[Taxacom] Recent research on polar bear origins

[Kenneth Kinman]

Dear All,
      I've been catching up on research on the origins of the polar bear
and hybridization with its mother species (the brown bear).  First we
had the 2010 paper by Lindqvist et al. (in Proc. Natl. Acad. Sci.) on
the mitochondrial sequence of the oldest known fossil polar bear (from
an island near Norway) which was about 120,000 years old.  The
conclusion was that this fossil was "relatively" close to when polar
bears split off from brown bears (perhaps 150,000 years ago).                           
      Then earlier this month, an article was published in Current
Biology indicating that the more restricted population of "living" polar
bears had a common ancestor (in the matrilinear line)  perhaps 45,000
years ago (long after the origin of the species as a whole), and that
their "mitochondrial Eve" was not a polar bear, but rather a brown bear
in or near the British Isles (extinct Irish brown bears in particular
were singled out, although I suppose this could change with even more
sampling).        
      The big surprise from both of these studies is that the origin of
polar bears as a total species, as well as the common ancestor of living
polar bears, were both in the area of northwestern Europe, NOT the "ABC"
islands of southeastern Alaska.     
      Anyway, it seems to me that somewhere along the line there were
one or more severe population bottlenecks of polar bears, the most
severe probably being shortly after the time of their "mitochondrial
Eve" around 45,000 years ago.  But the even bigger question is perhaps
whether there was a "Y-chromosomal Adam" of polar bears before or after
that time (which might have been close to yet another population
bottleneck), and whether that "Adam" was a brown bear or a polar bear.      
       Therefore, the ultimate question in my mind is how such
bottlenecks might have contributed to the continued maintenance of polar
bears as a separate species (morphologically in a number of different
ways) even though they interbred when climatic conditions forced them
back together geographically with brown bears.  Was it (1) a very small
population in a geographically small area that was just lucky to be
located where climate in particular was still favorable during the
bottleneck, or (2) was the bottleneck more spread out geographically
(and factors other than simple geography at work)?  
       This case seems more problematic than the human bottleneck(s)
which produced the living human population, where the "mitochondrial
Eve" and "Y-chromosomal Adam" were both in Africa (and although
different in predicted age, perhaps not all that far apart).  And the
origin of the human species (as a whole) is also clearly much older
(compared to the origin of the living populations), and it is not
paraphyletically derived from another living species.  So although human
origins and polar bear origins are problematic, it is probably for
somewhat different reasons.      
          ---------Ken Kinman