The authors of a new paper out in PLoS Genetics (Cahill et al. 2013, entitled “Genomic Evidence for Island Population Conversion Resolves Conflicting Theories of Polar Bear Evolution”) propose to explain how and why the brown bears (aka grizzlies) of the ABC islands of southeast Alaska (Admiralty, Baranof, and Chicagof – see previous post here), got to be so genetically distinct from brown bears on the Alaska mainland and so surprisingly similar (genetically) to polar bears. The authors determined (using a model) that this genetic pattern could be explained by an ancient hybridization event resulting from female polar bears cavorting with male brown bears in SE Alaska.
I had some issues with the way the paper was promoted by some of the co-authors, which I dealt with separately here. More importantly, I found the scenario these geneticists offered to explain how hybridization might have occurred to be patently implausible. Geological and fossil evidence from SE Alaska largely refutes their scenario, although another explanation may be more tenable. It is not impossible, in my opinion, that hybridization occurred in SE Alaska during the last Ice Age, but if it did, it almost certainly did not happen the way Cahill and colleagues suggest.
Posted in Evolution, Hybridization
Tagged ABC bears, black bears, brown bears, Cahill, genetics, Haida Gwaii, hybridization, Jim Baichtal, last glacial maximum, polar bear evolution, Prince of Wales Island, ringed seals, SE Alaska, sea ice extent, Tim Heaton
Most people would not question the species status of the polar bear (Ursus maritimus). While it is true that polar bears – under certain circumstances – have successfully interbred with brown bears (aka ‘grizzlies,’ Ursus arctos)[see previous post on hybridization], there are many characteristics that distinguish each of these species as unique entities (see diagram below).
Now, new genetic evidence adds weight to the balance on this issue. In this post, I’ll discuss very briefly the implications of this new paper:
Cronin, M. A. and MacNeil, M. D. 2012. Genetic relationships of extant brown bears (Ursus arctos) and polar bears (Ursus maritimus). Journal of Heredity 103 (6): 873-881. doi:10.1093/jhered/ess090 http://jhered.oxfordjournals.org/content/103/6/873.abstract
These are some of the points regarding this new paper by Miller et al. (2012, in press), on genetic evidence for the origin of polar bears, that I think deserve a critical look. See part 1, for my comments on Claim #1 (the fossil evidence)(see Doug Hoffman summary of the paper here).
In this post, I’ll elaborate on Claim #2. I’ve added one more to the original three listed.
Claim #1 Polar bears and brown bears (aka grizzlies) arose 4-5 million years old. [I countered with the fossil evidence]
Claim #2 Hybridization in both directions occurred repeatedly throughout the evolutionary history of polar bears and brown bears.
Claim #3 Svalbard may have been an important refugium for polar bears during warm interglacial periods.
Claim #4 Polar bear population numbers (population size estimates) over the last one million years tracked changes in climate (warmer/colder periods).
Claim #2 Hybridization in both directions occurred repeatedly throughout the evolutionary history of polar bears and brown bears [a claim also made in two other recent papers on polar bear evolution (Edwards et al. 2011; Hailer et al. 2012)].
A new genetic study (Miller et al. 2012, in press) suggests that polar bears arose between 4-5 million years ago and thus survived the more than 50 glacial/interglacial cycles of the 2.5 million year Pleistocene epoch. I will explore some of the claims of that paper later, suggesting why we might want to take them with a grain of salt.
But first, a bit of background and my personal caving adventure in ABC brown bear country.
Update: see new ABC bear photos added, courtesy Jim Baichtal, US Forest Service, Alaska (“real” ABC bears, he says)