Peppered moths, evidence for natural selection and evolution

In 2007, as part of the continuing series explaining evolution and how it should be taught in public schools, I wrote this piece on the controversy over peppered moths as part of the basic information pages for this blog.  With modest edits, I repeat it here, both to re-emphasize the importance of the examples, and to make it easier for internet search engines to index the material.

Among creationists, in the past decade or so it has become popular to claim that much of evolution is less than sound science. In order to support that claim, specific points of evolution theory are often distorted, and specific evidence that strongly supports the theory are questioned and denied.

In a famous hoax, Jonathan Wells wrote a book, Icons of Evolution, in which he claimed ten popular stories about evolution were false.

Wells spent a chapter denying the work of H. B. Kettlewell — the famous peppered moth research, in which Kettlewell discovered a classic case of natural selection at work over a 150-year period in England. When air pollution darkened the bark of trees, peppered moths in industrialized areas with the darkened trunks, also darkened. Kettlewell ran several experiments to see whether predation by birds might be a driver of this selection event, and concluded that the lighter moths stood out on darkened tree bark, and that it was likely that predation by birds or other moth predators pushed the rise of the darker moths.

Kettlewell’s conclusions were spectacularly borne out when cleaner air lightened the trunks of the trees, and lighter moths reappeared. The selective pressure ran back toward lighter moths.

The two most common variants of the peppered moth, the one on the left showing how it got is name, and the one on the right showing the variant that became more common when air pollution darkened the bark on trees where the moth rests.  BBC image

The two most common variants of the peppered moth, the one on the left showing how it got is name, and the one on the right showing the variant that became more common when air pollution darkened the bark on trees where the moth rests. BBC image

Kettlewell’s research was groundbreaking in its pioneering of new ways to study evolution in the wild. But because he was at the cutting edge, questions arose about the exact nature of his conclusions. Kettlewell tried several different methods to count moth predation, finally settling on a system of release and recapture, and counting the moths that were not recaptured as casualties to the predator. Kettlewell released moths in the presence of English titmice, who promptly found moths colored wrongly to hide, and ate them. Tits are not the chief predators of these moths, some argue, and critics wondered whether Kettlewell could accurately conclude what the predator was. Ultimately, it has become clear that Kettlewell’s conclusions are accurate regardless the predator. Scientists like Majerus and Jerry Coyne — fierce rivals now, showing the unanimity of science support for Kettlewell’s conclusions across the spectrum of science views — urge more research to refine what we know about the moths.

Critics claimed some of the steps in some of the experiments as faulty, and extrapolated that the entire conclusions are faulty. In reality, because Kettlewell got similar results with different methods, his conclusions are more robust.

When I took Wells’ chapter on moths and tracked down the citations, I discovered that each person he cited disputed Wells’ conclusions — some quite violently.  The hoax, it turns out, is Wells’s claim that the story is false.

Over the past decade, while Wells and the Discovery Institute have continued their assault on science, some of the refutations of his work have fallen by the wayside.

In this page, I hope to preserve the arguments showing Wells’ work’s problems, and preserve some of the publications that have become difficult to find.

Creationism controversy in Pratt, Kansas

In 1999 the school board in Pratt, Kansas, considered biology curriculum and book changes. In what now appears to be practice for future fights, the Discovery Institute of Seattle, Washington, descended on Pratt with Jonathan Wells and others , urging the school board to dilute evolution in the curriculum or eliminate it, and using a variety of unusual arguments on the science to buttress their claims that the science of evolution was somehow in question.

Specifically, Wells introduced his claims on Kettlewell’s work. When they heard of the incident, some researchers scrambled to present the view of science. Bruce Grant of William and Mary College, and Jerry Coyne of the University of Chicago, wrote letters to the local newspaper, The Pratt Tribune, defending Kettlewell’s work and questioning Wells’ tactics and conclusions. Evolution is solid science, they said, and should be taught undiluted to students.

