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Open almost any textbook dealing with biological evolution and you'll probably find photographs of peppered moths resting on tree trunks—illustrating the classic story of natural selection in action. A friend of mine says those photographs are all he remembers about evolution from his undergraduate days.
Before the mid-1800s, almost all peppered moths were light-colored, but during the industrial revolution dark-colored ("melanic") moths became more common—a phenomenon called "industrial melanism." In theory, industrial melanism was due to survival of the fittest: Dark moths were better camouflaged on pollution-darkened tree trunks, and thus more likely to avoid being eaten by predatory birds. For a long time, however, the theory lacked evidence.
In the early 1950s, British physician and amateur moth-collector Bernard Kettlewell released light and dark peppered moths onto nearby tree trunks and watched as birds ate the less camouflaged ones. He then released moths that had been marked on the underside with a tiny spot of paint. When he later recaptured some, the proportion of moths matching the color of nearby tree trunks was significantly higher than in the batch he had released, consistent with the camouflage-predation theory. Kettlewell called this "Darwin's missing evidence," and it quickly became standard fare in biology textbooks.
Most textbooks fail to mention, however, that the peppered moth story began to unravel in the 1960s, when biologists noticed that dark moths were unexpectedly plentiful in some unpolluted locations. When anti-pollution legislation led to cleaner air in the 1970s, light-colored moths made a comeback; but, contrary to theory, the comeback occurred without corresponding changes in tree trunks. Then, in the 1980s, biologists realized that peppered moths almost never rest on tree trunks (as Kettlewell wrongly supposed when he initially released the moths onto tree trunks, creating atypical conditions). Instead, these night-flying insects probably spend their days hiding underneath horizontal branches high up in the trees, where they can't be seen.
In 1998, University of Chicago evolutionary biologist Jerry Coyne wrote: "From time to time, evolutionists re-examine a classic experimental study and find, to their horror, that it is flawed or downright wrong." According to Coyne, the fact that peppered moths rarely rest on tree trunks "alone invalidates Kettlewell's release-and-recapture experiments, as moths were released by placing them directly onto tree trunks." Coyne concluded that this "prize horse in our stable of examples" of natural selection "is in bad shape, and, while not yet ready for the glue factory, needs serious attention" (Nature, Nov. 5, 1998).
Tipped off by a 1999 article in Whole Earth, author Judith Hooper decided to investigate the peppered moth story. The result is a fascinating book, Of Moths and Men: An Evolutionary Tale. Hooper deftly portrays the two men largely responsible for producing the classic story, E. B. Ford and Bernard Kettlewell—and one man who took the lead in debunking it, Ted Sargent.
Oxford geneticist Edmund Brisco Ford wanted to study evolution under natural conditions, and peppered moths seemed a good test case. Ford obtained grant support for Kettlewell, who was then able to do what he loved most—traipsing around the woods surrounded by moths. Strangely, Kettlewell's field notes were never found; since he usually worked alone, there was no independent verification of his data. Ford helped Kettlewell compile and interpret his results, but according to Ford's biographer the fit between theory and data was too good to be true.
