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Post by grrraaahhh on Mar 22, 2010 17:18:51 GMT -9
Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear aDepartment of Biological Sciences, University at Buffalo, Buffalo, NY 14260; bCenter for Comparative Genomics and Bioinformatics, Pennsylvania State University, University Park, PA 16802; cAlaska Science Center, US Geological Survey, Anchorage, AK 99508; dNational Centre for Biosystematics, Natural History Museum, University of Oslo, 0318 Oslo, Norway; ePolar Environmental Centre, Norwegian Polar Institute, 9296 Tromsø, Norway; fDepartment of Earth Sciences, University of Iceland, 101 Reykjavik, Iceland; and gUniversity Centre in Svalbard, 9171 Longyearbyen, Norway www.bx.psu.edu/miller_lab/dist/polar_bear.pdf
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Post by grrraaahhh on Mar 27, 2010 7:46:12 GMT -9
Ancient DNA from Rare Fossil Reveals That Polar Bears Evolved Recently and Adapted QuicklyOur results confirm that the polar bear is an evolutionarily young species that split off from brown bears some 150,000 years ago and evolved extremely rapidly during the late Pleistocene, perhaps adapting to the opening of new habitats and food sources in response to climate changes just before the last interglacial period," says Charlotte Lindqvist, Ph.D., research assistant professor in the UB Department of Biological Sciences and lead author on the paper with Stephan C. Schuster at Penn State's Center for Comparative Genomics and Bioinformatics. www.sciencedaily.com/releases/2010/03/100301141848.htm
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Post by grrraaahhh on Sept 4, 2010 17:47:26 GMT -9
One small fossil, one giant step for polar bear evolutionWhat is a polar bear? Evolutionarily speaking, polar bears (Ursus maritimus) are brown bears (Ursus arctos). That might seem counterintuitive, but modern biologists classify species according to their evolutionary history. Organisms that are more closely related are grouped together. In this system, only clades — groups of organisms that contain all the descendents of an ancestor — are named. When we look at the family tree of bears, we can see that not only are polar bears most closely related to brown bears, but they actually fall within the brown bear clade. There is no clade that includes all the brown bears and excludes the polar bears. From an evolutionary perspective, polar bears are simply a unique and highly specialized sort of brown bear! The polar bear jawbone fossil that revealed so much about the history of polar bears. The researchers compared the fossil's DNA sequence to those from different species of modern bears and from extinct cave bears. They used these sequences to reconstruct the animals' family tree. As expected, the modern and ancient polar bear DNA formed a tight group, a clade — and that clade was most closely related to the brown bears of Southeast Alaska.
The surprise came when the researchers used two different lines of reasoning to learn more about the relationship between the ancestral polar bear and the fossil. First, they used the DNA sequences to extrapolate back to the sequence that the ancestor of polar bears and brown bears would likely have carried. The fossil specimen's DNA was extremely similar to the likely sequence of the polar bear ancestor! Second, they used molecular clocks to estimate the date of the split between polar bears and brown bears. Molecular clocks are stretches of DNA that evolve at a fairly regular pace can be used as evolutionary tape measures to estimate how long ago different events happened. The more differences a clock-like stretch of DNA has accumulated since a divergence, the more time has probably passed since that divergence. The differences among the bears' sequences pointed to a split between polar bears and brown bears just 150,000 years ago. All this means that the fossil (which may be as many as 130,000 years old) likely represents one of the very earliest members of the polar bear lineage!evolution.berkeley.edu/evolibrary/news/100401_polarbears
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Post by grrraaahhh on Oct 15, 2010 19:27:07 GMT -9
Polar bears and ice go together in our minds as automatically as salt and pepper. The largest of the terrestrial carnivores, polar bears are uniquely adapted to live on Arctic sea ice. The significance of that relationship can be expressed quite simply: polar bears came into existence in the first place because of sea ice - if the ice disappears, so will the polar bears.
