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I am enjoying following this thread and wonder if anyone has followed the story regarding the cross-breeding between Alaskan Brown Bears and Polar Bears? There have been several killed by hunters which means there are probably many more in the wild. What I am wondering is if the cross- breed bears are sterile, or if they can pass on their genes to another generation? Also, I am wondering if this would be considered "Evolution, Adaptation or Something Else"? I look forward to hearing opinions and statistics regarding this.
The hybrids are not sterile, and an increasing frequency in their occurrence can indicate that the boundaries between the two species are blurring, probably as a result of the arctic warming. Female polar bears must range further inland searching for food, more often encountering brown bears. When a female is in-season, she attracts whatever males are in the area. And if she's increasingly "in the area" of brown bears, her chances of being bred by one will also increase. The hybrid offspring learn their "way of life" by following their mother, and so these hybrids will try to be polar bears. As ice flows melt earlier and form later, and polar bears feed inland for longer periods of time, the more extreme "pure-polar" phenotype may become less of an advantage for living in this changing area than before, and the hybrid offspring can "get by" being only half polar bear. They'll likely breed back into the polar bear population (being raised by a polar bear mother), and over time, brown bear genes will flow back into the polar bear population. This may result in polar bears being different in a few generations than they are now, but if that phenotype is more advantageous in a newly warming arctic, then those "mostly-polar" bears will be selected over "pure-polar" bears. And what you'll have is evolution -- a change in the inherited phenotype frequencies within a population over time.
I'm not aware if the reverse hybridization happens much in the wild -- male polar X female brown. I did read some articles and watch a documentary on the one that was shot a few years back, and genetic analysis showed that its mother was a polar bear. What I gathered from that (and remember in the time since I read them) was that the reverse is rarer or still unknown, but I suppose that they'd likewise follow a "brown-bear" lifestyle. Being as the white coat color seems to be dominant, however, leads me to think that these bears wouldn't do as well among their brown neighbors, and fewer would survive to reproduce.
ETA -- I checked to see what wikipedia had to say about this. See link below.
http://en.wikipedia.org/wiki/Grizzly–polar_bear_hybrid
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