These letters were available on the internet at the time, but have recently become difficult to find. I reproduce them here:

Dr. Jerry Coyne is professor of ecology and evolution at the University of Chicago.

The Pratt Tribune, December 06, 2000

Original: Accessed in 2003

Jerry A. Coyne: Criticism of moth study no challenge to evolution

I have learned that the Pratt school board, apparently responding to creationist pressure, has recently revised its tenth-grade biology curriculum to include material that encourages students to question the theory of evolution. In reading the standards, I see that one of my articles – an article constantly misrepresented by creationists – is included as a supplementary reading used to cast doubt on evolution.

Dr. Jerry Coyne's Skeptic Trump card.  He is a professor at the University of Chicago.

Dr. Jerry Coyne’s Skeptic Trump card. He is a professor at the University of Chicago.

Except for a few creationist dissenters, the community of professional biologists has long accepted evolution as an essential theory supported by innumerable pieces of evidence. To make students think otherwise is as harmful as urging them to question the value of antibiotics because there are a few people who believe in spiritual healing.

My article appended to the Pratt standards is a re-evaluation of a classic evolutionary story in which rapid changes in the proportions of color forms of peppered moths occurred in only about 100 years. This evolutionary change is thought to be a response to air pollution, changes in the colors of trees, and increased bird predation. My only problem with the peppered-moth story is that I am not certain whether scientists have identified the precise agent causing the natural selection and evolutionary change. It may well be bird predators, but the experiments leave room for doubt.

Creationists such as Jonathan Wells claim that my criticism of these experiments casts strong doubt on Darwinism. But this characterization is false. All of us in the peppered moth debate agree that the moth story is a sound example of evolution produced by natural selection. My call for additional research on the moths has been wrongly characterized by creationists as revealing some fatal flaw in the theory of evolution.

In reality, the debate over what causes natural selection on moths is absolutely normal in our field. It is not uncommon for scientists to reexamine previous work and find it incomplete, or even wrong. This is the normal self-correcting mechanism of science. Textbook examples may be altered as additional data are found. Creationists, on the other hand, neither air their disagreements in public or admit that they were wrong. This is because their goal is not to achieve scientific truth, but to expel evolution from the public schools.

It is a classic creationist tactic (as exemplified in Wells’ book, “Icons of Evolution”) to assert that healthy scientific debate is really a sign that evolutionists are either committing fraud or buttressing a crumbling theory. In reality, evolution and natural selection are alive and well, with new supporting evidence arriving daily.

I strongly object to the use of my article by the Pratt school board to cast doubt on Darwinism. And I feel sorry for the students who are being misled by creationists into doubting one of the most vigorous and well-supported theories in biology.

Jerry A. Coyne
Professor of Ecology & Evolution
The University of Chicago

Dr. Bruce Grant was professor of biology at William and Mary University, now emeritus.

The Pratt Tribune, December 13, 2000
Accessed 2003

Bruce Grant: Charges of fraud misleading

In recent weeks your newspaper has printed letters debating revisions in high school biology curricula. Some of the correspondents have leveled charges of fraud directed at evolutionists for attributing changes in the colors of peppered moths to natural selection. As I am one of the evolutionary biologists who study peppered moths, I feel obliged to comment. Charges of fraud cannot be left unchallenged.

Some background about peppered moths is necessary. The common form of this moth species is pale gray. About 150 years ago, a black specimen was discovered near an industrial city in England. Over the years, the black (melanic) form became ever more common as the pale form became rare. By 1900 the black form exceeded 90 percent in peppered moth populations throughout the industrialized regions of England. The phenomenon was dubbed industrial melanism.

Bruce Grant, Professor of Biology, Emeritus, College of William and Mary

Bruce Grant, Professor of Biology, Emeritus, College of William and Mary

Because people knew that birds eat insects, scientists as early as 1896 suspected that birds were eating the different color forms of peppered moths selectively based on their degree of conspicuousness in habitats variously blackened by industrial soot. Extensive experimental work supports this view, although questions remain. Other scientists proposed that moths responded to the presence of pollutants by developing darker body colors. We now know from genetic analysis that the colors of adult peppered moths are determined by genes; thus, the changes in the percentages of pale to black moths over generations reflect changes in the genetic makeup of moth populations.

