Polar bears (Ursus maritimus) are the largest and most well-adapted mammals to a frigid arctic environment. They spend the majority of their life on or near arctic sea ice and are characterized by many common phenotypic markers necessary for survival in this harsh ecosystem.
Despite many behavioural studies of the polar bear, researchers know very little about their evolutionary history. Fossilized remains may never be found if their ancestors lived and died, as modern day polar bears do, around sea ice. Most fossils are likely at the bottom of the arctic ocean or randomly scattered beneath arctic ice shelves, forever hidden from excavators.
Therefore, in order to gain a better understanding of how and when polar bears dispersed into the arctic researchers at the German Biodiversity and Climate Research Center (BiK-F) decided to analyze segments of the polar bears nuclear DNA. In the past, only mitochondrial DNA (mtDNA) had been analyzed, which gives a incomplete and potentially misleading picture of the polar bears evolutionary history.
|Polar bears are the largest arctic mammals|
The results provide us with a much more complete and plausible history of polar bear speciation and adaptation. Past mtDNA research suggested that polar bears separated from brown bears (Ursus arctos) approximately 150 kya, however this was suspected to be an early estimate based on the amount of time it was assumed it would take a large mammal to adapt so well to an inhospitable and alien clime. Nuclear DNA evidence pushed the date of the speciation back to over 600 kya, which is a much more realistic date in theory, and a much more accurate date methodologically.
This means that polar bears are more genetically distinct than previously believed and this makes their specific and distinct adaptations (e.g., black skin, white fur, fur-covered feet) more easily explained theoretically. The past mtDNA results that hinted at a much more recent speciation event ~150 kya likely indicates that in the past climatic fluctuations caused a brief overlap between polar bear and brown bear populations resulting in gene flow between the populations. However, this overlap would have been brief, since polar bears and brown bears are genetically distinct species today.
Although scientists may never recover skeletal remains of the polar bears ancestors, future genetic research should help us get a better understanding of this unique mammals evolutionary history.
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Hailer, F. et al. 2012. Nuclear genomic sequences reveal that polar bears are an old and distinct bear lineage. Science 336, 344-347.
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