You’ve probably heard the argument: animal populations that have been through a major decline in numbers often have such low genetic diversity that they are extremely vulnerable to subsequent extinction.
In an interview in late March regarding a new genetic paper on polar bear evolution (by Matt Cronin and colleagues), polar bear biologist and Polar Bears International spokesperson Steve Amstrup made a ridiculous statement: that polar bears have never experienced a rate of warming like they’ve seen over the last 30 years. I countered that easily here.
In that same interview about the Cronin et al. paper, fellow geneticist Charlotte Lindqvist offered an outdated argument against future polar bear survival that is as easy to refute as Amstrup’s “unprecedented rate of warming” nonsense.
I didn’t have time to deal with it back in April [where has the time gone?] but want to get back to it now because it’s important: there is lots of evidence to support my contention that polar bears are not more vulnerable to extinction just because they have low genetic diversity.
Here are Lindquist’s comments quoted by the Anchorage Daily News (published March 31, 2014):
“Another researcher, Charlotte Lindqvist, an evolutionary biologist at the University of Buffalo in New York, echoed that [Amstrup’s] view. Lindqvist co-wrote a 2012 paper that analyzed mitochondrial DNA and pegged the divergence between brown bears and polar bears at 4.5 million years.1
Lindqvist observed that modern polar bears have much lower genetic diversity than in the past, which suggests a genetic bottleneck may have been at work. That could mean that today’s polar bears have much less of a buffer to withstand environmental changes, she said.” [my bold, my footnote]
If by “could” she means a very, very low probability of this outcome happening, she might be correct.
However, in this case, it appears that Lindqvist has used “could” to imply she considers this outcome likely to occur, but not absolutely certain. Unfortunately, most conservation biologists (which includes most polar bear specialists) still accept this concept of low genetic diversity as a sort of unexploded, potentially lethal bomb just waiting to exterminate populations, which Lehman (1998) has called “the central dogma of conservation genetics.”
And even though follow-up studies on populations that have been the most severely reduced in size and genetic diversity have shown this is rarely (if ever) true, it doesn’t stop some scientists from trotting out the idea whenever it’s convenient.
Many populations that were reduced to very low numbers (i.e., gone through a ‘bottleneck’ ), ending up with low genetic variation, have subsequently recovered dramatically without adverse affects.
In other words, they not only recouped their population size after a population bottleneck but did so while dealing with subsequent environmental fluctuations and other natural threats to their survival (Lehman 1998:R723-724).
In some cases, genetic diversity increased after a population bottleneck, via mechanisms biologists are only just beginning to understand.
Here are some examples (with references, most of which are open access, and links to online information):
Northern elephant seals (Mirounga angustirostrus)
Rus Hoelzel and colleagues (Hoelzel et al. 1993) demonstrated more than 20 years ago that even though Northern elephant seals experienced an extremely severe population bottleneck in the 19th century (with numbers estimated to have fallen to 20-30 seals), their numbers had rebounded spectacularly despite extremely low genetic diversity. By the time of the last census (2005), there were ~170,000 seal total (with about 124,000 of those residing in the USA). There have been no apparent negative repercussions from that low diversity and the conservation status of this species is listed by the IUCN as ‘Least concern.’
Guadalupe fur seal (Arctocephalus townsendi)
A similar situation to the northern elephant seal has been described for the Guadalupe fur seal, which lives in the same region as the Northern elephant seal (Weber et al. 2000; 2004). A severe population bottleneck (due to over-hunting) that caused extremely low genetic diversity was followed by a dramatic increase in population without any noticeable negative effects.
San Nicolas Island foxes (Urocyon littoralis dickeyi)
Work by Andres Aguilar and colleagues (2004) showed that while this population of foxes, which lives on an island off the southern California coast, is “genetically the most monomorphic [similar] sexually reproducing animal yet reported,” due to having survived a population decline to less than 10 individuals, it has not suffered any reduced fitness.
Mouflon sheep (Ovis orientalis aka, Ovis aries) Sub-Antarctic Kerguelen Archipelago
This extremely isolated population of domestic sheep ancestor (the mouflon) on Haute Island in the southern Indian Ocean descends from just two individuals left there in 1957. Genetic diversity was found to have increased over time (Kaeffer et al. 2007) in a manner unexplained by current genetic theories.
North Atlantic right whale (Eubalaena glacialis)
Similar to the mouflon sheep example, North Atlantic right whales have also increased their genetic diversity since a population decline reduced their numbers severely. From the study by Tim Frasier and colleagues (Frasier et al. 2013):
“Combining the study of Bensch et al. (2006) and the results presented here shows that there is increasing evidence that the genetics of small populations is more complex than previously thought. Specifically, there are a number of widely documented natural mechanisms – such as mate choice for genetically dissimilar mates, or fertilization bias toward dissimilar gametes – that can have an increasing impact on overall patterns of genetic diversity as population size declines, that can serve to minimize the loss of genetic diversity, and the associated negative consequences, through time.” [my bold]
[the Bensch et al. 2006 study referred to above was on Scandinavian wolves, founded by three individuals]
What does this mean for polar bears?
Populations of polar bears almost certainly declined and recovered many times in the past in response to changing sea ice conditions.
LGM: Lindqvist et al. (2010:5054) suggested a rather severe bottleneck for polar bears during the height of the last Ice Age (~19,000-30,000 years ago) when extremely thick glacial ice would have pushed polar bears and their prey out to the edges of the pack ice in the North Atlantic and North Pacific.
Eemian: In addition, Miller and colleagues (2012) put another, much less severe decline in polar bear numbers prior to the beginning of the last Interglacial (the Eemian, ~115,000-130,000 years ago), when there was virtually no late summer ice in the Arctic and reduced winter ice (see previous post here).
