Teaching Evolution

A blog devoted to teaching evolution, both in our schools and in our communities.

Tuesday, May 24, 2005

Why are Some Squirrels Black?

Throughout my childhood, all the squirrels I ever saw were brown or gray. The first time I saw a black squirrel I thought something had happened to it -- it fell in some tar at a construction site or something.

Through the Washington Post via Chris C. Mooney, I learn why black squirrels are gaining in number:
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.

"It shows the spread of a gene within a population," said Richard W. Thorington Jr., a Smithsonian Institution researcher working on a book that includes a history of the District's black squirrels. "That is evolutionary change before your eyes."

...Here's why some scientists believe the black squirrels were multiplying: In winter, their dark coats allowed them to retain heat from sunlight, leaving them less desperate for warmth than their lighter-colored cousins.

"If you can do it with solar heat, you don't need quite as much metabolic heat," and, therefore, need less food, Thorington said.

In some cases, this advantage seems to have outweighed the potential downside of a black coat -- being more conspicuous to hawks and other predators.

Thorington believes that black squirrels were slightly more likely to survive and reproduce, and their genes were passed on to succeeding generations.

Fascinating stuff, and I agree with Chris that this is a wonderful sentence: "He used to smear a tree behind his Silver Spring home with a mixture of peanut butter and Valium and then tattoo the squirrels that he found passed out below."


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