Selective bird predation on the peppered moth: the last experiment of Michael Majerus

wikipedia_peppered_moths.jpgToday a paper came out that should get special attention from evolutionary biologists, evolution educators, and creationism fighters. It is:

Cook, L. M.; Grant, B. S.; Saccheri, I. J.; Mallet, J. (2012). “Selective bird predation on the peppered moth: the last experiment of Michael Majerus.” Biology Letters, Published online before print February 8, 2012. doi: Abstract at Journal, Supplementary Online Material.


Colour variation in the peppered moth Biston betularia was long accepted to be under strong natural selection. Melanics were believed to be fitter than pale morphs because of lower predation at daytime resting sites on dark, sooty bark. Melanics became common during the industrial revolution, but since 1970 there has been a rapid reversal, assumed to have been caused by predators selecting against melanics resting on today’s less sooty bark. 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.

As long-time readers of Panda’s Thumb know, I’ve had an axe to grind about the peppered moth case since the beginning of my serious involvement with creationism-fighting. Back in 2002 I wrote a long review of Jonathan Wells’s creationism/ID book Icons of Evolution for Wells’s strategy was very clever; rather than attacking the science of evolution head-on, he attacked high school biology textbooks. He engaged in a delicate dance of selective citation and quote-mining so as to make it appear that the criticisms of standard textbook examples used to introduce various evolutionary concepts were coming from scientists.

Since everyone, including scientists and science journalists, “knows” that introductory textbooks have problems, more than a few people reacted to Wells’s book with the defensive reaction “well, sure, textbooks have problems, but the science of evolution is well-supported”. However, this was giving away the game, because (a) Wells’s attacks, read carefully, were actually aimed at the credibility of the science of evolutionary biology and evolutionary biologists, and (b) his attacks were tendentious, question-begging, and most importantly based on an amazingly selective and misleading review of the evidence and the scientific community on each question.

One Wells chapter that was particularly annoying was on peppered moths. Everyone remembers something vague from high school biology about moths sitting on tree trunks and birds eating the ones that were the wrong color. In 1998, a leading peppered moth researcher, Michael Majerus from Cambridge, published a book called Melanism which included two long chapters reviewing scientific study of the peppered moth from the initial studies by Bernard Kettlewell in the 1950s through Majerus’s own work. One message of the chapters was that textbook accounts were oversimplified and that the full story was much more interesting. For example, Majerus presented field observations which indicated that peppered moths rest not just on tree trunks but also on tree branches. But the other message of the chapters was that Kettlewell’s basic hypothesis – that bird predation on moths had caused the shift in peppered moth color from light to dark and back again, through differential predation based on camouflage – was correct and confirmed by the work that had happened since Kettlewell’s initial studies, despite various criticisms of the details of some of his experiments.

The story of what happened after Majerus’s book came out is complex and bizarre and is briefly reviewed in Supplement 1 of Cook et al., entitled “A brief history of the peppered moth debacle.” The short version is that Jerry Coyne wrote a prominent review of the book in Nature, which concluded – somehow – that the peppered moth research was all highly questionable. Coyne was and is a prominent and respected evolutionary biologist, and his debunkings of pop-ev psych, creationism, etc. are often of high quality – but there is no way to avoid the conclusion that Coyne must have had an off-day, and his review of Majerus was uncareful and made many mistakes. For example, Coyne wrote:

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.

The only problems with this are that:

(a) Majerus himself, right there in Melanism, presented data showing that moths rested on trunks or trunk/branch joints in 32/47 moths Majerus had personally observed undisturbed in the wild, and 136/203 moths observed resting near light traps. Furthermore, Majerus’s photographs contained several unstaged photos, taken by him, of moths discovered in various natural positions, including on tree trunks.

(b) Birds that hunt on tree trunks are not somehow magically blocked from hunting on tree branches, and lichens are known to grow not just on tree trunks, but also on tree branches. Air pollution and soot, which darken trees both by killing lichens and by physically blackening surfaces (many critiques of the peppered moth example ignore that both processes happen), also manage to get to both places. (An aside – people forget what air pollution was like in 1950s England and before. Think Dickens. Black soot would fall out of the sky. That’s where the term “fallout” comes from, I believe. Sometimes the audience at the back of an opera house could not see the stage at the front. The death rate would spike on bad air days. Etc. This was not a subtle environmental change.)

(c) Not all of Kettlewell’s experiments relied solely on placing moths only on tree trunks. In fact it was Kettlewell himself who first noted in the 1950s that the moths also like branches, some of his experiments let the moths find their own resting spots.

(d) Sargent et al.’s review, which clearly influenced Coyne more than Majerus’s actual book, was on the phenomenon of melanism in moths in general – which likely does have diverse causes – and many of its criticisms did not apply to the specific case of the peppered moth. And as it turned out, Sargent and his coauthors had some very weird Lamarkian and anti-Modern-Synthesis views that have been aired in other venues.

Coyne added a few other choice quotes which rang around the world:

Depressingly, Majerus shows that this classic example is in bad shape, and, while not yet ready for the glue factory, needs serious attention.


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.


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.

Majerus and other peppered moth researchers were dismayed by Coyne’s review, and said so in various fora, but none of this attracted anything like the attention that Coyne’s review received, particularly when it was amplified by journalists and creationists. By the early 2000s, Wells and other creationists, and even some benighted journalists such as Judith Hooper, were alleging not just that the Kettlewell work was mistaken, but that it was fraud. Soon the peppered moth was disappearing from textbooks. The whole phenomenon was bizarre if one paid attention to the actual research literature by the actual people who had done fieldwork and experiments by peppered moths, e.g. Majerus himself, Cook, Bruce Grant, etc. Cook et al. write:

The attacks on the classic peppered moth story were promulgated almost entirely by people who never studied the peppered moth themselves. It is notable that no new fieldwork had ever been done that disproved the classical explanation.

