Do you remember where mammals have DNA? (hint- it isn’t just in the nucleus)
Mammal cells have DNA in both their cell nuclei and their mitochondria. While DNA in the nucleus is a combination of both parents, mitochondrial DNA is inherited directly from the mother. (For more information about nuclear DNA and mitochondrial DNA download: www.cbc.ca/fifth/2008-2009/the_girl_in…/dna-definitions.doc)
And what does this have to do with the age of Polar Bears?
Well, according to a recent article in the New York Times, scientists have been surprised to find that polar bears are not so closely related to brown bears as previously thought. For years, scientists thought that the polar bear specie evolved about 150,000 years ago. Adaptations, probably due to natural selection, include white fur and webbed paws – both of which are very helpful in the icy Arctic.
Researchers Axel Jenkle and Frank Haler, of the Biodiversity and Climate Research Center in Frankfurt studied 19 polar bears, 18 brown bears and 7 black bears. After analyzing the nuclear DNA of polar bears, they believe that brown bears and polar bears began taking different evolutionary paths as much as 600,000 years ago.
The old, incorrect, theory was based on mitochondrial DNA. The mitochondrial DNA of polar bears and brown bears are very similar. Because polar bears live on ice, and there aren’t many fossils saved in the icy arctic, it has been difficult to trace the evolution of these famous white bears.
Now scientists are trying to figure out why the mitochondrial DNA of brown and polar bears is so similar. One hypothesis is that polar bears mated with brown bears during time of global warming or climate changes. There is some evidence of the bottleneck effect, which helps support this theory.
Link to main article: http://www.nytimes.com/2012/04/20/science/polar-bears-did-not-descend-from-brown-bears-dna-study-indicates.html?_r=2&ref=science