If any of you have watched David Attenborough’s ‘Pole to Pole’ in the BBC’s Earth series, you’ll happily join the cause too.
Polar bears, Ursus maritimus, are one of eight members of the family Ursidae. The largest of their cousins, they inhabit the circumpolar north, which is unfortunately their demise
(Polar Bears International, 2012). Global warming has many ramifications; one being that it’s increasing the rate at which the Arctic sea ice melts, cutting short the regular hunt/feed/breed cycle of the Polar Bear. The question to their survival is, can they adapt?
“Norse poets described the polar bear as the seal's dread, the rider of icebergs, the whale's bane, and the sailor of the floe. They praised polar bears for having the strength of 12 men and the wit of 11”
(Polar Bears International, 2012).
© Daniel J. Cox/NaturalExposures.com
(Polar Bears International, 2012)
Recent studies of mitochondrial DNA (mtDNA) have dated Polar Bears to have had diverged from their closest cousin, the Brown Bear, Ursus arctos, in the late Pleistocene less than 111-166 thousand years ago
(Hailer, et al., 2012). This “potentially indicate[s] rapid speciation and adaption to arctic conditions” (Hailer, et al., 2012, p. 344) for such a ‘young’ species. However, a recent study conducted by Frank Hailer and colleagues of Germany’s Biodiversity and Climate Research Centre suggests otherwise (Hirschler Reuters, 2012). Through the analysis of multiple loci of the brown, black and polar bear nuclear genome, they conclude that Polar Bears evolved much earlier, in the middle Pleistocene about 600 thousand years ago (Hailer, et al., 2012).
Hailer and his colleagues argue that the maternal inheritance of mtDNA creates a biased representation of Polar Bear evolution, as it may be explained through “introgressive hybridisation”
(Hailer, et al., 2012, p. 346). This is certainly possible with recent sightings of Polar/Brown bear hybrids, suggesting it may have occurred multiple times throughout Polar Bears history*. When compared against Hailer’s time scale, geological evidence suggests that Polar Bears have gone through stages of increasing temperature before, bringing the two species within closer range to one another and thus having the opportunity to interbreed.
The most substantial evidence supporting their claim is the lack of shared alleles in the nuclear genome of the two species, which one would expect from recently diverged species. Rather, “out of 35 haplotypes in polar bears and 79 in brown bears, only 6 were shared”
(Hailer, et al., 2012, p. 346). This places Polar Bears in a distinct clade, rather than within the Brown Bear clade (Fig 1), indicting that they are a genetically differentiated species (Hailer, et al., 2012).
Figure 1: Comparison of Polar Bear and Brown Bear evolutionary lineage when ordered against Nuclear DNA and Mitochondrial DNA
(Hailer, et al., 2012, p. 345).
Speciation, in the case of the Polar Bear, can be inferred as the result of adaption to the Arctic environment, and from Hailer’s study, it seems this was a slower process than previously thought
(Hirschler Reuters, 2012). This has implications for the survival of the Polar Bear because it implies this species is highly specialised to its habitat. Rapid environmental change, as suggested by Polar Bears International, is the driving force behind the swift loss in population numbers. The Arctic is at the forefront of the impacts suffered from global warming. It’s causing the summer sea ice to melt before Polar Bears have to chance to hunt and breed. However, as previously mentioned, this new evidence calls attention to the possibility that these bears have endured through phases of increased temperatures before, and obviously survived. This holds hope that these creatures can push on, but as Hailer (in Hirschler Reuters, 2012) points out “the main difference this time is that humans are impacting polar bears as well… If they go extinct in this phase of warming, we're going to have to ask ourselves what our role in that process was".
Thanks for reading!
Sophie Smith 42365763
Hailer, F., Kutschera, E. V., Hallstorm, M. B., Denise, K., Fain, R. S., Leonard, A. J., et al. (2012). Nuclear Genomic Sequences Reveal that Polar Bears are an Old and Distinct Bear Lineage. Science , 336, 344-437.
Hirschler Reuters, B. (2012 йил 20-April). Poler Bears are no new kids on the block. (ABC Science) Retrieved 2012 йил 19-August from ABC Science: http://www.abc.net.au/science/articles/2012/04/20/3482016.htm
Polar Bears International. (2012). Polar Bears. Retrieved 2012 19-August from Polar Bears International: http://www.polarbearsinternational.org