Researchers have gained new insights into the intertwined evolutionary history of brown and polar bears. The genetic analyses, which also include the genome of a 115,000-year-old polar bear, show that the splitting into the two species began around 1.3 to 1.6 million years ago. After the species had become established, however, hybridizations occurred again, which primarily led to gene flow from brown bears to polar bears. It becomes clear that the evolutionary history of these two types of interactions was also marked – similar to what is known, for example, from modern humans and Neanderthals, the scientists say.
The evolutionary history of living beings is often represented as a tree: certain lines of development have produced branches because species have split into further species in the course of evolution. In some cases, this split was not very long ago, which means that two species are still relatively similar and can sometimes produce common offspring. An example of this is the brown bear (Ursus arctos) and polar bear (Ursus maritimus), which evolved from a common ancestor and subsequently adapted to their different habitats. They can also be crossed, as is known from rare cases of hybridization in the northern overlapping areas of their distribution areas.
Evolutionary history reflected in the genome
In order to shed more light on the evolutionary history, the scientists led by Charlotte Lindqvist from the University at Buffalo have now taken another look at the genome of the two bear species. For their study, they analyzed the genomes of 64 modern polar and brown bears and also provided special comparative data: They sequenced polar bear genome that comes from the jaw of a specimen that lived in the Norwegian Svalbard archipelago 115,000 to 130,000 years ago. Informative comparisons with modern genome data were thus possible.
Based on certain mutation rates in the genome, the scientists now estimate that polar bears and brown bears began to develop into different species around 1.3 to 1.6 million years ago. In the case of the polar bear, one thing is becoming clear: after it had become a species of its own, there was a significant loss of genetic diversity in its genome. This was probably due to drastic population drops that led to inbreeding. According to the researchers, the most important study result is the evidence of the interactions between polar and brown bears after species differentiation: The genetic data reflects that even after the two species had become established, there was a clear interbreeding that left lasting traces left in the populations.
Gene flow from brown to polar bear
In concrete terms, there are indications that around 150,000 years ago genetic transmission took place, which was probably bidirectional, but which shaped polar bears in particular. “We found evidence of interbreeding between polar bears and brown bears that predated the ancient polar bear we studied,” says Lindqvist. “Furthermore, our results reveal a complicated, intertwined evolutionary history between brown and polar bears, with gene flow primarily directed from brown bears to polar bears.” In concrete terms, it could therefore be that in warmer climatic phases there was an increased overlap in the habitat of the two bear species, which was accompanied by hybridization. Apparently, the legacy of the brown bears stuck with the polar bears rather than vice versa.
“It shows again that the emergence and maintenance of species can be a complex process,” says Lindqvist. The scientist draws particular attention to the interesting parallels to human evolution. As Lindqvist explains, it was once thought that modern humans and Neanderthals in Africa and Eurasia simply split into different species after emerging from a common ancestor. But then Neanderthal DNA was discovered in modern Eurasian humans, proving that modern human populations received an influx of genes from their archaic cousins at some point in their shared evolutionary history.
“What happened to the polar and brown bears is a nice analogy to what we’ve now learned about human evolution: that the segregation of species can be incomplete. Because genomes reflect the fact that there was multidirectional genetic mixing. Various groups of archaic humans mated with ancestors of modern humans. Polar bears and brown bears are another system where this can now be seen,” says Lindqvist. Her colleague Luis Herrera-Estrella from Texas Tech University in Lubbock concludes: “Our genetic study makes it clear that complicated evolutionary histories can be hidden behind species groups of mammals.”
Source: University at Buffalo, professional article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2200016119