The Peppered Moth Story: Vindicated!

by Richard Peachey

To the great relief of evolutionists everywhere, the beleaguered "peppered moth story" has now been reinstated to its former status of credible evidence for evolution by natural selection.

Up to 1998, many evolutionists had considered the peppered moth prime evidence for the Darwinian process. Within a few decades in the late 1800s, some British populations of this insect had changed from mostly light-coloured to predominantly dark. Oxford researcher Bernard Kettlewell called this "the most striking evolutionary change ever witnessed by man." Pulitzer prize-winning science writer Jonathan Weiner referred to the story as "the single best-known evolution watch in history." Evolutionary biologist Paul Brakefield, now at Cambridge, enthused: "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."

But the November 5, 1998 issue of Nature cast a disturbing pall over the case of the peppered moth. (One could say that the peppered moth story itself went from "mostly light-coloured to predominantly dark"!). Esteemed evolutionary biologist Jerry Coyne, of the University of Chicago, in a review of a book by peppered moth expert Michael Majerus, wrote emotion-laden sentences like these (bold print within the following quotation indicates emphasis added):

From time to time, evolutionists re-examine a classic experimental study and find, to their horror, that it is flawed or downright wrong. . . . 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. . . .

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 [the light-coloured] typica occurred well before lichens recolonized the polluted trees, and that a parallel increase and decrease of the melanic [dark] 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.

Ever since Coyne's article, evolutionists have been gnashing their teeth. Some have seen fit to denounce Coyne for what he wrote. (See, for example, this post by little-known entomologist Donald Frack.) Michael Majerus indulged in some of that denunciation (he cited approvingly from Frack's writeup), but he also undertook to revisit Kettlewell's experiments and re-do his procedures. That effort has now borne fruit. Majerus died in 2009, but his data have been written up by other lepidopterists in an article titled "Selective bird predation on the peppered moth" (http://rsbl.royalsocietypublishing.org/content/early/2012/01/27/rsbl.2011.1136.full.pdf+html). And Jerry Coyne himself has acknowledged this work in an engaging writeup titled "The peppered moth story is solid" (http://whyevolutionistrue.wordpress.com/2012/02/10/the-peppered-moth-story-is-solid/).

One certainly has to give credit where it is due. The peppered moth story is a highly interesting tale, and evolutionists have persevered to overcome criticism from within their own ranks as well as from outside. But their accomplishment must also be kept in perspective. The change that took place was all within one species. Nobody disputes that. The moths are evidence of evolution only if evolution is defined as a change in allelic (gene) frequencies — something that no one doubts takes place! As leading creationist John Morris had already written, prior to 1998:

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.


Additional Notes

• The article above serves as an update to my earlier discussion on Biston betularia, "The Peppered Moth Story: Prime Example of Evolution?" Most of the argumentation in that article remains current. In my view, only the section regarding Coyne/Majerus's critique of Bernard Kettlewell's experiments, and the appendix providing detailed citations from Coyne's 1998 article, required updating.

• Jerry Coyne's change of mind re the peppered moth story was brought to my attention by a retired UBC physicist, an evolutionist. I appreciate receiving the information, and acknowledge that he will likely disagree with my conclusion on the current value of the peppered moth story for evolutionists.

• Jerry Coyne has given no indication of backtracking from his original criticisms of Kettlewell. (That is, he clearly believes his critique was fully justified at the time he wrote it, in 1998.) In his recent update (http://whyevolutionistrue.wordpress.com/2012/02/10/the-peppered-moth-story-is-solid/), he makes the following noteworthy comments along such lines (bold print indicates emphasis added):

I was a notorious critic of Kettlewell’s experiments, and in a review in Nature of a book on melanism by Michael Majerus (download the book review “Not black and white” here), I suggested that Kettlewell’s experiments were so poorly designed that their results couldn’t be taken seriously. This, combined with the absence of much information on where the moths really rested during the day (when they are subject to bird predation), suggested to me that the Biston story was weaker than presented in textbooks, and needed more attention and—especially—more research. In my review, I wrote the following assessment, which was widely cited, especially by creationists:

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.

This drew not only the ire of British ecological geneticists, who thought I was both unfair and unnecessarily dismissive of a classic story (I stood by my guns here), but predictably attracted creationists and other evolution-deniers, who found in the weaknesses of the Biston story a lack of evidence for natural selection (ignoring all the other cases that were well supported), and, indeed, a conspiracy by evolutionists to prop up a tale they knew was wrong! . . . Kettlewell was not a fraud, just a naturalist who wasn’t that good at experimental design.

Despite the defensiveness of British evolutionists, I think my criticisms carried some weight, because Cambridge biologist Michael Majerus decided to repeat Kettlewell’s experiments, but doing them correctly this time. . . .

The authors [of the published study based on Majerus's data] conclude:

Factors other than predation have often been argued to play a substantial role in the rise and subsequent post-industrial fall of melanism in Biston. . . . Nonetheless, with this new evidence added to the existing data, it is virtually impossible to escape the previously accepted conclusion that visual predation by birds is the major cause of rapid changes in frequency of melanic peppered moths. . . . These new data answer criticisms of earlier work and validate the methodology employed in many previous predation experiments that used tree trunks as resting sites . . . . The new data, coupled with the weight of previously existing data convincingly show that ‘industrial melanism in the peppered moth is still one of the clearest and most easily understood examples of Darwinian evolution in action’.

