The subfossil jawbone of a polar bear that lived 115,000 to 130,000 years ago in Norway’s Svalbard archipelago. A genomic study includes an analysis of DNA extracted from a tooth attached to this jawbone, which is now housed at the Natural History Museum at the University of Oslo.
Credit: Photo by Karsten Sund, Natural History Museum (NHM), University of Oslo
A new study by scientists led by Charlotte Lindqvist at University at Buffalo (UB) in the U.S.; Luis Herrera-Estrella at the National Laboratory of Genomics for Biodiversity (LANGEBIO) in Mexico and Texas Tech University in the U.S.; and Kalle Leppälä at the University of Oulu in Finland has shown that the evolutionary divergence of brown bears, Ursus arctos, and polar bears, Ursus maritimus, into different species, was every bit as complex as that of hominins into the single extant species, Homo sapiens.
This illustrates how evolutionary divergence is often a far from straightforward process, with several periods of partial speciation, remixing and convergence and separation again until, as today, we can end up with two species that can still interbreed, indicating that barrier to hybridization have not yet fully evolved.
This, of course, is exactly what we would expect of a slow process over time, but not speciation events such as creationists wrongly believe scientists think is how evolution works. This is perhaps best illustrated with the simple analogy of one colour changes into another in a rainbow spectrum where it is impossible to say exactly where one colour ends and the next begins:
A mother polar bear and her 2-year-old cubs in northwestern Greenland.
Credit: Øystein Wiig
An adult male polar bear in northwestern Greenland.
Credit: Øystein Wiig
The study will be published the week of June 6 in the Proceedings of the National Academy of Sciences. It was led by Lindqvist at UB in the U.S.; Luis Herrera-Estrella at the National Laboratory of Genomics for Biodiversity (LANGEBIO) in Mexico and Texas Tech University in the U.S.; and Kalle Leppälä at the University of Oulu in Finland. Tianying Lan, PhD, a former UB postdoctoral researcher now at Daicel Arbor Biosciences, was co-first author with Leppälä.The teams findings are published, open access in Proceedings of the National Academy of Science (PNAS):
The concept of Arctic-adapted polar bears capturing genetic material from brown bears, which are adapted to life in lower latitudes, is one of several findings of possible interest for scientists concerned with climate change impacts on threatened species.
As the world warms and Arctic sea ice declines, polar bears and brown bears may run into each other more frequently in places where their ranges overlap. This makes their shared evolutionary history a particularly intriguing subject of study, Lindqvist says.The formation and maintenance of species can be a messy process. 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.
We find evidence for interbreeding between polar bears and brown bears that predates an ancient polar bear we studied, and, moreover, our results demonstrate a complicated, intertwined evolutionary history among brown and polar bears, with the main direction of gene flow going into polar bears from brown bears. This inverts a hypothesis suggested by other researchers that gene flow has been unidirectional and going into brown bears around the peak of the last ice age
Charlotte Lindqvist, PhD, lead author
Associate professor of biological sciences and an expert in bear genetics
College of Arts and Sciences
University at Buffalo, Buffalo, NY, USA.
It’s exciting how DNA can help reveal ancient life history. Gene flow direction is harder to determine than merely its presence, but these patterns are vital to understanding how past adaptations have transferred among species to give modern animals their current features.
Kalle Leppälä, PhD, co-first author
Postdoctoral researcher
Research unit of mathematical sciences
University of Oulu, Oulu, Finland.As Lindqvist explains, scientists once thought modern humans and Neanderthals simply split into separate species after evolving from a common ancestor. Then, researchers found Neanderthal DNA in modern Eurasian people, implying that modern human populations received an influx of genes from Neanderthals at some point in their shared evolutionary history, she says.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.
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.