Only a million or so years ago ¬-- a blink of an eye in evolutionary time -- the barren ground grizzly bears roamed the coast of the Arctic mainland, adjacent to a vast new habitat: sea ice. And the ice was home to an unexploited abundance of prey – seals. At some point, grizzlies ventured out onto the sea ice and, as they learned to prey upon the ice-breeding ringed seals, they rapidly evolved into the present day polar bears.Body shape comparison between polar bear (up) and grizzly (down). Narrower head, longer neck and lower shoulder hump differentiate the polar bear morphology from its closest relative the grizzly - © Julien Norwwod - Le Cercle Polaire Ours blanc : chronique d'une extinction annoncéewww.lecerclepolaire.com/articles_archives/Stirling-Polar_Bear.htmlIt is not surprising that one of the most predatory sub species of brown bear, the "Tundra Grizzly/Brown Bear" would survive (enough of them) their frozen isolation and evolve into the Polar bear. It is also ironic that one of the smallest sub species of brown bear morphed into the largest bears to ever walk the Earth.
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Post by grrraaahhh on Apr 19, 2012 19:27:39 GMT -9
The latest research explains the following: Nuclear Genomic Sequences Reveal that Polar Bears Are an Old and Distinct Bear LineageAbstract Recent studies have shown that the polar bear matriline (mitochondrial DNA) evolved from a brown bear lineage since the late Pleistocene, potentially indicating rapid speciation and adaption to arctic conditions. Here, we present a high-resolution data set from multiple independent loci across the nuclear genomes of a broad sample of polar, brown, and black bears. Bayesian coalescent analyses place polar bears outside the brown bear clade and date the divergence much earlier, in the middle Pleistocene, about 600 (338 to 934) thousand years ago. This provides more time for polar bear evolution and confirms previous suggestions that polar bears carry introgressed brown bear mitochondrial DNA due to past hybridization. Our results highlight that multilocus genomic analyses are crucial for an accurate understanding of evolutionary history. www.sciencemag.org/content/336/6079/344From the BBC: DNA reveals polar bear's ancient originsThe polar bear is much older than previously thought, according to new genetic evidence. DNA studies suggest the Arctic predator split from its ancestor, the brown bear, about 600,000 years ago. Previous estimates put the polar bear at about 150,000 years old, suggesting the mammal adapted very rapidly to Arctic life. Conservationists say the new study, published in Science, has implications for bear conservation. Polar bears are listed as threatened under the US Endangered Species Act. Conservationists say their survival is at risk, mainly due to the loss of the Arctic sea ice on which they spend much of their lives. Dr Frank Hailer of the German Biodiversity and Climate Research Centre in Frankfurt, who led the international study, said the genetic information shed new light on conservation issues. "It fundamentally changes our understanding of polar bears and their conservation today," he told BBC News. "They have survived previous warm phases but they carry scars from these times - they must have been close to extinction at times." Nuclear DNAThe researchers looked at DNA from modern bears to study the history of the species, analysing genetic information from the cell nucleus of more than 40 brown, black and polar bears. Past work has relied mainly on mitochondrial DNA, the fragments of genetic material contained within tiny cell components called mitochondria. The latest findings suggest the polar bear evolved in the mid Pleistocene, about 600,000 years ago. This scenario paints a new picture of the bear's evolutionary history. The mammal would have had more time to colonise and adapt to life in the high Arctic, and lived through various cycles of warming and cooling. The polar bear's lack of genetic diversity suggests that changes in the environment, such as warm phases, led to dramatic falls in numbers at times. The researchers say polar bears face many other threats to their survival today, including habitat destruction, hunting and the effects of environmental pollutants. Writing in Science, they explain: "Although polar bears have persisted through previous warm phases, multiple human-mediated stressors (eg habitat conversion, persecution, and accumulation of toxic substances in the food chain) could magnify the impact of current climate change, posing a novel and likely profound threat to polar bear survival." Commenting on the research, Dr Steven Amstrup, chief scientist of Polar Bears International, said he suspected the age of the polar bear was not entirely settled yet. Even if they did split from their ancestor 600,000 years ago, they would have lived through only two periods noticeably warmer than today, he said, and spent most of their existence in cooler times. He added: "It's continuing evidence that polar bears are adapted to a cold environment and they haven't experienced anything of the warming that we are likely to experience in the next 100 years." www.bbc.co.uk/news/science-environment-17762196
