Polar Bears May Have Survived 4-5 million Years of Climate Change
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.
The newest offering, from another research group, has taken a different approach. Reporting in a paper published on July 23, 2012, Miller et al. (2012) looked at the entire nuclear genome ("deep genome sequencing") of a single polar bear, two brown bears from the ABC islands, one typical brown bear (a grizzly from mainland Alaska) and one black bear (augmented by nuclear genomic data from additional individuals for some analyses). The authors then undertook a multi-component study that included computer simulations designed to explore the genetic history of the three bear species. The computer models they used were constrained by assumptions about how genes change, how long they take to change and how genes might recombine when animals mate. The results of the computer analyses allowed the researchers to make inferences about when the different species arose, how many breeding animals might have been in the various populations at any given time, and whether there had been gene flow between species. In other words, the models were designed to suggest how a distant ancestor might have evolved into an ancient polar bear and then into the polar bear we know today, genetically speaking.
The group also utilized nuclear data from the oldest-known polar bear fossil from Svalbard (mtDNA of this specimen described by Lindqvist et al., 2010), estimated to be 130,000-110,000 years old, in some of their analyses. However, they did not include ancient sequence data from younger fossil specimens of polar bear, ancient brown bears (Davison et al., 2011), or ancient cave bears (Bon et al., 2008). Miller et al. also compared mtDNA sequences from some individuals of the three species, as other studies had done, but in addition, they looked in detail at some specific nuclear genes. Finally, these researchers compared their results to climate data representing the last million years of the Pleistocene epoch.
As for results, among other things, the computer models suggested that polar bears may have suffered a "prolonged and dramatic decline" in population, which the authors propose track some climate events during the mid to late Pleistocene (the last 1 million years). Miller and colleagues also found, in one component of the nuclear genome analyses, what they interpret to be evidence of ancient hybridization between polar bears and ABC brown bears: 5.5% of one ABC brown bear genome and 9.4% of the other were more polar bear-like than mainland grizzly-like (only 1.5% of the mainland grizzly genome was polar bear-like). A similar analysis of a subset of the genome for a larger, geographically diverse sample of bears (58 polar bears, nine ABC bears and 51 non-ABC brown bears) found that up to 11% of the genome segments were polar bear-like in some ABC brown bear genomes. In addition, while as much as 20% of the genome was brown bear-like in two of the modern polar bears, as much as 25% of the genome was brown-bear-like in the ancient polar bear from Svalbard. The authors conclude (in their abstract): "We demonstrate that brown bears and PB [polar bears] have had sufficiently independent evolutionary histories over the last 4-5 million years to leave imprints in the PB nuclear genome that likely are associated with ecological adaptation to the Arctic environment." In addition, they contend (pg 7) that "we also show that gene flow between bear species, possibly as a result of shifts in distribution of formerly isolated populations, has occurred during their evolutionary history."
Given the dynamic nature of genetic investigations on this topic, it is unlikely this paper is the last word on polar bear evolution. But if the results hold up, Miller and colleagues suggest that rather extensive hybridization between polar bears and brown bears, in both directions, may have taken place over their history. Also, if these authors are correct that polar bears have existed as a distinct species for 4-5 millions years, it means they must have persisted through all of the more than 50 glacial cycles of the Pleistocene epoch, which lasted 2.5 million years (Gibbard et al., 2005). A number of the more recent cycles - during the last 1 million years - involved extreme swings in climate from quite warm, with much less summer sea ice in the Arctic than today, to extremely cold, with a mass of thick ice in the Arctic Basin and sea ice extending into the north Atlantic and north Pacific (Polyak et al., 2010). This suggests that the Arctic adaptations that make polar bears unique includes an ability to persist as a species through extreme swings in sea ice conditions.
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