The
New York Times
June 18, 2002
By Nicholas Wade
A leading example of evolution given in biology textbooks has come unglued, evoking jeers and jubilation in the camp of creationists, who have been trying for years to expel Darwin from the classroom.
The case is that of the peppered moth, which over the course of a few decades has changed its wing color from pale-peppered to black and back to peppered again in parallel with the rise and fall of industrial pollution.
The moth, a furtive citizen of Britain and the United States, flies only at night. During the day, it supposedly hides on the trunks of lichen-encrusted trees, where the normal pale form is almost invisible.
Textbook writers have long held that the dark form of the moth grew much more common when soot from industrial activity blackened the trees and killed the lichens, making the pale form more conspicuous to birds. But with the passage of clean air laws, the lichens returned, the pale form regained its camouflage, and the black form reverted to rarity.
This account of events became an instant hit with Darwinian advocates. The story caught evolution in unusually speedy action, and flagged bird predation as the mechanism of natural selection that drove it. The moths made a striking illustration because in a typical pair of photographs, one with lichen covering a tree trunk and the other with soot, the reader could hardly spot the pale moth in the first or the dark form in the second, and it was easy to imagine a bird being similarly deceived.
For generations of biologists reared on the peppered moth story as perfect proof of Darwin's theory, it came as a shock to learn of certain problems the textbooks ignored and which a new book is interpreting in sinister light.
For one thing, the moths in the famous photos were not alive. Like the parrot in the Monty Python skit, they were ex-moths, winged members of the choir invisible, firmly glued or pinned to their perches.
And they were glued in place for good reason: the peppered moth almost never rests on tree trunks, its preferred hideaway probably being under twigs in the high canopy of trees.
"My own reaction resembles the dismay attending my discovery, at the age of 6, that it was my father and not Santa who brought the presents on Christmas Eve," wrote Dr. Jerry A. Coyne, an evolutionary biologist at the University of Chicago, in a 1998 review of a book, "Melanism: Evolution in Action," which noted the moth photos were staged.
But not everyone sees the peppered moth story as a black-and-white case of deception. Dr. Michael Majerus, a moth man at Cambridge University in England and the author of the book reviewed by Dr. Coyne, is a staunch supporter of the textbook version, despite all the flaws he laid out. So too are American moth experts like Dr. Bruce S. Grant of the College of William and Mary.
The moth's defenders concede that there were serious design problems with the original peppered moth experiments, conducted from the mid-1950's onward by Dr. Bernard Kettlewell of Oxford University. But they say that he and his successors have tried in good faith to correct the problems and that the basic story holds up.
True, many biologists who tested Dr. Kettlewell's findings, though not Dr. Kettlewell himself, used dead moths to test birds' feeding preferences, but it was not done with intent to deceive, and the textbook writers who omitted the detail are at fault for oversimplifying, Dr. Majerus said. "Many of Kettlewell's experiments were not perfect," he said, "but I think they were right qualitatively."
Dr. Theodore D. Sargent of the University of Massachusetts has a less-forgiving interpretation. He believes that Dr. Kettlewell's experiments created an entirely artificial situation, with moths in an unnatural position and birds that quickly learned they were being served a free lunch in the woods.
Dr. Sargent is a central figure in "Of Moths and Men," by Judith Hooper, to be published by Norton in August. Ms. Hooper portrays the poisonous relationship between Dr. Kettlewell and his eccentric supervisor at Oxford, E. B. Ford, known as Henry.
"Sensitive to slights and always desperately insecure, Bernard became increasingly intimidated by Henry's basilisk gaze and his nuanced but lacerating put-downs," she writes on evidence from Dr. Kettlewell's son David. Both Dr. Kettlewell and Dr. Ford, like the moths in the pictures, are departed.
Reflecting Dr. Sargent's deep skepticism, Ms. Hooper suggests Dr. Kettlewell may have fudged his peppered moth counts so as to please his overbearing mentor. "I wouldn't want to go on record as saying he cooked his results," she said in an interview, but the failure by others to confirm some of Dr. Kettlewell's findings was "quite damning."
But Dr. Majerus rejected the notion that the two biologists had ever fudged their experiments, noting that he had trained with their students and never heard any suggestion of improper scientific behavior.
Creationists have not been downcast at the confusion in the evolutionists' ranks, assailing the peppered moth story as another typical myth from the fairy tale book of evolution.
And Dr. Jonathan Wells, who belongs to the "intelligent design" school, which sees a designer giving a helping hand to evolution, argued that the case should no longer be presented as a textbook example of evolution in action.
"Part of my gripe with evolutionary biologists is that they make their case sound so much stronger than it really is, and I would prefer to see a good deal more agnosticism," said Dr. Wells, a member of the Discovery Institute in Seattle with doctorates in biology and religious studies.
Perhaps the present truth about the peppered moth is too complicated for textbook treatment. The famous photographs are certainly misleading without mention that they are staged. But the creationists are crowing too early.
The pale form of the peppered moth clearly gave way to the dark, or melanic, form as industrialization and air pollution increased in England and the United States. Biologists agree that one form of the moth's color-determining gene became more common than the other. The process then reversed in the two countries, a compelling example of evolution in action, after clean air laws reduced pollution.
But in some areas the pale moths returned to prominence before the lichens that Dr. Kettlewell argued were their camouflage.
Both Dr. Majerus and Dr. Grant remain convinced that the principal mechanism whereby natural selection acts on the peppered moth is predation by birds, and the two are working hard to prove it before the melanic form disappears altogether.
Dr. Majerus is spending 100 days this year on a bird and peppered moth feeding experiment. Dr. Grant, though now retired, is also in pursuit. From a phone booth in Alaska, where he is hunting for peppered moths, he said the role of lichens had been overemphasized and that grime alone had probably been enough to give the black form of the moth its transient advantage.
In retrospect, biologists may have accepted the simple version of the peppered moth story too eagerly. The Kettlewell experiments on lichen camouflage may have been just an enormous diversion. But the melanic form of the moth did rise and fall, for whatever reason. The moth is no myth, and the moth men's continuing efforts may one day get it ready for evolutionary prime time again.
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