Of moths and men

The changing colours of the peppered moth have long been held up as the perfect example of evolution in action. But is the insect's iconic status based on fraudulent research? Steve Connor reports on a raging Darwinian controversy
Click to follow
The Independent Online

This is the story of the moth that turned black when Britain had its Industrial Revolution. It is a story told in any school biology book as the canonical example of evolution in action. The light and dark varieties of this moth were key players on the Darwinian stage. That was until someone decided that it was time to rewrite scientific history and declare the story of the peppered moth a myth. A myth, furthermore, based on fraudulent research.

Doubts about the veracity of the peppered moth story first surfaced about five years ago. Leading evolutionists began publicly to question the landmark experiments that were supposed to demonstrate how the dark and light forms of the moth were each better camouflaged against being eaten by birds. When unpolluted trees were covered in lichen - which is very sensitive to pollution - it was the light or "peppered" form of the moth that more easily escaped the notice of predatory birds. When trees were covered in soot or devoid of lichen, the black "melanic" form was better disguised.

Last year these niggling doubts about the details of the experimental design mutated into a full-blown conspiracy. Judith Hooper, an American science writer, questioned not just the methodology of the experiments carried out 50 years ago but, more seriously, whether the scientific data they produced were indeed genuine.

In her book, Of Moths and Men, Hooper suggests that the (now deceased) scientist involved in these studies may have actually perpetrated an outright fraud in order to "prove" that the rise of the melanic form of the peppered moth in 19th-century industrial Britain was the result of better camouflage against birds. If true, it would mean that every schoolchild who has ever been taught the story of the peppered moth has effectively been fed a lie.

Although Hooper insists she is not a creationist - she argues that to be uncritical of science is to turn it into dogma - her arguments are widely cited in creationist circles. Believers in the literal truth of the Bible, or in evolution by "intelligent design", have been quick to seize on the controversy, claiming that it fatally undermines an important plank of Darwinist teaching. Schools in America have come under pressure to drop the peppered moth from their science curriculum, and creationists in Britain are exploiting the doubts to justify their own aberrant position.

And so this unprepossessing little insect, which sleeps by day and flits from tree to tree by night, has become the latest weapon in the creationist war against Darwin. Now, with Hooper's book reaching the best-seller list, the evolutionists have decided to fight back. Accepting the limitations of the original experiments, they are still adamant that the peppered moth remains a perfect example of evolution by natural selection.

Ironically, the roots of the dispute can be traced to a man who arguably knows more about the peppered moth than anyone. Michael Majerus, reader in genetics at Cambridge University, has made industrial melanism one of his specialisms and has spent hours poring over scientific papers - and many more hours scrambling around trees at all times of day and night, watching and wondering about Biston betularia.

"For 45 years I have bred, collected, photographed and recorded moths, butterflies and ladybirds in Britain," he says. "I have run one or more moth traps almost nightly for 40 years. I bred my first broods of the peppered moth in 1964. I found my first peppered moth at rest in the wild in the same year. As far as I am aware, I have found more peppered moths at rest in their natural resting position than any other person alive. I admit to being, in part, a moth man."

Oxford University Press asked Majerus to write a book on industrial melanism for publication in 1998 to mark the 25th anniversary of another book, The Evolution of Melanism by Bernard Kettlewell. It was Kettlewell who carried out the seminal experiments in the 1950s that were supposed to have demonstrated the role of predatory birds and pollution in the evolution of the two forms of peppered moth.

Dressed in khaki shorts and fortified with a supply of gin and cigars, Kettlewell would camp out for weeks doing what he enjoyed most - studying moths and butterflies. Although he carried out the earliest and most important field experiments on the peppered moth, and was widely viewed as a brilliant naturalist, this former medical doctor with a lacklustre degree in zoology was not considered a particularly good scientist.

"Bernard Kettlewell was a highly gifted amateur lepidopterist," says Professor Bryan Clarke, a geneticist at the University of Nottingham who knew him personally. "He was not a trained scientist. He never got to understand the refinements of theory, as can be seen in his book, which is embarrassingly bad. None the less, he had an extraordinary capacity for organising and executing studies in the field. More or less single-handedly, he accomplished what was then the largest and most demanding set of experiments ever carried out under natural conditions."

Kettlewell conducted his fieldwork under the auspices of a towering intellectual figure within Oxford University's zoology department in the 1950s. EB Ford, known as Henry to his colleagues, was the father of ecological genetics, a new discipline in the 1920s and 1930s that attempted to place Darwinism on a more solid footing. For all his brilliance, Ford was an eccentric, a misogynist who refused to give lectures to female students, a social snob and a gentleman scientist who would probably have been happier living in Victorian England.

According to Judith Hooper, Ford was also a megalomaniac intent on getting the results he wanted from Kettlewell's experiments. She insinuates that Ford pressured Kettlewell into producing the data that would fit the accepted but still unproven theory of how the melanic form of the peppered moth rose to prominence during the Industrial Revolution. (It rose from virtually zero to 100 per cent frequency in the most polluted parts of Britain prior to the Clean Air Act of 1956.)

