Henry Bernard Davis Kettlewell, MD, an outstanding physician, lepidopterist and geneticist, is best known for his work on industrial melanism that so elegantly illustrates evolution in action it is now a feature of almost all basic biological texts. Kettlewell was also an energetic field researcher and co-founder of the significant Rothschild-Cockayne-Kettlewell Lepidoptera Collection in the British Museum.
Kettlewell was born on February 24, 1907 in Howden, Yorkshire, UK. He was a schoolboy at Charterhouse, trained in medicine and zoology at Gonville & Caius College, Cambridge, and then was a postgraduate clinical trainee at St. Bartholomew’s Hospital, London. He later practiced general medicine as well as anesthesiology before service at Woking War Hospital. In 1949, he immigrated to the University of Cape Town, where he initiated research and from which he took expeditions to the Kalahari, Knysna Forest, Congo, and Mozambique. In 1952 he repatriated as a Nuffield Research Fellow in Genetics and Zoology at Oxford. In 1958 he led an expedition to Brazil marking the centennial of Darwin’s Origin of the Species. From 1965 on, he was a Fellow of Iffley (now Wolfson) College, Oxford. Kettlewell died in 1979 of an accidental overdose.
In Britain prior to the industrial revolution, Biston betularia was a common moth of which a light-colored typica form predominated. A new, dark-colored phenotype—Biston betularia carbonaria—was first reported in 1848; remarkably, by 1895, this novel type comprised 98% of populations near Manchester. Such dramatic increase in carbonaria subspecies caused many to deduce that this was due to deposition of black coal soot throughout the landscape, notably on tree bark.
Manifestly, carbonaria is easiest to see against a light background but nigh invisible on a dark background; the reverse is true of typica. Indeed, typica was more commonly seen in the country, whereas carbonaria was more commonly seen in besooted urban areas. Moreover, with the advent of modern, antipollution practices, there has been a marked decline in environmental soot and, simultaneously, a sharply reduced frequency of carbonaria. In fact, some lepidopterists worry carbonaria will soon be extinct.
For decades it was widely assumed that the rise and decline of carbonaria evidenced evolution in action; however, Kettlewell, seeking empirical proof, embarked on his classic research in the 1950s. His results confirmed the hypothesis that camouflage congruent with the usual landscape surfaces on which the moths often alit was an essential driving force via natural selection. In 1998, geneticist Michael E. N. Majerus of the University of Cambridge reviewed the original studies of melanism by Kettlewell and others. Majerus endorsed Kettlewell’s basic finding that phenotype color directly affected differential moth survival. Majerus also reported that many experiments-including some of Kettlewell’s—were either not rigorous enough or not properly designed. For example, one main study compared predation of moths congruent and noncongruent with tree trunk bark coloration, but the moths do not often perch on tree trunks. Likewise, control for both the ultraviolet visual acuity of predator birds and the effects of migration was not adequate. Unfortunately, Majerus’s review has been much misconstrued— sometimes rather polemically—by nonspecialists and creationists among others. Kettlewell’s work remains a paragon of excellent field study.
- Kettlewell, H. B. D. (1952). Use of radioactive tracer in the study of insect populations (Lepidoptera). Nature, 170, 584-586.
- Kettlewell, H. B. D. (1956). A résumé of investigations on the evolution of melanism in the Lepidoptera. Proceedings. Royal Society of London. (Series B). Biological Sciences, 145,297-303.
- Majerus, M. E. N. (1998). Melanism: Evolution in action. New York: Oxford University Press.