National Geographic has a short but informative article on an unusual animal: the Kermode or “spirit” bear. This is a subspecies of the regular black bear (Ursus americanus kermodei) that occurs on the coast of British Columbia. The subspecies has about 400 individuals, and about 10% of these are homozygous for a mutation that turns their coats whitish. (Populations that contain two or more genetic variants like this are called “polymorphic,” meaning “many forms” in Greek.) Here are the two morphs:
These bears are not albinos, for their eyes and noses are pigmented. Nor are they hybrids with polar bears, although polar bears have been known to hybridize with a different species, the grizzly bear. The mutation that turns normally black bears white occurs at the MC1R locus (short for “melanocortin receptor”), a gene involved in the synthesis of the pigment eumelanin. A mutation at the same gene is what gives humans red hair. The white color gene is recessive; that is, it takes two copies of that gene to make the bear white. A bear carrying one copy of a black and one copy of a white gene is black.
Curiously, a similar MC1R polymorphism has been found in woolly mammoths using DNA sequences extracted from their bones. This has led to speculation that mammoths were polymorphic like these bears, with both light and dark-colored individuals.
What’s the evolutionary significance of this polymorphism (if any)? It’s been known for a while that white bears are better at catching fish than black ones:
Researchers have recently proved that the spirit bear’s white coat gives it an advantage when fishing. Although white and black bears tend to have the same success rate after dark—when bears do a lot of their fishing—scientists Reimchen and Dan Klinka from the University of Victoria noticed a difference during the daytime. White bears catch salmon in one-third of their attempts. Black individuals are successful only one-quarter of the time. “The salmon are less concerned about a white object as seen from below the surface,” Reimchen speculates. That may answer part of the question about why the white-fur trait continues to flourish today. If salmon are a coastal bear’s primary fat and protein source, a successful female can feast on salmon to store more fat for winter, potentially increasing the number of cubs she can produce.
Carriers that have two copies of the white gene have an additional advantage: according to the Geographic article, the local people, the Tsimshian, consider the white morph sacred and do not hunt white bears.
So, if the color gives an advantage at fishing and protection from being hunted, why aren’t all the bears in the area white? Even if black bears have an unknown countervailing advantage (like camouflage in the forest), that wouldn’t necessarily keep both color variants in the population: you’d expect the color conferring the highest net fitness to sweep through the population.
If the polymorphism is maintained by natural selection rather than being “neutral” (i.e. considering all factors, black and white bears have equal lifetime reproductive fitness), then it would have to be a special kind of natural selection, involving either one color being better in one area of the habitat and the other color in a different area (unlikely, since both forms live in the same place), or perhaps a reproductive advantage of the heterozygote: the black-colored bear that carries one copy of the white gene).
Paul Nicklen’s other photos of the spirit bear are here.
Mother and cub. What you can say with certainty here is that mother carries one copy of the white gene, and one copy of the black:
h/t: Matthew Cobb
The mammoths’s range included both snow-covered tundra and forests, perhaps on a seasonal cycle, right? If so, it’d make sense that some would fare better with a white coat agains the snow and others with a dark coat against the trees.
I’d think one would also have to consider the social structure of a species with regards to any sort of selection mechanism. If the dark and light forms are likely to help each other, the advantage of the mutation gets spread throughout the population, including those individuals who lack that particular gene.
Cheers,
b&
Not so sure Ben – why would a mammoth need to hide? I am not saying that they could not have had different colour forms (I think the red of surviving hair is not a ‘true’ colour).
Well, their primary predator (us) us a highly visual predator. One can imagine it being harder to spot a small herd on the other side of the valley with better camouflage. Of course, once we started using traps, it was game over….
b&
“…..(unlikely, since both forms live in the same place),….”
This appears true now. Natural influences (weather, over-population of predators, river diversions, etc.) may have occurred over thousands of years that forced the two populations to exist in closer proximity to each other.
How interesting, any of your suggestions for the persistence of the gene seem plausible.
There may be benefits/risks that we don’t know about. We know, for example, that Redheads are more resistant to some diseases (rickets and tuberculosis for example), but are more susceptible to others (skin cancer for example)**. It’s possible that something similar occurs with the bears. Even in the same habitat it may be that there are situational advantages which sometimes favor one color over the other (sticking with the example above, disease cycles spring to mind), leading to persistence of both.
**Incidentally they also require more anesthesia than most other people
Thanks for the comfort of knowing I won’t get rickets – though I am a big milk drinker so I might have avoided that anyway! Skin cancer – I hate the summer! And it is only when you are in pain that you really know you are alive!
😉
Haven’t read the article yet, but considering that most black bears are black across North America, presumably the fishing advantage of being white is not enough to overcome whatever the advantages of being black are, in most places most of the time.
The human hunting preference could explain it, maybe, but I don’t know if there is all that much human hunting of bears.
I think a key question is how much introgression this population has with others. If there is some, then white alleles could have a local advantage, but nevertheless be swamped by gene flow from the outside. And bears are actually pretty mobile I think.
