Peppered moth

The English peppered moth (Biston betularia) was the subject of an early study by H. B. D. Kettlewell and is often cited as an example of Charles Darwin's theory of natural selection in action, although Kettlewell's work is also criticized by opponents, including some scientists.

Table of contents

1 Background 2 Kettlewell's studies 3 Critique

Background

The "dark" or carbonaria form of the peppered moth constituted less than 2% of the population before 1848. The frequency of the dark moth increased relative to that of the "light" or typica form, until, by 1898, 95% of the moths in Manchester and other highly industrialized areas were the dark form. Their frequency was less in rural areas. It is known that the dark and light phenotypes of the moth are controlled by a single gene.

Long before Kettlewell's research in the 1950's, Tutt, in the 1890's, proposed the "differential bird predation" theory for the rapid change in moth colors. According to the theory, coal burned during the early decades of the industrial revolution produced soot that darkened the Birch trees (a favorite habitat of the moths) in industrial areas of England between London and Manchester. Several naturalists noted that the light form was more common in the countryside, while the dark moth prevailed in the sooty regions. The natural conclusion was that the darker moths had some sort of survival advantage in the newly-darkened landscape. Tutt proposed that the dark moths were better comouflaged against the darker background, making it harder for birds (the moth's primary predator) to see them. The lighter months, in contrast, were more difficult to see against the lighter-colored trees in the countryside.

Kettlewell's studies

Kettlewell conducted several sorts of experiments, including direct observation of bird predation on moths (under controlled conditions and in the wild), and mark-release-recapture experiments, where moths are released into a forest and then recaptured the next night with moth-attracting light traps (moths are attracted to lights and pheromones). Kettlewell performed reciprocal experiments in polluted and unpolluted woods, and found that typica moths were more likely to survive predation in unpolluted woods, and carbonaria moths in polluted woods. These results were published in 1953 and 1955 and have been widely cited ever since. Numerous follow-up studies appear to have confirmed the basic conclusion of Kettlewell's research, although minor quantitative differences (in estimated selection coefficients, etc.) are common.

Along with his conclusions, Kettlewell published striking photographs of each type of moth against the trunk of different trees. These photographs are often reproduced in biology textbooks.

Critique

Various critiques of Kettlewell's work have been made. The critiques in the peer-reviewed literature are mostly moderate and do not challenge his conclusion. However, a combination of poor journalism and creationist propaganda has taken the critiques out of context and created the widespread impression that the peppered moth example has been debunked. The actual peppered moth researchers do not agree with this view, but they seem unable to get a hearing in the popular press. The recent (2002) book Of Moths and Men, by journalist Judith Hooper, took things to new levels, basically charging Kettlewell with fraud, a completely novel claim entirely of her own making. Hooper's book has, predictably, been praised in the science press and the creationist literature, but the book reviews in scientific journals, by scientists familiar with the peppered moth case and with Kettlewell, have been mostly hostile.

The various controversies surrounding Kettlewell's work are most digestibly reviewed by peppered moth researcher Bruce Grant, in a paper hosted at his webpage, "Fine tuning the peppered moth paradigm": http://faculty.wm.edu/bsgran/melanism.pdf

One of the strongest points in favor of Kettlewell's theory is due to recent events. The burning of cleaner fuels and the advent of Clean Air laws has eliminated much of the sootiness in industrial areas of England. The prevalence of the dark form of the moth has declined dramatically, and indeed some even fear its extinction.

In 1998, Michael E. N. Majerus of the University of Cambridge Department of Genetics reexamined Kettlewell's studies and the more recent ones, and reported: "Differential bird predation of the typica and carbonaria forms, in habitats affected by industrial pollution to different degrees, is the primary influence on the evolution of melanism in the peppered moth." (Melanism - Evolution in Action, M. E. N. Majerus, Oxford University Press, New York, 1998).