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In Context #8 (Fall, 2002, pp. 8-10); copyright 2002 by The Nature Institute

The Tyranny of a Concept:
The Case of the Peppered Moth

Craig Holdrege

The peppered moth is one of the most celebrated textbook examples showing natural selection in action. A dark variety of the otherwise light-colored peppered moth appeared in England in the mid-nineteenth century and its numbers continued to grow in the industrial areas of England. In the 1950s Bernard Kettlewell set out to show experimentally that the dark variety of moth prospered because it was better camouflaged against the soot-darkened, lichen-free tree trunks in industrial areas. He released light and dark peppered moths onto tree trunks in the polluted forests. He not only recaptured (via lamps at night) proportionately more dark moths than light moths, but also observed birds feeding predominantly on the light moths. He obtained exactly the opposite results in an unpolluted forest with much lighter tree trunks. He concluded that natural selection via bird predation in polluted forests was causing peppered moth populations to evolve from the light to the dark variety.

This story quickly became a standard textbook example of Darwinian evolution in action. No other example showed so simply and elegantly that variation and natural selection were the two primary factors guiding the evolution of organisms. Like many other biology teachers around the globe, I taught the case of the peppered moth.

In the mid 1980s I had a peppered moth epiphany. A peppered moth researcher described in an article that during twenty-five years of research he had found exactly two peppered moths resting on trees. How could that be? How could the moths' color in relation to the tree bark figure so prominently in evolution if the moths almost never rested on trees? Something strange was going on here. What had I been teaching? I began a search of the primary literature and over the next decade the solid and gleaming edifice of the peppered moth story dissolved into a shimmering illusion. What the textbooks were presenting and what all of us teachers were teaching was simply not true. This led me to write an article on the peppered moth (Holdrege, 1999). During the process I discovered that a growing number of scientists were writing about the same problems I'd discovered. The time was ripe for the myth of the peppered moth to be shattered.

A free-lance science writer, Dick Teresi, having read my article in Whole Earth, became interested in the story and interviewed me for an article in the New York Times Magazine. The article was never published. (At the very moment it was to appear, the Kansas School board "outlawed" the teaching of evolution, and an article critical of a central Darwinian example might have given the appearance that the Times supports creationism. Creationists have latched onto the peppered moth in their efforts to discredit evolutionary thought.) But Judith Hooper, a writer and Teresi's wife, became fascinated with the moth story and embarked upon a couple of years of research, the fruits of which have now appeared in her new book, Of Moths and Men (Hooper, 2002). I was thrilled that my article had such ripple effects and that Hooper corroborated what I'd found. As she shows in great detail, the peppered moth illustrates how an idea can distort the way facts are seen. Anyone who is interested in learning about how science actually works should read this book.

Hooper describes how Kettlewell—who, while eccentric, was a gifted naturalist—was part of E.B. Ford's school of "ecological genetics" at Oxford. Ford and his colleagues strived to demonstrate that natural selection—at a time when the concept was under attack by other biologists—was a primary driving force in evolution. They concentrated on moths and butterflies. Kettlewell's experiments became their showcase example, "proving" the efficacy of natural selection. Through Hooper's research it becomes clear that the Ford school had a mission, and when you're on a mission you marshal all the facts that support your idea and leave everything else by the wayside. You have a counterintrigue for every possible criticism and you build up a well-fortified edifice.

Moths on blackened tree trunk

Light and dark forms of the peppered moth were photographed against the trunk of an oak tree blackened by the polluted air of Birmingham, England. The dark form is hard to see, while the light form is conspicuous. (See next illustration for credit.)

For example, Kettlewell knew that peppered moths are rarely found on tree trunks, are never found in high concentrations, and never fly during the day. But in his experiment he placed the moths on tree trunks, two to four specimens per tree (much higher than any normal concentration might be), and exposed them during the day. As moth expert Ted Sargent put it, Kettlewell's experiment was "a bird feeder" and we shouldn't be surprised at the outcome. The experiment creates a highly artificial situation and has splendid results. The problem is, they tell you nothing about what goes on in nature. And an experienced naturalist did the experiment! This shows, I believe, how a person (and a whole segment of the scientific community) can be swayed by an idea that colors everything they see.