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Post by grrraaahhh on Jan 30, 2013 12:05:16 GMT -9
The latest research explains the following: Nuclear Genomic Sequences Reveal that Polar Bears Are an Old and Distinct Bear LineageAbstract Recent studies have shown that the polar bear matriline (mitochondrial DNA) evolved from a brown bear lineage since the late Pleistocene, potentially indicating rapid speciation and adaption to arctic conditions. Here, we present a high-resolution data set from multiple independent loci across the nuclear genomes of a broad sample of polar, brown, and black bears. Bayesian coalescent analyses place polar bears outside the brown bear clade and date the divergence much earlier, in the middle Pleistocene, about 600 (338 to 934) thousand years ago. This provides more time for polar bear evolution and confirms previous suggestions that polar bears carry introgressed brown bear mitochondrial DNA due to past hybridization. Our results highlight that multilocus genomic analyses are crucial for an accurate understanding of evolutionary history. www.sciencemag.org/content/336/6079/344From the BBC: DNA reveals polar bear's ancient originsThe polar bear is much older than previously thought, according to new genetic evidence. DNA studies suggest the Arctic predator split from its ancestor, the brown bear, about 600,000 years ago. Previous estimates put the polar bear at about 150,000 years old, suggesting the mammal adapted very rapidly to Arctic life. Conservationists say the new study, published in Science, has implications for bear conservation. Polar bears are listed as threatened under the US Endangered Species Act. Conservationists say their survival is at risk, mainly due to the loss of the Arctic sea ice on which they spend much of their lives. Dr Frank Hailer of the German Biodiversity and Climate Research Centre in Frankfurt, who led the international study, said the genetic information shed new light on conservation issues. "It fundamentally changes our understanding of polar bears and their conservation today," he told BBC News. "They have survived previous warm phases but they carry scars from these times - they must have been close to extinction at times." Nuclear DNAThe researchers looked at DNA from modern bears to study the history of the species, analysing genetic information from the cell nucleus of more than 40 brown, black and polar bears. Past work has relied mainly on mitochondrial DNA, the fragments of genetic material contained within tiny cell components called mitochondria. The latest findings suggest the polar bear evolved in the mid Pleistocene, about 600,000 years ago. This scenario paints a new picture of the bear's evolutionary history. The mammal would have had more time to colonise and adapt to life in the high Arctic, and lived through various cycles of warming and cooling. The polar bear's lack of genetic diversity suggests that changes in the environment, such as warm phases, led to dramatic falls in numbers at times. The researchers say polar bears face many other threats to their survival today, including habitat destruction, hunting and the effects of environmental pollutants. Writing in Science, they explain: "Although polar bears have persisted through previous warm phases, multiple human-mediated stressors (eg habitat conversion, persecution, and accumulation of toxic substances in the food chain) could magnify the impact of current climate change, posing a novel and likely profound threat to polar bear survival." Commenting on the research, Dr Steven Amstrup, chief scientist of Polar Bears International, said he suspected the age of the polar bear was not entirely settled yet. Even if they did split from their ancestor 600,000 years ago, they would have lived through only two periods noticeably warmer than today, he said, and spent most of their existence in cooler times. He added: "It's continuing evidence that polar bears are adapted to a cold environment and they haven't experienced anything of the warming that we are likely to experience in the next 100 years." www.bbc.co.uk/news/science-environment-17762196 Follow up... Intimate Relations: The nuclear genomes of bears (black outline) suggest that polar bears and brown bears diverged from one another 4 to 5 million years ago, and that occasional exchange of genes between the two species (shaded gray areas) followed. Results from maternally inherited mitochondrial DNA (dotted line) indicate extinction (marked with an "X") and replacement of polar bear mitochondrial DNA around 160,000 years ago due to interbreeding between the two species. (Credit: Penn State University).Bear demographic history during the Pleistocene. Text Extract:"We generated a first-rate set of data, including deep sequence coverage for the entire genomes of a polar bear, three brown bears and a black bear, plus lower coverage of 23 additional polar bears, including a 120,000-year-old individual; very few vertebrate species have such comprehensive genomic resources available," Schuster said. Using this vast amount of data, the scientists discovered that polar bears are actually an older species than previously thought -- indeed, far more ancient than suggested by a recent study that placed the species' age