Peppered moth

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

Kettewells study and his interpretation

The basic findings of his study are generally well-accepted. The "dark" or carbonaria form of the 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.

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. Kettewell believed 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.

Kettewell published his beliefs, along with striking photographs of each type of moth against the trunk of different trees. These photographs are often reproduced in biology textbooks.

Critique

At the time, scientists did not know that birds see a different spectrum of light than humans, particularly in the ultraviolet, so what appears well-camouflaged to humans may not to birds. Secondly, the moth almost never lights on the trunks of the trees as pictured, but prefers to light on its leaves. In fact Kettewell pinned the moths to the tree trunks for his photographs (some call the photographs "faked"; "staged" would be a better term). Lastly, Kettewell made no effort to account for the migration of the moths among the different areas.

Because of these flaws in Kettewell's work, it is now believed that he was rash to conclude that the observed variations were adaptations due to better avoidance of predation. When University of Chicago evolutionary biologist Jerry Coyne[?] learned of this in 1998--more than a decade after it was announced in the scientific literature--he wrote that he was "embarrassed" to find that the peppered moth story he had been teaching his students for years was seriously flawed. He compared his reaction to "the dismay attending my discovery, at age 6, that it was my father and not Santa who brought the presents on Christmas Eve."

In recent years, though, more careful studies have been performed that vindicate the result, if not the man. 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 Kettewell'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).