Okay, so "taking over" may be a bit of an exaggeration. But according to the Washington Post black squirrels now make up 5-25% of the squirrel population. They are descended from a small number of Canadian black squirrels released about 100 years ago.Black squirrels are much more common in Toronto than gray squirrels so there's always been speculation about why that is. So far there's no evidence of a selective advantage so it's probably due to random genetic drift. In fact that's what the result in Washington suggests to me. But that's not what the newspaper article says, ...
Scientists say it's a real-life example of natural selection at work, which has rolled on for a century here without much public notice.The IDiots have jumped all over this one and for once they may be partly right.
Denyse O'Learly criticizes the article in The Washington Post Thinks It Has Discovered Natural Selection. I agree with Denyse that this is probably not natural selection. It's a clear and wonderful example of evolution—because allele frequencies in the squirrel population are changing—but the mechanism is probably random genetic drift.
I wish people would understand the difference between evolution and natural selection. If you are a pluralist, you recongize several different mechanisms of evolution and natural selection is only one of them. That's why pluralists (i.e. most evolutionary biologists) are not Ultra-Darwinians.
[The photo is borrowed from the definitive website on blackskwerls. Don't go there unless you're prepared to be really, really frightened. The truth about the black squirrel invasion is ....]
Are the black and grey squirrels different species, or just colour morphs? We have both kinds here in Ottawa. However, we have noticed (by way of completely unsystematic observations) that the greys seem to be increasing -- they used to be quite rare relative to the blacks (We approve: the greys are very pretty -- lovely salt-and-pepper shading, with long white guard hairs on the tail).
ReplyDeleteWe have a vague theory that the black colour survives Ottawa winters better, and the increase in greys reflects climate change. OTOH, maybe it's just some kind of inter-capital squirrel exhange program ;-),
They are the same species. I agree with you that the gray ones are better looking.
ReplyDeleteI'm not sure about your explanation of the change in frequency. As a former Ottawa native, I was under the impression that nothing can survive an Ottawa winter. :-)
If I'd clicked twice before posting, I could have partially answered my own questions ;-).
ReplyDeleteFrom http://animaldiversity.ummz.umich.edu/site/accounts/information/Sciurus_carolinensis.html:
"Some interesting clines occur in both skull size and coat color.....Also, more black-coated squirrels occur in the north. Studies have shown that black animals have 18% lower heat loss in temperatures below -10 degrees Celcius, along wth a 20% lower basal metabolic rate, and a nonshivering thermogenesis capacity 11% higher than grey morphs."
So it seems our hunch that black is the preferred colour for surviving Ottawa winters might be right. Now whether the increasing proportion of greys reflects climate change (says he, advancing an adaptationist hypothesis with fear and trembling in Larry's presence ;-), or drift, or is a figment of our imagination in the first place......
No, it's not random drift. I did some quick simulations with the following parameters:
ReplyDelete--initial number of black squirrels: 18 (from the article)
--genetics: assume black is dominant over gray; assume the 18 black squirrels were all homozygotes
--total number of squirrels in D.C.: 10000 (an order of magnitude or two too low, but I don't have all day for the simulations)
--relative fitnesses: all 1
--number of generations: 100
I did 1000 replicate simulations, and the highest the black allele got in frequency was 2%. In 99% of the simulations, the allele frequency was less than 1%. Even if the black allele is dominant, it would be unlikely to get "5% to 25% black squirrels" (allele frequency 2.5% to 13.4%, assuming Hardy Weinberg) by random drift. With a more realistic population size of 100,000 or more, the initial allele frequency is smaller and random drift is less effective, so the chance of the allele frequencies increasing that much by drift in 100 years would become infinitesimal.
Since 80% of urban squirrels die from being hit by cars, perhaps the greater visibility of black squirrels on gray pavement plays a role in selection.