Luis Herrera-Estrella, PhD, C-corresponding author
President’s Distinguished Professor of Plant Genomics
Director of the Institute of Genomics for Crop Abiotic Stress Tolerance
Department of Plant and Soil Science
Texas Tech University, Lubbock, TX, USA
And professor emeritus at LANGEBIO
Only later did scientists realize that this genetic intermingling also supplemented Neanderthal populations with modern human genes, Lindqvist adds. In other words, interbreeding can be complex, not necessarily a one-way street, 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. Earlier research proposed the inverse pattern only, Lindqvist says.
SignificanceWhat we have here then is exactly what the Theory of Evolution predicts. Against a background of Northern Hemisphere glaciers advancing and retreating in response to climate change, so the specialist polar bears spent period in isolation from the brown bears, and periods when they came back into contact and could exchange genetic material. The process never progressed to the point where interbreeding became impossible and there were few pressures to establish barriers to hybridization because both species could cohabit in the same inter-glacial environment, so their hybrid offspring were not at a strong disadvantage.
Interspecific hybridization is a widespread phenomenon, but measuring its extent, directionality, and adaptive importance remains challenging. Ancient genomes, however, can help illuminate the history of modern organisms. Here, we present a genome retrieved from a 130,000- to 115,000-y-old polar bear and perform genome analyses of modern polar and brown bears throughout their geographic ranges. We find that the principal direction of ancient allele sharing was from brown bear into polar bear, although gene flow between them has likely been bidirectional. This partially inverts the current paradigm of unidirectional gene flow from polar into brown bear, and it suggests that polar bears were recipients of external genetic variation prior to their extensive population decline.
Abstract
The polar bear (Ursus maritimus) has become a symbol of the threat to biodiversity from climate change. Understanding polar bear evolutionary history may provide insights into apex carnivore responses and prospects during periods of extreme environmental perturbations. In recent years, genomic studies have examined bear speciation and population history, including evidence for ancient admixture between polar bears and brown bears (Ursus arctos). Here, we extend our earlier studies of a 130,000- to 115,000-y-old polar bear from the Svalbard Archipelago using a 10× coverage genome sequence and 10 new genomes of polar and brown bears from contemporary zones of overlap in northern Alaska. We demonstrate a dramatic decline in effective population size for this ancient polar bear’s lineage, followed by a modest increase just before its demise. A slightly higher genetic diversity in the ancient polar bear suggests a severe genetic erosion over a prolonged bottleneck in modern polar bears. Statistical fitting of data to alternative admixture graph scenarios favors at least one ancient introgression event from brown bears into the ancestor of polar bears, possibly dating back over 150,000 y. Gene flow was likely bidirectional, but allelic transfer from brown into polar bear is the strongest detected signal, which contrasts with other published work. These findings may have implications for our understanding of climate change impacts: Polar bears, a specialist Arctic lineage, may not only have undergone severe genetic bottlenecks but also been the recipient of generalist, boreal genetic variants from brown bears during critical phases of Northern Hemisphere glacial oscillations.
Lan Tianying; Leppälä Kalle; Tomlin Crystal; Talbot Sandra L.; Sage George K.; Farley Sean D.; Shideler Richard T.; Bachmann Lutz; Wiig Øystein; Albert Victor A.; Salojärvi Jarkko; Mailund Thomas; Drautz-Moses Daniela I.; Schuster Stephan C.; Herrera-Estrella Luis; Lindqvist Charlotte
Insights into bear evolution from a Pleistocene polar bear genome
Proceedings of the National Academy of Sciences 119 (24), e2200016119; DOI: 10.1073/pnas.2200016119
Copyright: © 2022 The authors. Published by PNAS, Open access
Reprinted under a Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
Just like modern humans, Neanderthals and Denisovans, each time they came into contact after several hundred thousand years of separation, they were still able to interbreed, so Neanderthals had remnants of modern and Denisovan DNA and modern humans now have remnants of Neanderthal and Denisovan DNA.
To dd to the embarrassment of creationists, there is also evidence that proto-humans and proto-chimpanzees occasionally interbred over a prolonged period as the two species diverged.
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