Genetic Study Reveals the Polar Bear is Old Enough to Have Survived Several Glacial Cycles

The polar bear is an ice-dependent species that is well-adapted to the Arctic, suggesting that the number of glacial cycles in its evolutionary past may be pertinent to its future survival. Extremely cold glacial and warm interglacial periods cycled regularly throughout the 2.7 million years of the Pleistocene (Gibbard et al. 2007): first one, then the other, with associated changes in sea ice conditions. Some of these periods were warmer than today. For example, we are presently in an interglacial period (Marine Isotope Stage 1, MIS 1, also known as the Holocene), that began about 11,500 years ago with a three thousand year period (the Holocene Thermal Maximum) that was warmer than today (Kaufman et al. 2004). In the Middle Pleistocene, one glacial period lasted from about 160,000 to 140,000 years ago (MIS 6), while the following interglacial (which was warmer than today) lasted from 130,000 to 115,000 years ago (MIS 5e, sometimes called the Eemian or Sangamonian).

Writing as background for their paper, Hailer et al. (2012) state that "recent studies have shown that the polar bear matriline (mitochondrial DNA) evolved from a brown bear lineage since the late Pleistocene, potentially indicating rapid speciation and adaption to arctic conditions," but would analysis of nuclear genes (the kind found on chromosomes) yield the same answer? Two previous estimates for the timing of this brown bear/polar bear split, based on mitochondrial DNA (mtDNA), suggest a relatively recent speciation event, approximately 134,000 and 160,000 years ago, respectively (Lindqvist et al. 2010 and Davison et al., 2011). Both of these estimates seem consistent with the date of the oldest polar bear fossil (a mandible found in Svalbard, Norway), about 110,000-130,000 years old.

In exploring this issue further, Hailer and colleagues sampled nuclear DNA from 19 polar bears, 18 brown bears (also called grizzlies) and 7 black bears representing the main mitochondrial lineages (clades) of living bears. They also sequenced 14 different nuclear genes (9116 nucleotides total) and constructed a species tree from the data generated. Then, they sequenced and analyzed a short segment of mtDNA (640 base pairs, from the control region). So what did they learn?

Hailer et al. found less genetic variation within polar bears than within brown bears or black bears and also that "numerous nuclear haplotypes were unique to polar bears." These results suggest that the polar bear is a genetically distinct lineage. The calculated median divergence date for polar bears and brown bears was approximately 603,000 years, based on a range of 334,000-934,000 years, suggesting that polar bears are older than previously estimated from mtDNA and fossil data.

Given such findings, the authors conclude that "an evolutionary origin several hundred thousand years ago implies that polar bears as a species have experienced multiple glacial cycles and have had considerable time to adapt to arctic conditions." The study indicates that polar bears have not only survived several very cold glacial and warm interglacial periods (with the associated changes in sea ice conditions) but also the transitions between them. In other words, the results of the Hailer et al. study indicate that the polar bear has survived many changes in its sea ice habitat over its evolutionary history, which suggests it may be more resilient to future changes than has been predicted (Derocher et al. 2004).

Additional References
Davison, J., Ho, S.Y.W., Bray, S.C., Korsten, M., Tammeleht, E., Hindrikson, M., Østbye, K., Østbye, E., Lauritzen, S-E., Austin, J., Cooper, A., and Saarma, U. 2011. Late-Quaternary biogeographic scenarios for the brown bear (Ursus arctos), a wild mammal model species. Quaternary Science Reviews 30: 418-430.

Derocher, A.E., Lunn, N.J. and Stirling, I. 2004. Polar bears in a warming climate. Integrative and Comparative Biology 44: 163-176.

Edwards, C.J., Suchard, M.A., Lemey, P., Welch, J.J., Barnes, I., Fulton, T.L., Barnett, R., O'Connell, T.C., Coxon, P., Monoghan, N., Valdiosera, C.E., Lorenzen, E.D., Willerslev, E., Baryshnikov, G.F., Rambaut, A., Thomas, M.G., Bradley, D.G. and Shapiro, B. 2011. Ancient hybridization and an Irish origin for the modern polar bear matriline. Current Biology 21: 1251-1258.

Gibbard, P. L., Boreham, S., Cohen, K. M. and Moscariello, A. 2005. Global chronostratigraphical correlation table for the last 2.7 million years, modified/updated 2007. Boreas 34(1) unpaginated and University of Cambridge, Cambridge Quaternary http://www.qpg.geog.cam.ac.uk/

Kaufman, D.S., Ager, T.A., Anderson, N.J., Anderson, P.M., Andrews, J.T., Bartlein, P.J., Brubaker, L.B., Coats, L.L., Cwynar, L.C., Duvall, M.L., Dyke, A.S., Edwards, M.E, Eisner, W.R., Gajewski, K., Geirsdóttir, A., Hu, F.S., Jennings, A.E., Kaplan, M.R., Kerwin, M.W., Lozhkin, A.V., MacDonald, G.M., Miller, G.H., Mock, C.J., Oswald, W.W., Otto-Bliesner, B.L., Porinchu, D.F., Rühland, K., Smol, J.P., Steig, E.J. and Wolfe, B.B. 2004. Holocene thermal maximum in the western Arctic (0-180°W). Quaternary Science Reviews 23: 529-560

Lindqvist, C., Schuster, S.C., Sun, Y., Talbot, S.L., Qi, J., Ratan, A., Tomsho, L., Kasson, L., Zeyl, E., Aars, J., Miller, W., Ingólfsson, Ó., Bachmann, L. and Wiig, Ø. 2010. Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. Proceedings of the National Academy of Sciences USA 107: 5053-5057.