THE PEPPERED MOTH STORY:
PRIME EXAMPLE OF EVOLUTION?

The peppered moth (Biston betularia) has often been used as a prime example of “evolution in action.” This case first came to prominence when H. B. D. (Bernard) Kettlewell published a key Scientific American article in 1959. The story can be summarized as follows: Up to about 1850, the peppered moth populations in England were mostly light-coloured, with no more than a very small minority of melanic (dark-coloured) individuals. During the daytime, these moths would rest on light lichen-covered tree trunks. The light-coloured moths would be well camouflaged, but any dark moths would be easily seen by predators (birds) and would be picked off (“selected out” by “natural selection”). The light moths survived to pass on their genetic traits, and continued as the predominant form. But as a result of the Industrial Revolution, tree trunks became darkened by soot particles, and the light-coloured lichens covering the trees were killed by polluted air containing sulfur dioxide and other contaminants. No longer was it advantageous for peppered moths to be light-coloured. Now the melanic moths were better camouflaged against the darkened tree trunks; the predators began to eat more of the light-coloured moths, and the dark ones lived to reproduce, passing on their traits to their offspring. By the end of the 19th century, the vast majority of the peppered moths in regions affected by the Industrial Revolution were dark-coloured. A huge shift in the population’s average colour had taken place, from light to dark, within a few decades. Kettlewell called this “the most striking evolutionary change ever witnessed by man” (Kettlewell 48).

Many other evolutionists have made similarly enthusiastic comments:

The peppered moth story is “the single best-known evolution watch in history” (Weiner 271).
“The peppered moth, Biston betularia, is rightly regarded as a striking example of adaptive change through natural selection and as one of the foundation stones for the modern synthesis of evolutionary theory” (Brakefield 376).
“Typical [light], intermediate and melanic [dark] forms of the Peppered moth Biston betularia . . . furnish one of the best known examples of evolution taking place before our eyes” (Blaney 137).
“One of the most intensively studied examples of cryptic coloration [i.e., camouflage] is that of the peppered moth Biston betularia. . . . This is perhaps the most frequently quoted example of evolution in action” (Rowland-Entwistle 76).
“Organisms can adapt to moderate amounts of certain forms of pollution: the spread of industrial melanism [darkening] in peppered moths and other insects, and of heavy metal tolerance . . . in certain plants, have provided some of the most convincing evidence for evolution by natural selection” (Skelton 954).

In 1956, a Clean Air Act was passed in Britain. Within about fifteen years, studies were showing that the percentage of light-coloured moths in many populations was increasing again (Berry 308). The original colour change, though attention-grabbing, had been simply a short-term fluctuation, proving nothing about large-scale evolution. As Harvard paleontologist Stephen Jay Gould comments:

“. . . biologists have documented a veritable glut of cases for rapid and eminently measurable evolution on timescales of years and decades. . . . but, to be visible at all over so short a span, evolution must be far too rapid (and transient) to serve as the basis for major transformations in geological time. Hence, the ‘paradox of the visibly irrelevant’—or, if you can see it at all, it’s too fast to matter in the long run. . . . Most cases of rapid microevolution represent the transient and momentary blips and fillips that ‘flesh out’ the rich history of lineages of stasis. . . . Small local populations and parts of lineages make short and temporary forays of transient adaptation but almost always die out or get reintegrated into the general pool of the species” (Gould 12, 14, 64).

Recently, questions have been raised about Kettlewell’s experimental procedures. He released moths during the day (they normally choose resting places at night), placing them directly onto tree trunks (which is probably not where they normally rest). Later experimenters found no tendency for the moths to choose matching backgrounds. Upon learning of these (and other) flaws in Kettlewell’s experiments, one evolutionary biologist lamented: “Until now . . . the prize horse in our stable of examples has been the evolution of ‘industrial melanism’ in the peppered moth, Biston betularia, presented by most teachers and textbooks as the paradigm of natural selection and evolution occurring within a human lifetime. . . . My own reaction [to the true story] resembles the dismay attending my discovery, at the age of six, that it was my father and not Santa who brought the presents on Christmas Eve. . . . for the time being we must discard Biston as a well-understood example of natural selection in action, although it is clearly a case of [micro]evolution. . . . It is also worth pondering why there has been general and unquestioned acceptance of Kettlewell’s work. Perhaps such powerful stories discourage close scrutiny. . . . our field [of evolutionary biology] is not self-correcting because few studies depend on the accuracy of earlier ones” (Coyne 35-36).

