A new study offers enhanced insight into the intertwined evolutionary histories of polar bears and brown bears.
Becoming separate species did not entirely prevent these animals from mating with each other. Scientists have known this for some time, but the new research relies on an expanded dataset — including DNA from an ancient polar bear fang — to elicit more detail.
The story that emerges reveals complexities similar to those that complicate human evolutionary history.
“Species emergence and maintenance can be a chaotic process,” says the study’s lead author, Charlotte Lindqvist, associate professor of biological sciences at the University of Buffalo and an expert in bear genetics.
“What happened to polar bears and brown bears is a nice analogy to what we’re learning about human evolution: that the splitting of species can be incomplete. As more ancient genomes were recovered from ancient human populations, including Neanderthals and Denisovans, we see that there was multidirectional genetic mixing as different groups of archaic humans mated with ancestors of modern humans. Polar bears and brown bears are another system where this happens.”
“We find evidence of interbreeding between polar bears and brown bears that is older than an ancient polar bear that we studied,” she says. “Moreover, our results reveal a complicated, intertwined evolutionary history between brown and polar bears, with the main direction of gene flow being from brown bears to polar bears.
“This reverses a hypothesis proposed by other researchers that gene flow was unidirectional and transitioned into brown bears around the peak of the last Ice Age.”
Polar bear, brown bear hybridization
The concept of Arctic-adapted polar bears scavenging genetic material from brown bears adapted to life at lower latitudes is one of several findings of interest to scientists studying the impact of climate change on threatened species could be.
As the world warms and Arctic sea ice decreases, polar bears and brown bears may encounter each other more frequently in places where their ranges overlap. This makes their shared evolutionary history a particularly fascinating subject to study, says Lindqvist.
Scientists once thought that modern humans and Neanderthals simply split into separate species after evolving from a common ancestor. Then researchers found Neanderthal DNA in modern Eurasian humans, suggesting that modern human populations received an influx of genes from Neanderthals at some point in their shared evolutionary history, says Lindqvist.
It was only later that scientists realized that this genetic mixing also supplemented Neanderthal populations with modern human genes, she adds. In other words, intersections can be complex and not necessarily one-way, she says.
The new study on bears reveals a remarkably similar story: 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, researchers say. Previous research only suggested the reverse pattern, says Lindqvist.
“It’s exciting how DNA can help uncover the history of ancient life. The direction of gene flow is harder to determine than just their presence, but these patterns are crucial to understanding how past adaptations have transmitted between species to give modern animals their current traits,” says Kalle Leppälä, a postdoctoral researcher in the Mathematical Sciences Research Unit at the University of Oulu.
“Population genomics is an increasingly powerful toolkit for studying plant and animal evolution and the impact of human activities and climate change on endangered species,” says Luis Herrera-Estrella, professor of plant genomics and director of the Institute of Genomics for Crop Abiotic Stress Tolerance in Texas Tech Department of Plant and Soil Science.
“Bears don’t provide simple speciation histories any more than human evolution does. This new genomic research suggests that mammalian species groups may hide intricate evolutionary histories.”
Old polar bear teeth
The study analyzed the genomes of 64 modern polar and brown bears, including several new genomes from Alaska, a state where both species are found.
The team also produced a new, more complete genome for a polar bear that lived in Norway’s Svalbard archipelago between 115,000 and 130,000 years ago. Researchers extracted DNA for the ancient polar bear from a tooth attached to a subfossil jawbone now housed at the University of Oslo Museum of Natural History.
Using this dataset, the researchers estimate that polar bears and brown bears began becoming distinct species around 1.3 to 1.6 million years ago, updating earlier estimates by some of the same scientists. The age of the split has been and remains a subject of scientific debate, with previous interbreeding and limited fossil evidence of ancient polar bears among the factors making it difficult to pinpoint the exact timing, Lindqvist says.
In any case, after becoming their own species, polar bears suffered dramatic population declines and an ongoing genetic bottleneck, leaving these bears with much lower genetic diversity than brown bears, the new study finds. The results confirm previous research pointing to the same trends and add evidence to support this hypothesis.
Together with the analysis of gene flow, these results provide new insights into the chaotic, intertwined evolutionary history of polar bears and brown bears.
The study appears in Proceedings of the National Academy of Sciences.
Other researchers come from 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, Aarhus University, the University of Oulu and the National Laboratory of Genomics for Biodiversity in Mexico, Texas Tech and the University at Buffalo.
The work was funded by the National Fish and Wildlife Foundation, the National Science Foundation, the Alaska Department of Fish and Game, and the US Geological Survey.
Source: University of Buffalo