There is more that could be said about the details of the criticisms leveled against Kettlewell and the peppered moth work over the years, but this would take a published article to sort out. It suffices to say that many of the criticisms contradicted other criticisms, most or all “alternative explanations”, even on the rare occasions when a critic bothered to propose one, could not explain how peppered moth color changed from light to dark and then back to light again, and many of the criticisms were obviously armchair “in the bad way” of assuming things that would be obviously wrong to anyone who went out to the field and looked at the relevant forests a bit. (I realized this when I looked at the forests around Cambridge – forests of relatively small and short British hardwoods are rather different than forests on the West Coast of the U.S. Trunk versus canopy is a huge difference in a redwood forest, but literally a matter of an arms length and a second or two of flying for a moth or bird in an English forest.)

All in all, I feel that my assessment of the peppered moth work as of 2002 was right on, and has been confirmed by subsequent developments.

However, a fantastic feature of science is that even overwrought and unreasonable criticisms can benefit knowledge and science in the end, because they aggravate scientists enough to spur them to gather more data. To this end, Majerus conducted experiments and observations on peppered moths for seven summers from 2001-2007, and did it while deliberately avoiding the criticisms that had been leveled at previous experiments – Majerus’s moths were at low density, in natural resting positions, etc. And the result? The selection coefficient against dark moths was statistically significant and approximately 0.1. This is a huge value (huge in that much smaller selection coefficients can easily be relevant in population genetics), of the same order of magnitude and direction estimated in previous work, and sufficient and adequate to explain the change in frequency of the dark morph of the peppered moth, which dropped from 12% to 1% over the course of the study, continuing the trend which had been observed ever since the clean-air laws went into effect in the 1950s. As an aside, we are very lucky that Majerus did this work when he did, since (as the classical explanation predicted), the dark morph is now almost extinct.

Majerus’s data were in by 2007 and he released the results in various talks and in an online article on his website, and reviewed the work in a 2008 article in Evolution: Education and Outreach. Jerry Coyne, to his great credit, went on the air with Majerus in a radio interview and announced that Majerus’s new work had convinced him.

The only step left would have been for Majerus to formally publish the results in a peer-reviewed journal, but Majerus unexpectedly and shockingly died of a rare illness in 2009. Such an event causes chaos for a researcher’s family and laboratory, and I was beginning to worry that Majerus’s final experiment would never be published, and thus we would be subjected to endless cycles of rehashing of the same old half-baked arguments from creationists and the like for decades to come, each time someone rediscovered the charges of scandal and fraud from the late 1990s and early 2000s. Fortunately, however, a group of Majerus’s former colleagues assembled his results and methods and conducted a new statistical analysis, which resulted in the Cook et al. paper.

Whether or not this means that peppered moths will go back into the textbooks is, perhaps, not the most important question. The most important question is getting the science correct and then conforming our beliefs and confidence to whatever the best evidence says. And the science is continuing – researchers have recently identified (van’t Hof et al. 2011) the region of the moth genome responsible for producing the melanism trait, and presumably it is just a matter of time before we know the mutation(s) responsible for producing the trait. Interestingly, Majerus (1998) argued that the evidence argued for a single origin of melanism in British peppered moths. In this he was disagreeing with Kettlewell, who argued that melanism was a “recurring necessity” that had come and gone with climate change and the like (e.g., cryptic moths in general tend to be darker in wetter regions, probably because water darkens surfaces and cloud cover reduces the amount of light on surfaces). Hopefully soon, molecular work will reveal whether or not Majerus was as correct about this as he was about other things. (Note: van’t Hof et al. 2011 already conclude this based on the linkage disequilibrium pattern they observe in the moth chromosomes, but I believe they haven’t drilled down to the specific mutation in the sequence which is responsible.)

But I have to confess that I have a soft spot for the moths and for their place in the textbooks. It is true, as is often said, that we now have many good examples of natural selection in action. So we don’t need the moths. However, that argument only has a point if you have some residual reason to doubt the quality of the evidence in the peppered moth case, probably because you “heard somewhere” that it was in doubt. Hopefully a careful review of the published research, and not second-hand armchair sources, would convince any reasonable observer that the science is perfectly decent in the case of the peppered moth. Once that conclusion is accepted, the peppered moth story lends itself to classroom use for many reasons: the evolutionary change is obvious and visual. The mechanism of bird predation, and the resulting adaptation of camouflage, is easy to understand and gives students a crucial link between the statistical action of natural selection, and the production of adaptations that “seem designed” to naive observers. And the change was the unintentional byproduct of human activity – air pollution, and furthermore the change back was due to legislation which reduced air pollution. And the change in the peppered moths back to their original peppered state was an evolutionary prediction made by Kettlewell and subsequent researchers, and which was dramatically confirmed.

Finally, the whole snafu over the peppered moth science, and the subsequent resolution of the controversy, first by review of already published work by moth researchers, and confirmed by the additional research by Majerus, is itself an excellent example of how science can succeed even in spite of the mistakes all humans and scientists make, and in spite of the difficulties imposed by inadequate journalism, pseudoscientific propaganda like creationism, etc. Since the oversimplification of the peppered moth story in textbooks originally led to some of the backlash, surely it would be fitting to make the practice of science, in all its complexity, accessible to students today, in the form of the peppered moth example along with the history of the rise, and fall, and rise of the peppered moth.