It's a lot more complicated than that, John. There are dozens of American and Canadian cities with mixed populations of gray and black squirrels. If natural selection were as strong as your results imply then why haven't black squirrels taken over the entire North American population? (I doubt very much whether all the black squirrels in Washington are descended from the original 18. I'm sure some of them came from Baltimore.)
ReplyDeleteSame applies to your "evidence," Steve. With that kind of presumed advantage it's a wonder that there are any gray squirrels left.
Do you really believe that the gene for black coat color also leads to a 20% lower basal metabolic rate? Because that's what your "evidence" implies. How would that work? In my back yard there are several litters of squirrels with both gray and black pups. Would all the black ones have a lower metabolic rate?
These are not distinct isolated populations. The segregation of gray and black alleles in squirrels is similar to the segregation of blue and brown eyes in humans.
Larry, Are you saying that the frequency of the black allele increased too rapidly for selection, so it must be random drift? That's only plausible for really, really small populations, which the squirrels in D.C. certainly aren't.
ReplyDeleteTo go from an allele frequency of 0.18% to 13% in 100 generations, the black allele would only need a fitness advantage of about 5% over gray squirrels (assuming black is dominant). Whether it's a thermal advantage, camoflauge, linkage to aggression, or something else, a 5% difference in fitness between black and gray seems quite plausible to me.
As for why black squirrels haven't taken over, it would be easy to think of all kinds of just-so stories. I'd start with greater visibility having a tradeoff--increased fitness where the main "predator" is cars, decreased fitness where natural predators are more important. This would predict a higher proportion of black squirrels in high-traffic urban areas, and a lower proportion of black squirrels in rural areas.
John H. McDonald asks,
ReplyDeleteLarry, Are you saying that the frequency of the black allele increased too rapidly for selection, so it must be random drift?
No, of course not. I'm skeptical of the numbers in the Washington Post article. If your numbers are correct (18 ancestral black squirrels giving rise to >5% today) then it should be possible to estimate a selection coefficient (s). I suspect that selection coefficient is far too high to account for the persistance of gray squirrels in other locations.
I base my hypothesis on the overall frequency of gray and black squirrels in all parts of North America. It doesn't look like selection to me.
Here are the selection coefficients against gray squirrels needed to increase the allele frequency of black from 18/N to 5%, where N is the squirrel population size. This assumes black is dominant and all 18 initial squirrels were homozygotes.
ReplyDeleteN s
1000 0.011
10000 0.034
100000 0.056
1000000 0.078
These selection coefficients seem quite plausible for a major coat color difference. As for why such selection wouldn't have wiped out all gray squirrels, it may be that selection favoring black squirrels only occurs in urban areas, and selection favors gray squirrels in other areas; or it may be frequency-dependent selection (rare black squirrels don't fit predators' search image for squirrels); or heterozygote advantage. Or maybe selection favoring black squirrels only arose recently (the urban squirrel's habitat is far different from anything its ancestors experienced), and they're on their way to taking over.
Meanwhile, grey squirrels are overtaking red ones in the UK.
ReplyDeleteMeanwhile, grey squirrels are overtaking red ones in the UK.
ReplyDeleteAre UK reds like North American reds? We have both red and gray squirrels in most parts of Ontario. The reds are smaller (about 1/2 the size of the grays), and tend to inhabit more heavily forested areas, as opposed to the sparse trees-in-lawns which are characteristic of urban and suburban yards and parks, and the grays seem to thrive in. But the reds make up for their diminutive size in feistiness. They are agressively territorial - on one camping trip, a red squirrel spent the entire weekend dropping pinecones on the top of our tent trailer. On another occasion, a female red decided that our car's engine compartment was the ideal place to build a nest (she kept turning up carrying piles of grass then disappearing under the car - we eventually opened the hood to discover a lovely pile of grass arranged on top of the engine - fortunately, there were not any babies yet). By comparison, the grays are positively mellow rodents, who appear to understand about the impermeability of windows rather better than cats do - we often see them calmly sunning themselves stretched out on our windowsills, with our very excited cats stuck on the other side of the glass.