Sept/Oct 2002

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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. ...