On the other hand, if the population is small and well-isolated, then nothing more than founder effects need be invoked to explain a high frequency of some otherwise rare allele.
Maybe an example of balanced polymorphism? Has anyone compared gene frequency distribution to Hardy-Weinberg? I recall a paper on the famous white squirrels of Olney, Illinois, which found that gene frequencies for the trait were not significantly different from Hardy-Weinberg.
Humans have similar variations with hair color, eye color etc. Probably natural selection is not a significant factor here.
If blondes took home 33% more pay than brunettes for the same amount of work, you’d probably consider that a significant advantage.
actually if you take into account that (in most cases) lighter hair colors also mean that the body produces less melanin, in higher latitudes where sun light is less intense, light hair translates to more vitamin D which can have a positive effect on survival.
Which could be why in areas of northern Europe blonde hair was normal (greater than 50%) and in some cases the vast majority (>~80% in some areas).
But sexual selection could 😉
I am with Jay here.
An additional possible factor is environmental variations year to year that affect relative fitness. E.g., in a lean salmon year followed by a particularly harsh winter, white coats fare slightly better; whereas normally black coats have the advantage. (Similar to the long-beak / short-beak fluxuations in the Darwin finches.)
That is my thought. We tend to assume a constant environment, but of course it can change over time and gene frequencies can reflect that. I was thinking the exact same thing about reduced salmon runs temporarily increasing the fitness of the white morph…
Should the “spirit bear” even be considered a sub-species, considering it’s distinguishing characteristic is defined by a single loci? Sounds sketchy. I suspect there must be more to it than that…
The white form of the bear is produced by a change at one locus, but that doesn’t mean that the whole Kermode population (including the 90% of them that are black) doesn’t have other significant differences from the other types of black bear.
Ah, ok. So it is the “Kermode” population that is considered a sub-species, not the white variant, specifically. Got it.
Even if black bears have an unknown countervailing advantage (like camouflage in the forest), that wouldn’t necessarily keep both color variants in the population: you’d expect the color conferring the highest net fitness to sweep through the population.
A bear that doesn’t look like a bear – at least to a fish – will have a fishing advantage. So I’d expect that the fishing advantage of having an abnormal coat color would increase as the number of bears that have it grow smaller.
What this implies is that when a single coat color sweeps through the population, the ‘equilibrium point'(the point at which neither coat color has a net advantage) is not at “all black” but at “most of the population is black.”
At least, that’s the way it seems to me.
Jerry, amazing article.
Can’t believe they are still able to hunt them though. I boggles my mind that someone would think of shooting an animal as a good time.
to be fair the Tsimshian are a local indigenous people and the hunting is for food not for “a good time”
No one here has mentioned sexual selection yet. Do blonde bears have more fun ?
Also is the Griz less likely to push the dark bear off the Griz bears range ?
Perhaps there are other allied differences to which we are blind such as odour (pheromones), parasite control & ‘temperament’
Eric (#9), that’s a good point. Also important is the amount of gene flow between this population and interior populations. The vast majority of black bears don’t fish for salmon, so would not be under any selection pressure to be white, and under some pressure to be black. (I think that the other, less geographically based fitness advantages suggested for white bears, such as disease resistance, are unlikely, because the white polymorphism is extremely rare in other populations.) Without knowing the gene flow, I think it is hard to come to any conclusions about this.
I have read about these bears before – I am sure there was a TV programme a few years ago. This population survived long ice age isolation so the genes would not have been able to spread out of the locality. Read this abstract -http://www.ncbi.nlm.nih.gov/pubmed/11972757
“Kermode bears are part of a coastal or western lineage of black bears whose existence predates the Wisconsin glaciation, but microsatellite variation gave no evidence of past population expansion. We conclude that Kermodism was established and is maintained in populations by a combination of genetic isolation and somewhat reduced population sizes in insular habitat, with the possible contribution of selective pressure and/or nonrandom mating.”
Another thing – bears rely more on smell than sight so I would doubt sexual selection at work. If it were I would have expected it to spread surely? It suggests to me a trait that is not significant in that it survives but does not show signs of increasing particularly. I suppose the population went through a bottleneck in the Ice age then had new bear blood come in before inbreeding did any harm – does that make sense?
Actually read the Nat Geo article. It says different proportions of white bears on two different islands. Sounds like small sub populations, probably too small for meaningful Hardy-Weinberg analysis. Maybe some genetic drift going on here.
Nice blog Dr. Coyne.. I just subscribed 🙂
Bet the White Bears are more prized by poachers.
To do Hardy-Weinberg just for fun: The phenotype ratio for Gribbell Island is said to be one bear in three is white. So q*2 is .33, and q is @.56, where q is the frequency of the white allele. Thus p, the frequency of the black allele is 1 – .56 = .44. Frequency of homozygous black bears is p*2 = .19. Frequency of heterozygous black bears is 2pq = .49.
So here is a testable hypothesis of gene frequencies and distribution among the Gribbell Island bears. Note that the white gene is more common than the black gene, and white bears are more common than homozygous black bears. The phenotype numbers add up to 1.01 due to rounding off.