Polar bears - older past, same future?
Posted April 25, 2012
A new genetic study suggesting that polar bears have been around longer than previously thought has raised questions regarding the future of the species.
Although there have been many estimates as to how long polar bears have existed – varying from 150,000 to 1.7 million years – the most recent school of thought held that polar bears were relatively “young” evolutionarily and were, in fact, still very closely related to brown bears despite their differences in appearance and specialized ecological traits. This hypothesis suggested that polar bears had undergone unusually rapid evolution in their arctic habitat over a relatively short period of time (150K years). The new study, however, suggests that polar bears are indeed distinct from brown bears and have evolved over the last 600K years – which is more in line with other estimates of mammalian speciation and would have allowed a longer time period for the bears to adapt to arctic conditions.
Part of the uncertainty over the duration of the polar bear’s existence is the lack of a good fossil record, which can help date organisms more precisely. The oldest known polar bear fossil dates back to approximately 110 - 130K years ago, which informed earlier hypotheses of the polar bear’s young evolutionary history. Because polar bears are found on ice that shifts, floats, melts, and freezes again, it makes sense that there aren’t many discoverable fossils.
The other reason for uncertainty regarding the duration of the polar bear’s existence is that each estimate has been based on different lines of evidence. Indeed, an earlier study, which estimated a date of 150K years, was based on mitochondrial DNA, whereas the recent study, which estimated a date of 600K years, was based on nuclear DNA. These two types of DNA are different in a number of ways, including that they are inherited differently – mitochondrial DNA is inherited only from mothers, while nuclear DNA comes from both parents. Sometimes these two types of DNA tell a similar story about a critter’s history and sometimes they tell very different stories. In the case of the polar bear, they are telling very different stories.
Why so different? Interestingly, both types of DNA indicate that polar bears have limited genetic diversity, indicating that, in the past, the species has been reduced to small numbers of individuals (a phenomenon referred to as a “bottleneck”). But the fact that the mitochondrialDNA shows a close relationship with brown bears suggests that polar bears have hybridized with brown bears in the past. Specifically, it’s likely that male polar bears hybridized with female brown bears – a situation expert Linda Gormezano says could very well repeat itself in Hudson Bay as warming conditions favor the survival of adult male polar bears while at the same time driving more brown bears into this area. In fact, a few hybrids have recently been reported in the wild. Based on cumulative lines of evidence, including overlaying the timing of the genetic estimates with temperature curves, the authors of the new genetic study hypothesize that historic hybridization with brown bears may have coincided with a past warming event and population bottleneck, indicating that hybridization may have helped polar bears withstand interglacial warm phases and counteract the loss of genetic diversity from small population sizes.
All of this brings us to the question of whether this new interpretation of the polar bears’ evolutionary history changes the predictions for their future fate in a warming climate. If polar bears are older than we thought, that means they have survived more interglacial warming cycles in the past than had been accounted for. If they are only 150K years old they would have survived one warming period that was warmer than today’s current climate, and if they are 600K years old they would have survived two such events according to Steven Armstrup, scientist emeritus with USGS. Does this tell us anything different about their future?
Steven Armstrup says, “The short answer is that it really does not change the view we have long held…. (W)hether polar bears survived one or two warming periods of the past, they never have faced the kind of warming we will see in the next century.” Further emphasizing his point he says, “(I)f we continue on our current greenhouse-warming path, it will be far warmer in 50 years than at anytime in the existence of polar bears. That is, by the latter half of this century, polar bears will be off the charts of anything they have experienced during their evolutionary history.”
Similarly, Andrew Derocher, polar bear biologist with the University of Alberta, summarizes his view of the new findings, “I don’t think there is much in this that changes my view of what polar bears have done and what the future holds for the species. Polar bears haven’t seen what the ice models are predicting if we don’t deal with the warming patterns and sea ice loss.”
In terms of polar bears being able to adapt to these predicted changes in such a short period of time, Brendan Kelly, an evolutionary biologist at the University of Alaska, has explained, “(T)he kind of adaptation that’s necessary is a change toward genes that fit the new climatic environment better than the old genes. Individuals don’t adapt genetically. Populations do. That requires generations, which requires time. Bears, seals, whales -- these are long-lived animals. They need decades and centuries to adapt. But we’re talking about losing the Arctic summer sea ice in a matter of a few decades.”
As for the new time estimate of 600K years for the existence of polar bears, the scientists also agreed – the jury is likely still out. Due to the changing nature of genetic techniques and data availability, Steven Armstrup explains, “I believe it may be some time before geneticists settle on a time of separation on which they all can agree.”
Overall, the bottom line of this new study seems clear: the evolutionary history of polar bears is likely older and more complicated than some previously thought, but their future is still as endangered as ever.
Photo credit: USGS