Unfortunately, natural selection is still an idea that tyrannizes many minds. For every phenomenon a ready explanation is quick at hand to "explain" why this bird has a short beak, that bird a bright plumage, or the giraffe its long neck. As the recently deceased evolutionary scholar Stephen Jay Gould put it, textbooks, biology classes, and journal articles are rife with "just so" stories à la Kipling. They tell a neat story; the problem is that there are many other equally "compelling" stories we could use to explain the same phenomena.

Moths on
unpolluted tree trunk

The same two forms shown in the previous illustration, but now photographed against the lichen-covered trunk of a tree in an unpolluted area of England. The light form is hard to see; the dark form is very conspicuous. (Both illustrations from H.B.D. Kettlewell's 1959 article, "Darwin's Missing Evidence." In Evolution and the Fossil Record. San Francisco: W.H. Freeman and Company, 1978, pp. 28-33.)

How strongly the idea of natural selection has taken hold of scientists' minds is well illustrated by two negative reviews of Hooper's book in the premier English-language scientific journals Nature (4 July 2002, p. 19ff.) and Science (9 August 2002, p. 940ff.). The reviewers—both evolutionary scientists and one a peppered moth researcher—take Hooper to task on various issues, but one criticism that both reviewers brought forth interested me most. They stated that Hooper was failing to make an essential distinction: although the peppered moth example may not show the exact agent (mechanism) that causes the observed changes, it clearly does show the fact of evolution through natural selection. As the reviewer in Nature stated, the peppered moth is a "compelling case for natural selection. No other force could have caused such striking and directional change."

The problem is that the concept of selection includes at its core a selective agent. You can't speak about selection without there being something doing the selecting. The "force" of natural selection is nothing more and nothing less than the specific agent(s) of selection; to speak of the force of natural selection in general terms is vacuous (and borders on the mystical). Until scientists know that predation by birds, the effects of air pollution on larvae, or other factors are at work, the concept of natural selection has no concrete content. Why, then, do both reviewers stress the fact of natural selection, when no agent of selection has been clearly determined?

It's as if at all costs they want to save natural selection and feel that if they lose it, they lose all grip on evolution. Natural selection is felt to be the best weapon against the threat of creationism, and if they don't have this weapon, then what? But evolution—as opposed to a narrow, Darwinian interpretation of it—doesn't stand or fall with the idea of natural selection; the evidence that organisms change over time is not at risk. Both Darwinians and creationists are stuck in ideologies—fixed mental frameworks through which they interpret the phenomena. Wouldn't it be much more in the spirit of scientific inquiry to leave different avenues of explanation open as one continues to investigate the phenomena? Scientists should enjoy uncertainty and discovery as much as they enjoy fitting facts into a neat conceptual framework.

A book like Judith Hooper's can be a help to overcome the tyranny of ideas by putting the development of scientific concepts into a human context. We see the rigidity of an idea that resists all attempts at modification despite a legion of facts weighing against it. What might have begun as a useful idea becomes despotic. We all, I think, can at times find ourselves in the grip of an idea. But as we become more aware of the role we play in forming the ideas through which we illuminate the world, we are less likely to be ruled by them. Sometimes a prick of conscience will give us pause to stop and ask, "wait a minute, who's in the driver's seat here—an idea I've come to serve, or my search for truth?"

In my own life the case of the peppered moth has been very significant. Never again will I approach scientific "evidence" with a na´ve surety that what's presented can be taken at face value. In actual practice, what's presented is a web of ideas, emotions and phenomena woven into a fabric that warrants wakeful scrutiny. Unraveling this web is always a revelatory undertaking.


Holdrege, Craig (1999). "The Case of the Peppered Moth Illusion," Whole Earth (Spring), pp. 66-69. A longer version with references is on our website under the title, "Science as Process or Dogma? The Case of the Peppered Moth" ( It was originally published in Elemente der Naturwissenschaft #70 (1999), pp. 39-51.

Hooper, Judith (2002). Of Moths and Men. New York: W. W. Norton.

Original source: In Context #8 (Fall, 2002, pp. 8-10); copyright 2002 by The Nature Institute

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