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Miller, W., Schuster, S.C., Welch, A.J., Ratan, A., Bedoya-Reina, O.C., Zhao, F., Kim, H.L., Burhans, R.C., Drautz, D.I., Wittekindt, N. E., Tomsho, L. P., Ibarra-Laclette, E., Herrera-Estrella, L., Peacock, E., Farley, S., Sage, G.K., Rode, K., Obbard, M., Montiel, R., Bachmann, L., Ingolfsson, O., Aars, J., Mailund, T., Wiig, O., Talbot, S.L. and Lindqvist, C. 2012. Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1210506109 in press.
There has been considerable research effort expended recently on determining when and where polar bears arose (e.g. Davison et al., 2011; Edwards et al., 2011; Lindqvist et al., 2010). For example, a paper published earlier this year (Hailer et al., 2012) examined 14 nuclear genes of 19 polar bears (Ursus maritimus), 18 brown bears (Ursus arctos) and 7 black bears (Ursus americanus) and found less genetic variation within polar bears than within brown bears or black bears. Hailer and colleagues found enough haplotypes unique to the polar bear to suggest that it is a genetically distinct lineage and calculated a median divergence date for polar bears and brown bears of approximately 603,000 years – much older than previously estimated from mitochondrial DNA (mtDNA) and fossil data. They found no evidence of recent or on-going hybridization between brown and polar bears. However, they did find a polar bear haplotype of one nuclear gene in brown bears from the ABC Islands of Southeast Alaska, suggesting that this might be evidence of an ancient hybridization event, while their mtDNA analysis found a signal of at least one or two major hybridization events in the Late Pleistocene. (more…)