In perhaps his most famous field experiment, Kettlewell released large numbers of light and dark peppered moths - marked with dots of paint - into two woods, a polluted one with no lichens near Birmingham, and a lichen-festooned wood in Dorset. After recapturing the marked moths using a light trap, Kettlewell found that the dark melanics had survived better in the Birmingham wood and the light form had survived better in Dorset. When he released moths on to the trunks of polluted and unpolluted trees, he witnessed how easy it was for birds to eat the melanics against the lichen-covered bark, and the light form against the sooty, lichenless bark. His colleague Niko Tinbergen even managed to record the predation on 16mm film.

"It was the reciprocal nature of the results from the two woods, together with the visual record on film, that had such an impact on the scientific community and finally convinced the sceptics," Majerus says. There was no doubt that the two forms were better suited to the different environments, with the melanics having a greater chance of survival in a polluted environment. Furthermore, Majerus says: "The mechanism of selection - differential bird predation - had been identified and demonstrated."

For Kettlewell and Ford, the experiments were a triumph. They showed that the rise of the black moth since the 19th century was due to the spread of environmental pollutants, which had progressively blackened British trees, so giving the melanic moth a cryptic advantage over its light cousin, which was mostly confined to unpolluted woodland in the West Country until its recent re-emergence after the Clean Air Act. It became the standard story of evolution by natural selection, illustrated with photographs of the two moths on the trunks of polluted and unpolluted trees.

But nearly 50 years later, Majerus began to spot flaws in the design of Kettlewell's experiments and the way they had been simplified for schools. Peppered moths do not usually rest during the day on the trunks of trees - where Kettlewell released them in the bird predation experiment - preferring higher branches tucked out of sight. Photos in schoolbooks showing peppered moths resting on tree trunks are staged, sometimes using dead moths. They bear little resemblance to what occurs in nature.

Then there was the problem of how Kettlewell did his experiment. He released far too many moths in a small area for natural population densities to be represented, making any feeding trial highly unnatural. The moths were also a mixture of laboratory-bred and wild-caught individuals, which he failed to distinguish: an important omission, as each might behave differently. He released his moths in daylight rather than during the night, when moths are normally active. Worse, he began to release more moths halfway through his experiment when he failed to recapture enough individuals to make his results valid. It is a cardinal error in science to change an experiment's design midway through.

When Majerus listed these deficiencies in his 1998 book, Melanism: Evolution in Action, one reviewer for the journal Nature, Professor Jerry Coyne, an evolutionist at Chicago University, concluded that for the time being evolutionists must discard the peppered moth as a well-understood example of natural selection. "My own reaction resembles the dismay attending my discovery, at the age of six, that it was my father and not Santa who brought the presents on Christmas Eve," Coyne wrote.

With such damning words, Coyne had set in train a series of events that would culminate in the great peppered moth revisionism. One British Sunday newspaper put it like this: "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." As Coyne was to remark later on, he had unwittingly provided grist for the creationist mill.

Jonathan Wells of the University of California at Berkeley, who receives funds from the Discovery Institute in Seattle - which promulgates creationism - cited Coyne in his critical onslaught. "The classical story, elegant and appealing though it may be, should no longer be presented as a textbook example of evolution in action. If the purpose of science education is to teach students how to do good science, then instead of retelling the classical story, textbooks would do better to focus on how science revealed its flaws," Wells wrote.

With sentiments such as these gaining wider currency in the United States and in Britain, Majerus has decided that the time has come to set the record straight. "Judith Hooper displays her ignorance on virtually very page of her book," he says.

In a forthcoming scientific publication, Majerus intends to describe in detail why, with all the inherent faults of the Kettlewell field studies, the story of the peppered moth is still the quintessential example of Darwinism in action. "It is irrefutable proof of biological evolution through the process of natural selection. What there is no 'scientific' proof of is that predation by birds has caused it - but there is a tremendous amount of circumstantial evidence to suggest that it is the cause," he says. In other words, the definitive proof that birds are responsible has yet to be gathered, although what evidence there is would amount to guilt beyond reasonable doubt in any court of law.

Majerus says that eight experiments have since been carried out to emulate Kettlewell's field studies, and each has addressed one or more of Kettlewell's shortcomings, apart from one - proof that birds in the wild are responsible for driving the changes in frequency of the dark and light moths that are allowed to land where they like on a tree while it is still night.

This weakest link in the peppered moth experiments is now being tested by Majerus in a five-year experiment he has begun in his own rambling back garden. He hopes that within three years he will have enough data to address the final obstacle in the argument over what precisely has caused the rise and fall of the moth that turned black. If his hunch is right, birds will be shown to be the force of natural selection.

But even with this final piece of evidence, will the anti-evolutionists be convinced that the peppered moth remains a perfect example of Darwinism in action? "Sadly," admits Majerus, "I doubt it."

Comments