Untangling the history of polar bears and brown bears

A new study led by the University of Buffalo has explored the intertwined evolutionary history of polar bears and brown bears. The experts estimate that polar and brown bears started to become distinct species between 1.3 and 1.6 million years ago, but they continued to mate with each other for quite some time. Thus, rather than simple splitting events, the species histories of polar and brown bears hide convoluted stories of divergence and interbreeding, similar to those that complicate human evolutionary history.

“The formation and maintenance of species can be a messy process,” said study lead author Professor Charlotte Lindqvist. “What’s happened with polar bears and brown bears is a neat analog to what we’re learning about human evolution: that the splitting of species can be incomplete.” 

“As more and more ancient genomes have been recovered from ancient human populations, including Neanderthals and Denisovans, we’re seeing that there was multidirectional genetic mixing going on as different groups of archaic humans mated with ancestors of modern humans. Polar bears and brown bears are another system where you see this happening.”

By analyzing the genomes of 64 modern polar and brown bears, as well as that of a polar bear which lived 115,000 to 130,000 years ago in Norway’s Svalbard archipelago – its DNA was extracted from a tooth attached to a subfossil jawbone – Professor Lindqvist and her colleagues discovered a complicated, intertwined evolutionary history among brown and polar bears, with the main direction of gene flow going into polar bears from brown bears. 

Moreover, after becoming a distinct species, polar bears endured dramatic population decline and a prolonged genetic bottleneck, leaving them with much less genetic diversity than brown bears.

The discovery that Arctic-adapted polar bears have continued capturing genetic material from brown bears long after their species diverged could be of high interest for scientists investigating the impact of climate change on threatened species. Since global warming and subsequent Arctic sea ice declines could make polar and brown bears run into each other more frequently in places where their ranges overlap, a better understanding of their tangled evolutionary histories could help scientists predict how climate change may affect these species in the near future.

“Population genomics is an increasingly powerful toolbox to study plant and animal evolution and the effects of human activity and climate change on endangered species,” concluded study co-author Luis Herrera-Estrella of Texas Tech University. “Bears don’t provide simple speciation stories any more than human evolution has. This new genomic research suggests that mammalian species groups can hide complicated evolutionary histories.”

The study is published in the journal Proceedings of the National Academy of Sciences.

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By Andrei Ionescu, Earth.com Staff Writer