– Dava Sobel
natural selection is crumbling.
The peppered moth, biston betularia, is a species of moth familiar to anyone who has studied biology, for this ordinary moth provided evidence which proved Darwin’s theory of evolution by natural selection.
That’s what everyone thought.
In Darwin’s time, the word evolution implied a pre-existing, presumably a divine plan, and this is just what Darwin wanted to rule out. He pointed out that organisms vary spontaneously, offspring tend to inherit the characteristics of their parents, and in the competition for survival, the unfit are eliminated by natural selection. Thus natural selection could account for both the adaptations to their environment shown by plants and animals and the progressive development of new forms of life. This concept was summarized in the title of his most famous book, The Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.
The problem with Darwin’s theory of evolution by natural selection is that it occurs over thousands of years, and is therefore difficult to prove empirically. What was needed was an example of rapid environmental change, forcing organisms to adapt quickly.
In 1848, a black or melanic form of the peppered moth appeared in Manchester. At the time, industrial areas in England were subjected to high levels of atmospheric pollution. These deposits killed the lichens on tree bark, and in 1896 this was linked with the decline of the normal form of the moth. In polluted areas the black moths were better camouflaged against the dark tree trunks, and so less likely to be eaten by birds, a perfect demonstration of the survival of the fittest.
Where was the evidence? In the 1950’s Bernard Kettlewell of Oxford University made a study of peppered moths in polluted woodland near Birmingham.3 His results showed that black moths were twice as likely to avoid being eaten by birds in the polluted environment. Kettlewell’s experiment was what scientists had been waiting for: direct proof of Darwin’s theory of natural selection.
The peppered moth became the classic example of the phenomenon of industrial melanism, where all-dark individuals became the dominant form. The fact that, in later years, as industrial pollution began to decline, so did the melanic form f. carbonaria, was taken as supporting evidence.
The reign of the peppered moth was to prove short. Twenty years later the American lepidopterist Ted Sargent reported problems with Kettlewell’s experiment.5 Sargent’s criticisms received little attention at the time. But in 1998, Michael Majerus in his book Melanism: Evolution in Action strongly supported Sargent’s conclusions. 1,4 For example, the famous photos of moths on tree trunks were actually posed, using dead moths arranged on a log. Peppered moths do not alight on tree trunks long enough to be eaten, preferring the shady undersides of branches. It is even doubtful whether birds actually eat moths on tree trunks. Kettlewell’s evidence on the preference of birds for light coloured moths was clearly contrived. Alternative explanations for the appearance and subsequent decline of the melanic form are now available.
The journalist Judith Hooper, in Of Moths and Men: Intrigue, Tragedy and the Peppered Moth,2 puts the question of how such a flawed piece of research could become a cornerstone of evolutionary biology, presenting creationists with an unexpected gift.6
The implication is that scientists are just as prone as other humans to believe what they want to hear, and every bit as reluctant to accept inconvenient evidence to the contrary.
1. Jerry A. Coyne, ‘Not Black and White’, Nature, Vol. 396, Nov. 5, 1998, pp 35-36.
3. H. Kettlewell, ‘Darwin’s missing evidence’ (1959), Evolution and the fossil record, readings from Scientific American, W.H. Freeman and Co., San Francisco, 1978, p. 23.
4. Michael E. N. Majerus, Melanism: Evolution in Action, Oxford University Press, 1998.
5. Sargent T.R. et al., in M. K. Hecht et al., Evolutionary Biology, 30:299-322, Plenum Press, New York, 1998.
6. Wayne R. Spencer, Creation Education Materials.
See also on this website:
Second Thoughts About Peppered Moths
Jonathan Wells, Ph.D., Department of Molecular Cell Biology
University of California, Berkeley
The attempt to reclaim the peppered moth for Darwinism has not been an unqualified success.
Revenge of the Peppered Moths?
Jonathan Wells, Evolution News, February 12, 2012
Darwinism in a Flutter
Peter D. Smith, The Guardian, 10 May 2002.
“Did a moth show evolution in action? Peter D. Smith searches for answers in Of Moths and Men: Intrigue, Tragedy & the Peppered Moth by Judith Hooper.”