As industrial practices have changed in many regions, we have observed black moths plummet from 90 percent to 10 percent in the just the past few decades. Once again, we have observed significant genetic changes occur in moth populations. Evolution is defined at the operational level as genetic change over time, so this is evolution. Of the several factors known to produce evolutionary change, only natural selection is consistent with the patterns of the changes we see occurring in moth populations. Evolution examined at this level is as well established as any fact in science.

We still have work to do. We do not all agree about the relative roles of contributing factors, such as the flow of genes between moth populations in different regions, the importance of lichens on trees, where on trees moths might hide from predators, how important is differential predation, and so on. As in any branch of science, participants endlessly debate interpretations. Such wrangling is the norm, and it stimulates additional research. That is how we make progress.

Our debates have never been secret. For recent overviews of the controversies, please see [now at] or [ now, subscription required]. Yet, unwarranted charges of fraud, fakery and cover-ups repeatedly appear in letters printed in newspapers. In your paper, Ms. Katrina Rider “asserts” the peppered moth story is a hoax. She conveys the impression that dead moths were glued to trees as part of a conspiracy of deception. She seems unaware that moths were glued to trees in an experiment to assess the effect of the density (numbers) of moths on the foraging practices of birds. Taken out of the context of the purpose of the experiment, the procedure does sound ludicrous.

But, should we blame Ms. Rider for her outrage upon learning that moths were glued to trees? No. Instead, I blame Dr. Jonathan Wells, who wrote the article she cites as her source of information. While he has done no work on industrial melanism, he has written opinion about the work. To one outside the field, he passes as a scholar, complete with Ph.D. Unfortunately, Dr. Wells is intellectually dishonest. When I first encountered his attempts at journalism, I thought he might be a woefully deficient scholar because his critiques about peppered moth research were full of errors, but soon it became clear that he was intentionally distorting the literature in my field. He lavishly dresses his essays in quotations from experts (including some from me) which are generally taken out of context, and he systematically omits relevant details to make our conclusions seem ill founded, flawed, or fraudulent. Why does he do this? Is his goal to correct science through constructive criticism, or does he a have a different agenda? He never mentions creationism in any form. To be sure, he sticks to the scientific literature, but he misrepresents it. Perhaps it might be kinder to suggest that Wells is simply incompetent, but I think his errors are by intelligent design.

Bruce Grant
Professor of Biology
College of William and Mary
Williamsburg, Virginia


Update, September 2012: Creationists appear never to learn. The hoaxed “controversy” on peppered moths continued well after these exchanges back in 2000, which I collected when creationists tried to block science in textbooks in the great Texas State Board of Education Wars of 2003. Even after losing that fight, creationists continued to carp.

Publication of Of Moths and Men, nominally a history of Kettlewell and his experiments, revived the creationist claims of error — without evidence, as the author of the book noted, but since when have creationists ever changed course because of evidence?

In the midst of the flap, Michael Majerus went back to basics. He re-ran Kettlewell’s experiments, essentially. Of course, he discovered Kettlewell was right. I blogged about it earlier, but I failed to update this post. See these other posts at Millard Fillmore’s Bathtub:

Also, WordPress has improved since 2007; it finds relevant links, and related stories through Zemanta. I’ve added links above, and related articles below.

More, and Further Reading:

3 Responses to Peppered moths, evidence for natural selection and evolution

  1. […] Peppered moths, evidence for natural selection and evolution (Millard Fillmore’s Bathtub) […]


  2. Well done, Ed. As usual.


  3. Ed Darrell says:

    “Wintery Knight” will deny reality until the aurochs come home. Will a link to this article ever show up there? Don’t bet on it.


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