at 600,000 years old. That analysis looked only at small segments of DNA. "We showed, based on a consideration of the entire DNA sequence, that earlier inferences were entirely misleading," said study co-lead author Webb Miller, a Penn State professor of biology and computer science and engineering. "Rather than polar bears splitting from brown bears a few hundred thousand years ago, we estimate that the split occurred 4-5 million years ago." "This means polar bears definitely persisted through warming periods during Earth's history," UB's Lindqvist said. She cautions, however, that the species' endurance over several million years doesn't guarantee its future survival. www.sciencedaily.com/releases/2012/07/120723151028.htm Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. Proceedings of the National Academy of Sciences, 2012; DOI: 10.1073/pnas.1210506109 PDF LINK: www.pnas.org/content/109/36/E2382/suppl/DCSupplemental
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Post by grrraaahhh on Jan 31, 2013 19:16:26 GMT -9
The latest research explains the following: Nuclear Genomic Sequences Reveal that Polar Bears Are an Old and Distinct Bear LineageAbstract Recent studies have shown that the polar bear matriline (mitochondrial DNA) evolved from a brown bear lineage since the late Pleistocene, potentially indicating rapid speciation and adaption to arctic conditions. Here, we present a high-resolution data set from multiple independent loci across the nuclear genomes of a broad sample of polar, brown, and black bears. Bayesian coalescent analyses place polar bears outside the brown bear clade and date the divergence much earlier, in the middle Pleistocene, about 600 (338 to 934) thousand years ago. This provides more time for polar bear evolution and confirms previous suggestions that polar bears carry introgressed brown bear mitochondrial DNA due to past hybridization. Our results highlight that multilocus genomic analyses are crucial for an accurate understanding of evolutionary history. www.sciencemag.org/content/336/6079/344From the BBC: DNA reveals polar bear's ancient originsThe polar bear is much older than previously thought, according to new genetic evidence. DNA studies suggest the Arctic predator split from its ancestor, the brown bear, about 600,000 years ago. Previous estimates put the polar bear at about 150,000 years old, suggesting the mammal adapted very rapidly to Arctic life. Conservationists say the new study, published in Science, has implications for bear conservation. Polar bears are listed as threatened under the US Endangered Species Act. Conservationists say their survival is at risk, mainly due to the loss of the Arctic sea ice on which they spend much of their lives. Dr Frank Hailer of the German Biodiversity and Climate Research Centre in Frankfurt, who led the international study, said the genetic information shed new light on conservation issues. "It fundamentally changes our understanding of polar bears and their conservation today," he told BBC News. "They have survived previous warm phases but they carry scars from these times - they must have been close to extinction at times." Nuclear DNAThe researchers looked at DNA from modern bears to study the history of the species, analysing genetic information from the cell nucleus of more than 40 brown, black and polar bears. Past work has relied mainly on mitochondrial DNA, the fragments of genetic material contained within tiny cell components called mitochondria. The latest findings suggest the polar bear evolved in the mid Pleistocene, about 600,000 years ago. This scenario paints a new picture of the bear's evolutionary history. The mammal would have had more time to colonise and adapt to life in the high Arctic, and lived through various cycles of warming and cooling. The polar bear's lack of genetic diversity suggests that changes in the environment, such as warm phases, led to dramatic falls in numbers at times. The researchers say polar bears face many other threats to their survival today, including habitat destruction, hunting and the effects of environmental pollutants. Writing in Science, they explain: "Although polar bears have persisted through previous warm phases, multiple human-mediated stressors (eg habitat conversion, persecution, and accumulation of toxic substances in the food chain) could magnify the impact of current climate change, posing a novel and likely profound threat to polar bear survival." Commenting on the research, Dr Steven Amstrup, chief scientist of Polar Bears International, said he suspected the age of the polar bear was not entirely settled yet. Even if they did split from their ancestor 600,000 years ago, they would have lived through only two periods noticeably warmer than today, he said, and spent most of their existence in cooler times. He added: "It's continuing evidence that polar bears are adapted to a cold environment and they haven't experienced anything of the warming that we are likely to experience in the next 100 years." www.bbc.co.uk/news/science-environment-17762196 Follow up... Intimate Relations: The nuclear genomes of bears (black outline) suggest that polar bears and brown bears diverged from one another 4 to 5 million years ago, and that