These two events likely caused most of the low genetic diversity seen in modern polar bears.
Subsequent to those events, there were also marked declines in population size in many regions due to the wanton slaughter of bears by whalers during the late 1800s and early 1900s, as well as declines after the Second World War that brought the vulnerability of polar bears to hunting stress to the attention of the world.
Despite their low genetic diversity, polar bears numbers rebounded in a rather spectacular fashion — and show no apparent ill-effects with respect to their overall health.
The fact that living polar bear populations are currently healthy and widely distributed after recovering from both historically recent over-hunting and geologically recent climatic events is evidence that the species is very likely to be resilient to any future changes in climate despite their low genetic variability.
Abbitt and Scott (2001:1282) addressed this issue of adaptability, and put it this way:
“Natural threats alter our understanding of what comprises a “threat.” Species threatened by limited range, natural predators, or natural competitors have evolved with these “threats.” [my bold]
The polar bear has no natural predators, no real competitors, and does not have a limited range of habitat — except when it is limited by changing climate. As a consequence, climate change is virtually the only natural threat to which the polar bear has had to adapt over evolutionary time.
The extremes that polar bears have endured are almost unimaginable: from the deepest Glacial conditions that drove them out of the central Arctic altogether through several Interglacials warmer than today (with months-long periods of open water in summer and less ice in winter). Extreme changes in sea ice conditions have been an inseparable part of the polar bears’ past.
To suggest that polar bears cannot endure a bit of Arctic warming in the future (whether natural or due to human influences on climate, or a bit of both) is absurd: climatic extremes have defined the evolutionary history of polar bears, which means that climatic extremes have fine-tuned their biological adaptability.
Footnote 1. Note that I previously addressed Lindqvist and colleagues’ 2012 genetics paper, with references: Is it plausible that Polar bears are 4-5 million years old? Part 1 They came to a conclusion based on genetic results without consideration of the fossil record, which others continue to do.
Abbitt, R.J.F., and Scott, J.M. 2001. Examining differences between recovered and declining endangered species. Conservation Biology 15:1274-1284. http://onlinelibrary.wiley.com/doi/10.1111/j.1523-1739.2001.00430.x/abstract
Aguilar, A., Roemer, G., Debenham, S., Binns, M., Garcelon, D., and Wayne, R.K.. 2004. High MHC diversity maintained by balancing selection in an otherwise genetically monomorphic mammal. Proceedings of the National Academy of Sciences USA 101:3490-3494. Open Access http://www.pnas.org/content/101/10/3490.full
Bensch S, Andrén H, Hansson B, Pedersen HC, Sand H, Sejberg D, et al. 2006. Selection for heterozygosity gives hope to a wild population of inbred wolves. PLoS One.1:e72. Open access http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1762340/
Frasier, T.R., Gillett, R.M., Hamilton, P.K., Brown, M.W., Kraus, S.D. and White, B.N. 2013. Postcopulatory selection for dissimilar gametes maintains heterozygosity in the endangered North Atlantic right whale. Ecology and Evolution 3(10):3483-3494. Open access http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3797493/
Although small populations are expected to lose genetic diversity through genetic drift and inbreeding, a number of mechanisms exist that could minimize this genetic decline. Examples include mate choice for unrelated mates and fertilization patterns biased toward genetically dissimilar gametes. Both processes have been widely documented, but the long-term implications have received little attention. Here, we combined over 25 years of field data with high-resolution genetic data to assess the long-term impacts of biased fertilization patterns in the endangered North Atlantic right whale. Offspring have higher levels of microsatellite heterozygosity than expected from this gene pool (effect size = 0.326, P < 0.011). This pattern is not due to precopulatory mate choice for genetically dissimilar mates (P < 0.600), but instead results from postcopulatory selection for gametes that are genetically dissimilar (effect size = 0.37, P < 0.003). The long-term implication is that heterozygosity has slowly increased in calves born throughout the study period, as opposed to the slight decline that was expected. Therefore, this mechanism represents a natural means through which small populations can mitigate the loss of genetic diversity over time.
Hoelzel, A. R., Halley, J., O’Brien, S. J., Campagna, C., Arnbom, T., Le Boeuf, B., Ralls, K., and Dover, G. A. 1993. Elephant seal genetic variation and the use of simulation models to investigate historical population bottlenecks. Journal of Heredity 84:443-449. http://jhered.oxfordjournals.org/content/84/6/443.abstract
Kaeuffer, R., Coltman, D.W., Chapuis, J.-L., Pontier, D., and Réale, D. 2007. Unexpected heterozygosity in an island mouflon population founded by a single pair of individuals. Proceedings of the Royal Society B 274:527-533. Open access http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1766376/
Lehman N, 1998. Genes are not enough. Current Biology 8:1078-1081 [R722-R724]. Open access http://www.sciencedirect.com/science/article/pii/S0960982298704580
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. Open access http://www.pnas.org/content/107/11/5053.abstract
Weber, D. S., Stewart, B. S., Garza, J. C., and Lehman, N. 2000. An empirical genetic assessment of the severity of the northern elephant seal population bottleneck. Current Biology 10:1287-1290. Open access http://www.sciencedirect.com/science/article/pii/S0960982200007594
Weber, D. S., Stewart, B. S., and Lehman, N. 2004. Genetic consequences of a severe population bottleneck in the Guadalupe fur seal (Arctocephalus townsendi). Journal of Heredity 95:144-153. Open access http://jhered.oxfordjournals.org/content/95/2/144.full