A new study offers a more detailed look at the evolutionary histories of polar bears and brown bears. The fact that these animals became separate species did not prevent them from mating with one another. Scientists have known this for some time, but the new study uses a larger dataset, including DNA from an ancient polar bear tooth, to elicit more information.
A new study offers a more detailed look at the evolutionary histories of polar bears and brown bears.
The fact that these animals became separate species did not prevent them from mating with one another. Scientists have known this for some time, but the new study uses a larger dataset, including DNA from an ancient polar bear tooth, to elicit more information.
The emerging narrative reveals complexities similar to those that have a complicated human evolutionary history.
"The formation and maintenance of species can be a messy process," says Charlotte Lindqvist, PhD, an expert on bear genetics and associate professor of biological sciences in the University at Buffalo College of Arts and Sciences.
"What happened with polar bears and brown bears is a good analogy for what we're learning about human evolution: species separation can be incomplete. As more ancient genomes from ancient human populations, including Neanderthals and Denisovans, have been recovered, we're seeing evidence of multidirectional genetic mixing as different groups of archaic humans mated with ancestors of modern humans. Another system where this occurs is with polar and brown bears."
"We find evidence for polar bear-brown bear interbreeding that predates an ancient polar bear we studied," she says. "Furthermore, our findings show a complicated, intertwined evolutionary history between brown and polar bears, with gene flow primarily flowing into polar bears from brown bears. This contradicts another researcher's hypothesis that gene flow was unidirectional and went into brown bears near the end of the last ice age."
The findings will be published in the Proceedings of the National Academy of Sciences the week of June 6. It was led by Lindqvist at UB in the United States, Luis Herrera-Estrella at Mexico's National Laboratory of Genomics for Biodiversity (LANGEBIO), Texas Tech University in the United States, and Kalle Leppälä at Finland's University of Oulu. Tianying Lan, Ph.D., a former UB postdoctoral researcher now at Daicel Arbor Biosciences, collaborated with Leppälä as the co-first author.
The idea of Arctic-adapted polar bears stealing genetic material from brown bears adapted to life in lower latitudes is one of several findings that could be of interest to scientists concerned about the effects of climate change on threatened species.
Polar bears and brown bears may encounter each other more frequently in areas where their ranges overlap as the world warms and Arctic sea ice melts. Lindqvist describes their shared evolutionary history as "particularly fascinating."
Species separation can be a difficult process.
According to Lindqvist, scientists previously believed that modern humans and Neanderthals evolved from a common ancestor and then split into separate species. Then, she says, researchers discovered Neanderthal DNA in modern Eurasian people, implying that modern human populations received an influx of Neanderthal genes at some point in their shared evolutionary history.
Lindqvist adds that scientists only discovered later that this genetic intermingling also supplemented Neanderthal populations with modern human genes. In other words, she claims that interbreeding is not always a one-way street.
The new bear study reveals a startlingly similar story: According to the researchers, the analysis finds evidence of hybridization in both polar bear and brown bear genomes, with polar bears, in particular, carrying a strong signature of an influx of DNA from brown bears. Lindqvist claims that previous research only proposed the inverse pattern.
"It's fascinating how DNA can help uncover ancient life histories. The direction of gene flow is more difficult to determine than its presence, but these patterns are critical to understanding how past adaptations have transferred among species to give modern animals their current characteristics "says Leppälä, Ph.D., a postdoctoral researcher at the University of Oulu's mathematical sciences research unit.
"Population genomics is an increasingly powerful toolbox for studying plant and animal evolution, as well as the effects of human activity and climate change on endangered species," says Dr. Herrera-Estrella, President's Distinguished Professor of Plant Genomics and Director of the Institute of Genomics for Crop Abiotic Stress Tolerance in the Texas Tech Department of Plant and Soil Science. He is also an emeritus professor at LANGEBIO. "Bears, like human evolution, do not provide simple speciation stories. According to this new genomic study, mammalian species groups can conceal complicated evolutionary histories."
Modern bear genome evidence – and DNA from an ancient tooth
The researchers examined the genomes of 64 modern polar and brown bears, including several new genomes from Alaska, where both species live.
The researchers also created a new, more complete genome for a polar bear that lived in Norway's Svalbard archipelago 115,000 to 130,000 years ago. The DNA for the ancient polar bear was extracted from a tooth attached to a subfossil jawbone, which is now housed at the University of Oslo's Natural History Museum.
Using this dataset, researchers estimate that polar bears and brown bears split up around 1.3 to 1.6 million years ago, updating previous estimates made by some of the same scientists. The age of the split has been and continues to be a source of scientific debate, with past interbreeding and a lack of fossil evidence for ancient polar bears among the factors complicating the timing, according to Lindqvist.
In any case, the new study concludes that after becoming their own species, polar bears experienced a dramatic population decline and a prolonged genetic bottleneck, leaving these bears with far less genetic diversity than brown bears. The findings support previous research that pointed to similar trends and adds evidence to support this hypothesis.
These findings, along with the analysis of gene flow, provide new insights into the polar bear and brown bear evolutionary histories.
Scientists from UB, LANGEBIO, Texas Tech, the University of Oulu, the Far Northwestern Institute of Art and Science, the Alaska Department of Fish and Game, the Natural History Museum at the University of Oslo, Nanyang Technological University, the University of Helsinki, and Aarhus University comprised the international research team.
The National Fish and Wildlife Foundation, the National Science Foundation, the Alaska Department of Fish and Game, and the United States Geological Survey funded the study.
Tianying Lan, Kalle Leppälä, Crystal Tomlin, Sandra L. Talbot, George K. Sage, Sean D. Farley, Richard T. Shideler, Lutz Bachmann, Øystein Wiig, Victor A. Albert, Jarkko Salojärvi, Thomas Mailund, Daniela I. Drautz-Moses, Stephan C. Schuster, Luis Herrera-Estrella, Charlotte Lindqvist. Insights into bear evolution from a Pleistocene polar bear genome. Proceedings of the National Academy of Sciences, 2022; 119 (24) DOI: 10.1073/pnas.2200016119