The peppered moth (Biston botularia ) is an inconspicuous member of the family Geometridae, a night-flying species thought to spend its days resting camouflaged among the lichens that grow on tree trunks. The moth's predominant form has white wings, "peppered" with black specks or faint black lines, perfect for blending in with its tree bark environment. Less common is a variant, carbonaria, which is a black-winged moth, with increased levels of melanin (black pigment) causing the color change. The peppered moth has come to play a significant role in two important stories in science.
In 1859 British naturalist Charles Darwin proposed a theory of evolution in his book On the Origin of Species. He based his theory on three observations he made while collecting data on plants and animals during a five-year trip around the world in the survey ship Beagle: that living things vary, that they can pass on their characteristics, and that they are involved in a struggle for survival which favors genetic mutations that are better adapted to their environment.
The fossil record is tantalizingly full of what appear to be gradual changes from one mineralized skeleton to the next, charting the evolution of species. The only problem with Darwin's theory was that there was no evidence of natural selection in action. Then, in the mid-1800s a phenomenon occurred that seemed to indisputably prove natural selection. A small moth commonly called the peppered moth, common to British woodlands, underwent dramatic color changes with the advent of pollution-darkened skies. As the industrial revolution proceeded across Britain, covering towns and countryside with soot, blackening tree trunks, and killing the lichens, the melanic, or black, variety of the moth increased in number and the original peppered variety all but disappeared. Photographs of the two types of moths on sooty and clean tree bark were dramatic evidence of the power of camouflage, and experiments clearly showed birds predating the uncamouflaged moths when given a choice. Industrial melanism was the name given to this example of evolutionary adaptation to smoky air.
In the 1950s, Oxford University biologist H. B. D. Kettlewell bred peppered moths in a lab and released close to a thousand of them in polluted and nonpolluted woods. When the moths were recaptured several nights later, there was a clear correlation of more black moths in the dark woods and more white ones in the clean. Kettlewell further released hundreds of moths onto the bark of dark and light trees at dawn and photographed birds eating the more conspicuous species. He concluded, "The effects of natural selection on industrial melanism for crypsis (camouflage) in such areas can no longer be disputed. Birds act as selective agents as postulated by evolutionary theory. Had Darwin observed industrial melanism he would have seen evolution occurring not within thousands of years but in thousands of days." (Holdrege 1999, p. 66) By the 1970s, following the passage of legislation that resulted in cleaner air, the population of dark moths decreased and light ones made a dramatic comeback. This seemed to provide proof of natural selection.
Also during the 1970s, some surprising evidence was introduced by British biologist Cyril Clarke that called into question some of the previous research involving the peppered moth. In twenty-five years of studying the peppered moth, Clarke found only two in daylight. The moth is notoriously difficult to locate and in fact no one knows where it lives by day, but it is certainly not on the lower trunks of trees. The peppered moth is nocturnal and its chief predators are bats. All of the peppered moths experimented with had been collected in traps at night and many of the ones eaten from tree trunks had been glued to the trees where they were found by opportunistic birds. The ones not glued had been released in early morning when they typically would fall asleep on the bark. Further confounding the earlier research was the discovery that an increase in the original peppered variety around both Liverpool and Detroit, Michigan, occurred despite no increase in the dark lichens assumed to be their hiding place.
The history of the peppered moth research is a reminder of how strongly people see what they look for. Kettlewell's field experiments showed that birds feed on moths released onto tree trunks preferentially by degree of camouflage. Since the moths are not normally found on lower tree trunks during the day, this experiment created, as all experiments do, an artificial situation and then appeared to prove a hypothesis. Some evolutionary scientists such as Stephen Jay Gould are highly critical of the unwillingness of researchers to consider alternative concepts. If Kettlewell had not been so convinced of the truth of bird predation, he might have been more willing to question his results. When scientists have an uncritical acceptance of a certain theory there is a real danger of seeing what one believes and turning science into dogma. Dogmatic knowledge, teaching what is only an opinion as absolute fact, is the antithesis of science's basic tenet of observation and questioning.
Biologist Craig Holdrege believes that instead of using experiments as a way of proving or disproving an idea, scientists could come to see them as a way of interacting with phenomena. To keep science alive, scientists need to remember to be aware of their own preconceptions and be wary of drawing general conclusions from a specific and contrived event. Experiments help scientists clarify ideas and formulate new questions. As such, they become more of a jumping-off point than an end. The peppered moth story points to the need for much greater basic natural history observation, difficult as that is. Where does the moth rest by day? How far does it fly? What do the larvae eat and could the melanism be an effect of a change in the larvae's diet?
The peppered moth is a reminder that science is an evolving process. Vitality comes from doubting conventional dogma, making new observations, and thinking with originality. Science is an ongoing exploration and renewal of ideas. Just as Darwin's hypotheses added to the richness of scientific thinking, so the peppered moth story is an excellent teacher of the evolution of the scientific process.
see also Camouflage; Genetic Variation in a Population; Selective Breeding.
Holdrege, Craig. "The Case of the Peppered Moth Illusion." Whole Earth Spring (1999):66.