Polar bears (Ursus maritimus) share a close ancestry with the brown bear (U. arctos), and are currently distributed as a metapopulation, viz., an assemblage of smaller populations with limited gene flow between them. These disjunct populations are found within an Arctic land matrix dominated by sea-ice and island habitats (Kurten 1964, Ferguson et al. 1998). Genomic analysis has revealed that polar bears represent a surprisingly ancient lineage, with divergence estimates for the common ancestor of polar and brown bears likely occurring 934,000–338,000 years ago (Hailer et al. 2012); occasional hybridization between the two lineages is evident in introgressed mitochondrial DNA (mtDNA) sequences, suggesting that male brown bears occasionally mixed with isolated polar bear populations during warm interglacial periods when sea-ice receded (FIG. 1, Cahill et al. 2013). Scientific evidence suggests that the circumpolar distribution of the modern polar bear is the result of a rapid range expansion that followed a population bottleneck, perhaps induced, as Lindqvist et al. (2010) suggest, by variability in habitat and climate during the Eemian interglacial (~130,000–114,000 years before present).
If the early divergence estimates of Hailer et al. (2012) are accurate, the ancient polar bear lineage that squeezed through the Eemian bottleneck necessarily survived multiple prior bottlenecks as well (during previous interglacial periods, FIG. 2). The modern polar bear has also thus far survived the Holocene interglacial (10,000 years ago until present day), during which time the extent of sea-ice has fluctuated with an amplitude that far exceeds the current trend of sea-ice loss documented in the late 20th and early 21st centuries (McKay et al. 2008). The polar bear has persisted during these periods because a few subpopulations in ice-covered refugia made it through.
Although the polar bear may be unlikely to persist through a period of no sea-ice (i.e., not even refugia), the metapopulation is likely to exhibit long-term resiliency to climate change as long as some sea-ice remains. Indeed, it has previously demonstrated this resiliency in more extreme conditions (FIG. 2). It is the metapopulation structure and genetic variability (yes, even those introgressed brown bear genes) that have allowed the species to persist through periods of extreme environmental change. Theoretically, genetic bottlenecks should also have made the species more adapted to climate fluctuations, as those surviving subpopulations carry well-adapted genes and behaviors that improve resiliency to future change. Of course, the metapopulation itself may become extinct if the environmental pressure is sufficiently great, just as so many species in the vast history of the Earth have perished.
To be sure, human-exacerbated climate change poses a threat to polar bear populations, and may end up extirpating some populations. But considering what they have previously been through, it seems that it would take a serious blow to destroy the entire metapopulation and render the species extinct. After taking a look at the last 2 million years of temperature changes (FIG. 2), it seems hard to imagine that the current interglacial is here to stay; might the current warming trend just be the “calm before the storm”, a brief period of rapid warming before the next glacial period begins? To me, this scenario seems more likely than one of perpetual warming.
Is our current fossil fuel binge enough to stop the unfathomable momentum of the Pleistocene glaciation cycle? If yes, the future of the polar bear might truly be uncertain. If no, I bet it will make it through just fine until the next glaciation….and judging by the duration of the most recent interglacials, we’ve still got a few thousand years until the next ice age begins. Unless of course, humanity keeps priming the pump.
Cahill, J. A., et al. 2013. Genomic evidence for island population conversion resolves conflicting theories of polar bear evolution. PLOS Genetics 9:3, 1–8. doi:10.1371/journal.pgen.1003345.
Ferguson, S. H., M. K. Taylor, E. W. Born, and F. Messier. 1998. Fractals, sea-ice landscape and spatial patterns of polar bears. Journal of Biogeography 25:1081–1092.
Hailer, F., et al. 2013. Nuclear genome sequences reveal that polar bears are an old and distinct lineage. Science 336, 344–347.
Kurten, B. 1964. The evolution of the polar bear, Ursus maritimus. Phipps. Acta Zool. Fennica, 108, 1-30.
Lindqvist et al. 2010. Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. PNAS 107:5053–5057.
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