Evolution experts are quietly admitting that one of their most cherished examples of Darwin's theory, the rise and fall of the peppered moth, is based on a series of scientific blunders. Experiments using the moth in the Fifties and long believed to prove the truth of natural selection are now thought to be worthless, having been designed to come up with the "right" answer. Scientists now admit that they do not know the real explanation for the fate of Biston betularia, whose story is recounted in almost every textbook on evolution.
According to the standard account, only one version of Biston existed before the mid-19th century: a white variety, peppered with black spots. During the Industrial Revolution its numbers plummeted because it became easy prey for birds as it rested on the pollution-blackened trunks of trees.In its place a mutant, pitch-black form of the peppered moth began to thrive, as it could rest on tree trunks without fear of being eaten. Precisely as predicted by Darwin's theory of natural selection, this "fitter" mutant moth rapidly outnumbered the white version, reaching 100 per cent levels in some industrial areas.
During the Fifties, however, naturalists discovered a resurgence of the white variety, prompting the belief that Darwin had struck again through the Clean Air Acts, which had led to the return of unpolluted trees. These allowed the white moths to regain their Darwinian ascendancy, while the numbers of the now all-too-visible black variety fell.
This neat example of Darwinian evolution in action has been thought to be supported by solid evidence in the form of experiments begun in the Fifties by the late Oxford University scientist Bernard Kettlewell. But now evolution experts are pointing to blunders in Kettlewell's research that undermine the theory about the rise and fall of Biston. Scientists are beginning to concede that the white variety flourished again well before the return of pollution-free trees, while the black type continued to thrive in areas unaffected by industry. Experiments have also shown that neither moth chooses resting places best suited to its camouflage. Most damning of all, despite 40 years of effort, scientists have seen only two moths resting on tree trunks - the key element of the standard story and Kettlewell's experiments.
According to Michael Majerus, a Cambridge University expert on the moth, Dr Kettlewell tried to confirm the standard story simply by pinning dead moths on to parts of the trees where they could be seen easily by birds. Dr Majerus said: "He stuck them on low branches because he wanted to sit in his hide and watch them being eaten. They actually seem to rest in the shadows under branches, which makes even the black ones difficult to spot by birds."
Scientists are now beginning to doubt even the basic presumption that birds were responsible for the changing fortunes of the different types of Biston. According to Prof Jerry Coyne, an expert on evolution at the University of Chicago, when Dr. Kettlewell could not get the moths he needed naturally, he bred them in his laboratory. Prof Coyne said: "That could affect their vigour, so the level of bird predation he saw was just due to the fact that his moths were raised in the lab. In one case, Kettlewell actually used to warm them up on the bonnet of his car."
Prof Coyne insisted, however, that the moths are almost certainly an example of natural selection: "I'm certainly not saying Darwin is wrong. The real cause is probably connected with pollution - but beyond that I wouldn't want to go." He said, however, that Dr. Kettlewell's widely-quoted experiments are essentially useless. "There is a lot of wishful thinking and design flaws in them, and they wouldn't get published today."
Some fear that the new theories will be seized on by creationists to fuel "sensationalist" claims questioning all evidence for Darwin. Richard Dawkins, the professor of the public understanding of science at Oxford University and author of The Selfish Gene, said: "The details of any experiments done 40 years ago are bound to be vulnerable to detailed criticism. But, in any case, nothing momentous hangs on these experiments."
By Robert Matthews, Science Correspondent, London Telegraph 14 March 1999.
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