inverse pattern.
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Polar bears show less genetic diversity than brown bears.
The study analyzed the genomes of 64 modern polar and brown bears, including several Alaskans, where both species are found.
Researchers have also produced a new and more complete genome of a polar bear that lived between 115,000 and 130,000 years ago in the Norwegian archipelago of Svalbard. The DNA of the ancient specimen was extracted from the teeth of the fossil jaw, now in the Natural History Museum of the University of Oslo.
Based on this data, researchers estimate that polar and brown bears began splitting into different species about 1.3 to 1.6 million years ago. However, the age of the split has been and remains a topic of scientific debate, due to various factors, such as the limited fossil evidence of ancient polar bears.
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Regardless of the timing of the split, the study concluded that after polar bears became a species of their own, they experienced a significant population decline and long-term genetic bottleneck, which made them less genetically diverse than grizzly bears.
Sync Agency
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