Aside from the issue of the validity of Kettlewell’s experiments, a number of evolutionists have now acknowledged that the peppered moth story is not exciting evidence for evolution on any significant scale; what has happened is really only “microevolution,” defined as a shift in the frequencies of genes within a population. This is not at all the kind of process that could lead to the production of new organs or any sort of large-scale change (“macroevolution”).

“Students should understand that this is not an example of evolutionary change from light-colored to dark-colored to light-colored moths, because both kinds were already in the population. This is an example of natural selection, but in two senses. First, temporary conditions in the environment encouraged selection against dark-colored moths and then against light-colored moths. But second, and just as important, is the selection to maintain a balance of both black and white forms, which are adaptable to a variety of environmental circumstances. This balanced selection increases the chances of survival of the species. This is in many ways the most interesting feature of the evolution of the peppered moth but one that is often misrepresented in textbooks” (California State Board of Education 103).
“We have said that evolution in the present is difficult, if not extremely difficult, to observe. Some biologists maintain that they can not only observe it but also describe it in action; the facts that they describe, however, either have nothing to do with evolution or are insignificant. At best, present evolutionary phenomena are simply slight changes of genotypes within populations, or substitution of an allele by a new one. For example, the mutant carbonaria [dark form] of the birch moth [or peppered moth], Biston betularia, replaces the regular [form] in polluted industrial areas” (Grassé 84).
“The [peppered moth] experiments beautifully demonstrate natural selection — or survival of the fittest — in action, but they do not show evolution in progress, for however the populations may alter in their content of light, intermediate, or dark forms, all the moths remain from beginning to end Biston betularia” (Matthews xi).

As creationist John Morris puts it: “The population shift has been hailed as proof of Darwinian evolution. Probably every student in public education has been taught it. But what really happened? At the beginning, there were light and dark shades. Once the pollution darkened the environment, there were light and dark shades. There are light and dark shades now. Throughout the entire time, both shades existed and comprised a single interbreeding species. There’s no evolution here. . . . the peppered moth[s] demonstrate what creationists have been saying all along. Variation within a specific created type occurs all the time. Natural selection can select the variant best suited for an environment, but natural selection doesn’t create anything new. Why, then, do evolutionists use this as Exhibit No. 1? This, obviously, must be the best evidence they have got” (Morris 4).

References
Berry, R. J. 1990. Industrial melanism and peppered moths (Biston betularia (L.)). Biological Journal of the Linnean Society, Vol. 39, pp. 301-322.
Blaney, Walter M. 1976. How Insects Live. Oxford: Elsevier-Phaidon.
Brakefield, Paul M. 1998. Receding black moths. Trends in Ecology and Evolution, Vol. 13, No. 9, p. 376.
California State Board of Education. 1990. Science Framework.
Coyne, Jerry A. 1998 (Nov 5). Not black and white. Nature, Vol. 396, pp. 35-36.
Gould, Stephen Jay. 1997. The Paradox of the Visibly Irrelevant. Natural History, Vol. 106, No. 11, pp. 12-18, 60-66)
Grassé, Pierre-Paul. 1977. Evolution of Living Organisms: Evidence for a New Theory of Transformation. New York: Academic Press.
Kettlewell, H. B. D. 1959. Darwin’s Missing Evidence. Scientific American, Vol. 200, No. 3, pp. 48-53.
Matthews, L. Harrison. 1971. Introduction to Darwin, Charles. Origin of Species. London: Dent & Sons.
Morris, John D. 1994 (Apr). Do Peppered Moths Prove Evolution? Back to Genesis leaflet, p. 4.
Rowland-Entwistle, Theodore. 1976. The World You Never See: Insect Life. Chicago: Rand McNally.
Skelton, Peter (ed.). 1993. Evolution: A Biological and Paleontological Approach. Workingham, U.K.: Addison-Wesley.
Weiner, Jonathan. 1995. The Beak of the Finch. New York: Vintage Books.