The peppered moth represents one of the most iconic examples of natural selection in action. The popular account of its evolution is both simple and intuitive. Nineteenth century industrialization in Britain dramatically altered the landscape. Pale lichen covering tree branches was killed and trees blackened with soot. On this background, white peppered moths that sheltered on the trees became conspicuous targets to visual predators and were eaten, causing their decline. Simultaneously, because they escaped detection, mutant black moths increased in frequency. The predation explanation was plausible and well tested. It was a classic textbook case.
In 1998, however, the story took an unexpected turn. Cambridge geneticist and lepidopterist Michael Majerus published a book that raised legitimate questions about Bernard Kettlewell's predation experiments, the foundation of the moth story. Were Kettlewell's experiments natural? Was it appropriate to study the responses of birds to dead moths arranged on trees? Did moths even rest on tree trunks where Kettlewell had put them? This questioning triggered a firestorm of controversy that raged.
A book review in Nature implied that the moth story was as disappointingly uncertain as Santa Claus, while another book accused Kettlewell of fraud. Despite the defence from scientists that it was not the predation hypothesis itself that was in doubt, creationists rejoiced. The curiously named Institute of Creation Research railed that ‘the supposed best proof of evolution in action is so flimsy that it cannot stand the test of truth’. However, this controversy has finally been put to rest in a Biology Letters paper based on Majerus's experiments by Laurence Cook from the University of Manchester and an international team of collaborators.
Working in his Cambridge backyard, Majerus overcame the flaws in Kettlewell's design and validated the predation hypothesis. Black and white moths indeed rest on tree trunks and branches. More importantly, predatory birds do feed preferentially on the less camouflaged morph. In today's cleaned-up post-industrial landscape, the black morph is more conspicuous and its survival level is about 10% lower than that of the white morph. However valid the doubts were about Kettlewell's experiments, Majerus's results should now silence the critics.
Science, unlike creationism, is self-correcting. Cherished ideas can be tested and amended or overturned. The strategy of creationists, in contrast, is to target single examples for scorn while offering neither ideas nor credible tests to support their challenges. Majerus was not the only person in Britain with moths in his backyard, but he was the only one carrying out the hard work to test predictions of the peppered moth story. Unfortunately, he died before he could prepare his work for publication. Now freely available, his data are both interesting and important. They also serve as a clear demonstration of how science works by building an ever more refined understanding of the natural world. Creationists, with their unscientific agenda, working at so-called ‘research’ institutes, can offer nothing of substance in response.