occasional exchange of genes between the two species (shaded gray areas) followed. Results from maternally inherited mitochondrial DNA (dotted line) indicate extinction (marked with an "X") and replacement of polar bear mitochondrial DNA around 160,000 years ago due to interbreeding between the two species. (Credit: Penn State University).Bear demographic history during the Pleistocene. Text Extract:"We generated a first-rate set of data, including deep sequence coverage for the entire genomes of a polar bear, three brown bears and a black bear, plus lower coverage of 23 additional polar bears, including a 120,000-year-old individual; very few vertebrate species have such comprehensive genomic resources available," Schuster said. Using this vast amount of data, the scientists discovered that polar bears are actually an older species than previously thought -- indeed, far more ancient than suggested by a recent study that placed the species' age at 600,000 years old. That analysis looked only at small segments of DNA. "We showed, based on a consideration of the entire DNA sequence, that earlier inferences were entirely misleading," said study co-lead author Webb Miller, a Penn State professor of biology and computer science and engineering. "Rather than polar bears splitting from brown bears a few hundred thousand years ago, we estimate that the split occurred 4-5 million years ago." "This means polar bears definitely persisted through warming periods during Earth's history," UB's Lindqvist said. She cautions, however, that the species' endurance over several million years doesn't guarantee its future survival. www.sciencedaily.com/releases/2012/07/120723151028.htm Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. Proceedings of the National Academy of Sciences, 2012; DOI: 10.1073/pnas.1210506109 PDF LINK: www.pnas.org/content/109/36/E2382/suppl/DCSupplementalNot to go unchallenged, a compelling argument to the new 'genetic' position of 4-5 million year ancestry in polar bears is made. Will there be more? A Canadian zoologist who specializes in polar bear evolution named Susan J. Crockford makes the following key points: - Fossil evidence to counter how polar bears and brown bears (aka grizzlies) arose 4-5 million years ago.
- Detailed history of polar bear and brown bear hybridization.
- Svalbard would not have been an “interglacial refugium” for polar bears
- Polar bear population size over time in no way mirrors the dramatic changes in climate that have occurred over the last one million years.
Read the full online articles links below: Part 1Part 2Part 3P.S. Sorry for all of the quotes.
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Post by grrraaahhh on Mar 5, 2013 5:15:43 GMT -9
I moved your post to this thread.
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Post by grrraaahhh on Apr 1, 2013 12:32:58 GMT -9
Another update is presented. Thank you Ursus for the earlier heads-up: AbstractDespite extensive genetic analysis, the evolutionary relationship between polar bears (Ursus maritimus) and brown bears (U. arctos) remains unclear. The two most recent comprehensive reports indicate a recent divergence with little subsequent admixture or a much more ancient divergence followed by extensive admixture. At the center of this controversy are the Alaskan ABC Islands brown bears that show evidence of shared ancestry with polar bears. We present an analysis of genome-wide sequence data for seven polar bears, one ABC Islands brown bear, one mainland Alaskan brown bear, and a black bear (U. americanus), plus recently published datasets from other bears. Surprisingly, we find clear evidence for gene flow from polar bears into ABC Islands brown bears but no evidence of gene flow from brown bears into polar bears. Importantly, while polar bears contributed <1% of the autosomal genome of the ABC Islands brown bear, they contributed 6.5% of the X chromosome. The magnitude of sex-biased polar bear ancestry and the clear direction of gene flow suggest a model wherein the enigmatic ABC Island brown bears are the descendants of a polar bear population that was gradually converted into brown bears via male-dominated brown bear admixture. We present a model that reconciles heretofore conflicting genetic observations. We posit that the enigmatic ABC Islands brown bears derive from a population of polar bears likely stranded by the receding ice at the end of the last glacial period. Since then, male brown bear migration onto the island has gradually converted these bears into an admixed population whose phenotype and genotype are principally brown bear, except at mtDNA and X-linked loci. This process of genome erosion and conversion may be a common outcome when climate change or other forces cause a population to become isolated and then overrun by species with which it can hybridize. Cahill JA, Green RE, Fulton TL, Stiller M, Jay F, et al. Genomic Evidence for Island Population Conversion Resolves Conflicting Theories of Polar Bear Evolution. PLoS Genet, 9(3). PDF LINK: www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1003345
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Post by grrraaahhh on Dec 11, 2013 7:36:46 GMT -9
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