I am delighted to agree with this conclusion, which answers my previous criticisms about the Biston story. But we have to remember that the evidence for natural selection never rested entirely—or even substantially—on the bird predation experiments, but rather on the datasets documenting allele frequency changes that were consistent, parallel on two continents, and then reversed when the environment changed. What was important about the bird-predation experiments (especially the one discussed here) is that they identified the agent of selection.

• Coyne's commentary (above) draws on a scientific research article that uses data acquired by the late Michael Majerus. The article, published online by the British Royal Society, was authored by noted biologists L. M. Cook, B. S. Grant, I. J. Saccheri, and J. Mallet. The title is "Selective bird predation on the peppered moth: the last experiment of Michael Majerus" (http://rsbl.royalsocietypublishing.org/content/early/2012/01/27/rsbl.2011.1136.full.pdf+html). This research paper also takes proper note of (and does not deny the validity of) the criticisms raised by Coyne and others (bold print indicates emphasis added):

Recently, these classical explanations of melanism were attacked, and there has been general scepticism about birds as selective agents. Experiments and observations were accordingly carried out by Michael Majerus to address perceived weaknesses of earlier work. Unfortunately, he did not live to publish the results, which are analysed and presented here by the authors. Majerus released 4864 moths in his six-year experiment, the largest ever attempted for any similar study. There was strong differential bird predation against melanic peppered moths. Daily selection against melanics (s ≃ 0.1) was sufficient in magnitude and direction to explain the recent rapid decline of melanism in post-industrial Britain. These data provide the most direct evidence yet to implicate camouflage and bird predation as the overriding explanation for the rise and fall of melanism in moths. . . .

Melanics were long believed to be advantageous in the face of bird predation against bark resting sites darkened by soot pollution . . ., a form of camouflage . . . . Classic experiments in the mid-twentieth century proved that birds attacked the moths. Furthermore, resting moths that failed to match their background were more vulnerable to bird predation in cage experiments. . . . Mark–recapture studies of live moths, as well as many bird predation experiments using dead moths pinned to tree trunks, supported the hypothesis that birds were the agents of selection on melanism. . . .

However, these procedures have drawbacks . . ., and critiques were increasingly aired. . . . In experiments, moths were often placed on tree trunks, which were argued to be abnormal resting sites; pinned carcases seemed particularly unnatural. Moths were often released at greatly inflated densities, potentially increasing predation. Reared insects from geographically distant sources were often used to supplement wild individuals, and may not have behaved as naturally in recapture experiments as wild moths. By the 1990s, considerable scepticism became evident. . . . Factors other than bird predation (e.g. migration, physiological differences among genotypes) were argued to play a substantial role in the evolution of melanism in Biston. . . . Caveats about the predation experiments discussed in Majerus's book . . ., critiques by other biologists, as well as points made particularly forcefully in [Coyne's] review of the Majerus book . . ., were soon exploited by non-scientists to promote an anti-evolution agenda and to denigrate the predation explanation. . . . Kettlewell's original mark–recapture experiments were later argued to be fraudulent . . . (quite groundlessly . . .). Judith Hooper, author of this claim, also suggested that bats rather than birds might be the agents of selection [19]. Soon, both the public in general and even evolutionary biologists began to doubt the bird predation story (electronic supplementary material, S1 . . .).

Majerus therefore decided to make key new field observations, and he also designed and carried out a massive new predation experiment, the largest predation experiment ever performed (4864 released moths) to answer his own and other criticisms of earlier work. . . . Previous authors had argued that moths rarely rested on tree trunks during the day, and that many predation experiments employing tree trunks were therefore unnatural. In these new observations by Majerus, 35% of the 135 moths observed, both melanic and typical, were indeed found resting on tree trunks. . . .

Factors other than predation have often been argued to play a substantial role in the rise and subsequent post-industrial fall of melanism in Biston. . . . Nonetheless, with this new evidence added to the existing data, it is virtually impossible to escape the previously accepted conclusion that visual predation by birds is the major cause of rapid changes in frequency of melanic peppered moths. . . . These new data answer criticisms of earlier work and validate the methodology employed in many previous predation experiments that used tree trunks as resting sites. . . . The new data, coupled with the weight of previously existing data convincingly show that ‘industrial melanism in the peppered moth is still one of the clearest and most easily understood examples of Darwinian evolution in action’. . . .

• It bears repeating that evolutionists are still wanting to present the peppered moth story as if it were evidence for significant "evolution." Certainly they are to be given credit for cleaning up a case study that had been widely criticized (by both creationists and evolutionists!). But multiplying (or clarifying) such intraspecific reversible instances of "Darwinian evolution in action" will never serve to persuade creationists (or other people of common sense) that life arose from nonliving chemicals or that unicellular organisms evolved into human beings.

Consider these discussions:

"Do Examples of 'Microevolution' Provide Support for Macroevolution?" <http://www.creationbc.org/index.php?option=com_content&view=article&id=130>

"Do Evolutionists Avoid the Terms 'Macroevolution' and 'Microevolution'?" <http://www.creationbc.org/index.php?option=com_content&view=article&id=149>