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Jonathan Wells

The Peppered Myth

Of moths and men: An evolutionary tale

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.

In addition to the fundamental false assumption that prompted Kettlewell to release moths onto nearby tree trunks, where they don't normally rest, Hooper lists various other problems with his experiments; for example, (1) Kettlewell's moth densities were too high, "in effect, creating a feeding tray," with the intensity of predation recorded in his experiments simply being "a learned response by the local birds"; (2) Kettlewell measured camouflage by his own eye, even though bird vision is quite different from human vision; (3) he and Ford disregarded evidence that selection might operate on caterpillars (through differential mortality when exposed to pollution) instead of on adult moths; and (4) the two men ignored the fact that the main predators of peppered moths are night-flying bats.

Despite problems with the classic story, critics have been notably scarce. One of the few has been University of Massachusetts biology professor Ted Sargent, now retired—the third major protagonist in Hooper's book, and the real hero of the story. A lepidopterist, Sargent has been critical for decades of the camouflage-predation explanation for industrial melanism in peppered moths. In 1998 he and two colleagues wrote in Evolutionary Biology that "there is little persuasive evidence, in the form of rigorous and replicated observations and experiments, to support this explanation at the present time."

For his efforts, Sargent has been "marginalized" and even "demonized" by what Hooper calls "the industrial melanism establishment." (His principal protection, it seems, was that he resisted the modern trend among scientists to get research grants, so he remained free of the political entanglements that muzzled so many of his contemporaries.)

Why was Sargent treated so badly? One reason may simply be the tendency of scientists to cling to theories that are mainstays of their careers. I suspect, though, that something more is involved: a desire to protect the classic story as an icon of Darwinian evolution in action. Although Jerry Coyne is an outspoken evolutionist, and Ted Sargent is no creationist, the evolution-creation controversy fosters a climate in which many Darwinists regard criticism of supposed evidence for evolution as giving aid and comfort to the Enemy.

Ironically, though, the truth or falsity of the peppered moth story is largely irrelevant to the evolution-creation controversy. If the story were true, it would show only a reversible shift in the proportions of two varieties in a preexisting species—a result that even the most uncompromising creationist could accept. And its falsity poses no threat to the most uncompromising evolutionist, because there are now other, better examples of natural selection within existing species.

Nevertheless, many defenders of Darwinian evolution rush to protect the peppered moth icon as though their religion depended on it. In 2000, I wrote a book pointing out that the peppered moth story—though of limited significance in itself—is part of a larger pattern of systematic misrepresentation serving to prop up Darwin's theory. Kevin Padian, a Berkeley professor and president of the National Center for Science Education, a militantly pro-Darwin advocacy group, responded by likening me to the sociopathic antihero of the film The Talented Mr. Ripley. According to Padian, "a particularly egregious example of Mr. Wells's talents is his treatment of the peppered moth." Padian then went on to defend the classic story by claiming that peppered moths "rest on tree trunks 26% of the time" (The Quarterly Review of Biology, March 2002).

Padian bases his astonishing claim (which contradicts the published scientific literature) on the fact that 47 moths were found resting in the wild between 1964 and 1996, and that one quarter of these were on tree trunks. During the same period, however, many thousands of moths were caught in nighttime traps, so the 47 found in natural resting positions were less than 1 percent of the moths studied, and much less than 1 percent of all peppered moths living in the wild. Padian might as well claim that a quarter of all ocean fish are visible to predatory birds because he did statistics on the few that can be spotted from a boat.

Character assassination supported by transparently bogus statistics—how does a highly placed scientist end up indulging in such tactics? Obviously, the peppered moth story involves more than objective science.

So, what about those textbook photographs that impressed my college professor friend? If peppered moths don't normally rest on tree trunks, how were the photographs obtained? It turns out that they were staged—often by pinning or gluing dead moths in place.

Jonathan Wells is a Senior Fellow at the Seattle-based Discovery Institute and the author of "Second Thoughts About Peppered Moths" (The Scientist, 1999) and Icons of Evolution (Regnery, 2000).

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