The Peppered Moth Story: Prime Example of Evolution?
With a special Appendix: University of Chicago evolutionist Jerry Coyne on his discovery of the many flaws in this "icon of evolution"! (following the conclusion of this article)
by Richard Peachey
IMPORTANT NOTE: The argumentation in the article below requires updating in light of new experimental work by lepidopterist Michael Majerus, and published by others in 2012, after Majerus's death. The updating involves specifically the questioned experimental work by Bernard Kettlewell. See our newer article, "The Peppered Moth Story: Vindicated!"
The peppered moth (Biston betularia) has often been used as a prime example of "evolution in action." This case first came to prominence in 1959 when H. B. D. (Bernard) Kettlewell published a key Scientific American article. 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.
• As a result of the Industrial Revolution, however, 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 survived 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 and historian of science Stephen Jay Gould has noted:
". . . 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) and placed 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 35f.).
Aside from the issue of the validity of Kettlewell's experiments, a number of evolutionists have 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).
Berry, R. J. 1990. "Industrial melanism and peppered moths (Biston betularia (L.))." Biological Journal of the Linnean Society 39:301-322.
Blaney, Walter M. 1976. How Insects Live. Oxford: Elsevier-Phaidon.
Brakefield, Paul M. 1998. "Receding black moths." Trends in Ecology and Evolution 13(9):376.
California State Board of Education. 1990. Science Framework.
Coyne, Jerry A. 1998 (Nov 5). "Not black and white." Nature 396:35f.
Gould, Stephen Jay. 1997. "The Paradox of the Visibly Irrelevant." Natural History 106(11):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 200(3):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.
For further reading:
"Evolutionists and E x t r a p o l a t i o n" <http://www.creationbc.org/index.php?option=com_content&view=article&id=64>
APPENDIX: Extract from University of Chicago evolutionist Jerry Coyne's classic article on flaws in the peppered moth story, "Not black and white." 1998 (Nov 5) Nature 396:35f. (Coyne's article is a book review of Michael Majerus's Melanism: Evolution in Action.) Bold print indicates emphasis added.
[page 35] "From time to time, evolutionists re-examine a classic experimental study and find, to their horror, that it is flawed or downright wrong. We no longer use chromosomal polymorphism in Drosophila pseudoobscura to demonstrate heterozygous advantage, flower-colour variation in Linanthus parryae to illustrate random genetic drift, or the viceroy and monarch butterflies to exemplify Batesian mimicry. Until now, however, 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. The reexamination of this tale is the centrepiece of Michael Majerus's book, Melanism: Evolution in Action. Depressingly, Majerus shows that this classic example is in bad shape, and, while not yet ready for the glue factory, needs serious attention.
"According to the standard textbook litany, before the mid-nineteenth century, all B. betularia in England were white moths peppered with black spots, a form called typica. Between 1850 and 1920, typica was largely replaced by a pure black form (carbonaria) produced by a single dominant allele, the frequency of which rose to nearly 100% in some areas. After 1950, this trend reversed, making carbonaria rare and typica again common. These persistent and directional changes implied natural selection. In a series of studies, this conclusion was verified by several investigators, most prominently Bernard Kettlewell of Oxford.
"According to these workers, the evolution of colour was caused by birds eating the moths most conspicuous on their normal resting site — tree trunks. The increase in black moths was attributed to pollution accompanying the rise of heavy industry. A combination of soot and acid rain darkened trees by first killing the lichens that festooned them and then blackening the naked trunks. The typica form, previously camouflaged on lichens, thus became conspicuous and heavily predated, while the less visible carbonaria enjoyed protection and increased in frequency. After the passage of the Clean Air Acts in the 1950s, trees regained their former appearance, reversing the selective advantage of the morphs. This conclusion was bolstered by a geographical correlation between pollution levels and morph frequencies (carbonaria was most common in industrial areas), and most prominently by Kettlewell's famous experiments which showed that, after releasing typica and carbonaria in both polluted and unpolluted woods, researchers recaptured many more of the cryptic than of the conspicuous form. The differential predation was supported by direct observation of birds eating moths placed on trees. Finally, Kettlewell demonstrated in the laboratory that each form had a behavioural preference to settle on backgrounds that matched its colour.
"Criticisms of this story have circulated in samizdat for several years, but Majerus summarizes them for the first time in print in an absorbing two-chapter critique (coincidentally, a similar analysis [Sargent et al., Evol. Biol. 30, 299-322; 1998] has just appeared). Majerus notes that the most serious problem is that B. betularia probably does not rest on tree trunks — exactly two moths have been seen in such a position in more than 40 years of intensive search. The natural resting spots are, in fact, a mystery. This alone invalidates Kettlewell's release–recapture experiments, as moths were released by placing them directly onto tree trunks, where they are highly visible to bird predators. (Kettlewell also released his moths during the day, while they normally choose resting places at night.) The story is further eroded by noting that the resurgence of typica occurred well before lichens recolonized the polluted trees, and that a parallel increase and decrease of the melanic form also occurred in industrial areas of the United States, where there was no change in the abundance of the lichens that supposedly play such an important role.
"Finally, the results of Kettlewell's behavioural experiments were not replicated in later studies: moths have no tendency to choose matching backgrounds. Majerus finds many other flaws in the work, but they are too numerous to list here. I unearthed additional problems when, embarrassed at having taught the standard Biston story for years, I read Kettlewell's papers for the first time.
"Majerus concludes, reasonably, that all we can deduce from this story is that it is a case of rapid evolution, probably involving pollution and bird predation. I would, however, replace 'probably' with 'perhaps'. B. betularia shows the footprint of natural selection, but we have not yet seen the feet. Majerus finds some solace in his analysis, claiming that the true story is likely to be more complex and therefore more interesting, but one senses that he is making a virtue of necessity. My own reaction 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. . . .
[page 36] "What can one make of all this? Majerus concludes with the usual call for more research, but several lessons are already at hand. First, 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 evolution. There are many studies more appropriate for use in the classroom, including the classic work of Peter and Rosemary Grant on beak-size evolution in Galapagos finches. It is also worth pondering why there has been general and unquestioned acceptance of Kettlewell's work. Perhaps such powerful stories discourage close scrutiny. Moreover, in evolutionary biology there is little payoff in repeating other people's experiments, and, unlike molecular biology, our field is not self-correcting because few studies depend on the accuracy of earlier ones. Finally, teachers such as myself often neglect original papers in favour of shorter textbook summaries, which bleach the blemishes from complicated experiments. . . ."