The Mothman Who Created An Evolutionary Controversy

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Every student of biology has seen photos of the peppered moths. In a forest polluted by smoke, dark peppered moths resting against darkened bark have a natural advantage and proliferate. In a pristine forest, light moths have the advantage. It's a science legend, but is it the truth?

Bernard Kettlewell learned entomology mostly via travel and independent study instead of formal training. When he secured a position at Oxford, he made up for his lack of formal education in entomology by orchestrating one of the most famous and enduring proofs of natural selection in progress.


The woods near Oxford were variegated. Some parts, situated near factories, had trees covered in soot. Other parts were clean and soot-free, and there the light bark of the trees showed clearly. Occupying these woods were tens of thousands of peppered moths, which came in two varieties. Light moths were a mottled, but generally light, gray-black; melanistic moths were almost uniformly dark. Kettlewell went to the woods and spent his days catching, marking, and releasing moths.

That was not so challenging. The challenge came in recapturing the same moths the next day. Moths that had been eaten by birds could not be recaptured, Kettlewell reasoned, but by recording what percentage of dark and light moths were recaptured he hoped to estimate which moths nature was selecting to survive. After a great deal of work, he enthusiastically announced that, in the polluted and dark parts of the woods, the dark moths survived at a higher rate than the light moths. They blended with the dark bark, and weren't spotted by birds. In the light parts of the woods, however, the light moths were the advantageously-colored ones. Here was natural selection in progress.


Kettlewell's theory was evidence for evolution, so it came under fire immediately. The fire has endured, but it's not always creationists leveling the criticism at the Peppered Moth experiment. Sometimes critics went after the methodology of the experiment, and sometimes Kettlewell himself. The most famous and influential attack on the experiment, however, didn't involve Kettlewell's reputation but that of another scientist.

In the book Of Moths and Men, author Judith Hooper alleged that Kettlewell's dream job at Oxford became a nightmare. She believed that Kettlewell's superior, Edmund Ford, bullied Kettlewell constantly. Relentlessly driving Kettlewell to improve his results, Ford might have caused Kettlewell to falsify, or at least exaggerate, his data. The most damning letter from Ford urges Kettlewell to increase his capture rates, after which Kettlewell dramatically increases his capture rates without any real explanation of how he did it.


As anyone would expect, them's fighting words, and groups of people have been fighting about them ever since Hooper published the book. Kettlewell comes off pretty well in the foray, as other people have duplicated his results, and some have extended them. As for the letters, while some readers consider them bullying, others see them as the kind of rough encouragement that one colleague might give to another if they were both on informal terms.


Even the recapture rate letter is relatively soft evidence. Recapture rates had begun improving before the letter was sent. No matter what, Kettlewell set down the guide for an experiment that could work. The exact nature of his relationship with Ford, or with anyone else, doesn't change that.

[Via Scientific Feuds]