The peppered moth story is solid

February 10, 2012 • 5:01 am

The paradigmatic example of “natural selection in action” is the case of industrial melanism in the peppered moth, Biston betularia (see the Wikipedia article for a good summary). Briefly, the moth has several genetic forms, the most famous being the “typica” or white form, which is ivory colored with peppery black spots:

And the carbonaria form, which is pure black.

These forms differ by mutations at a single gene, with the carbonaria allele (gene form) dominant over the typica form. (That is, if you carry one typica allele and one carbonaria allele, you’re a black moth.)

During industrialization in 19th-century England, the black form increased from very low frequencies to nearly 100% in some locations, with the most polluted woods having the highest frequency of the black form. In unpolluted woods, as in the picture below, moths were said to rest on the light-colored trunks, and the typica form was more camouflaged from bird predators (note that both types of moths are in the picture).

When woods became polluted during industrialization, the trees got darkened from both soot deposition and the acid-rain-induced death of light-colored lichens. The typica moths, previously camouflaged, were now conspicuous, while the carbonaria ones were more camouflaged.  Differential bird predation based on camouflage was said to explain why the black allele reached such high frequencies, especially in industrial areas. And this, of course, was natural selection, which is defined as repeatable genetic change based on differential reproduction/survival of alleles.

After pollution-control laws were passed in the 1950s, the typica form again began to increase in frequency, presumably because the woods returned to their more pristine condition, giving the typica form a selective advantage once again.  Now in many places that form is predominant, reaching frequencies of 95% or more.  Thus we saw, over less than a century, a reversal of selection pressures coupled with a reversal in the direction of gene-frequency change.

Here is a color photo of both forms on the trunk of an unpolluted tree, showing the camouflage of the typica form.  The classical pictures are in black and white, but of course birds see in color, and in fact in the ultraviolet, so someone should do a picture like this photographed with UV light.

This became the classic case of natural selection in action, and appeared in nearly all evolution textbooks.  It was supported by predation experiments using dead moths of different colors pinned to tree trunks of different colors; these showed that contrasting moths were always attacked by birds at higher rates.  Lab experiments using moths caged with birds showed the same thing. And there were parallel reductions in the frequency of melanic forms of a subspecies (B. betularia cognataria) in the northeastern United States with the decline of pollution in the latter half of the 20th century.   This parallelism strongly suggests parallel selective pressures, though not necessarily birds.

The most famous evidence, however, involved Bernard Kettlewell’s release-recapture experiments beginning the 1950s, in which he released both light and dark moths into both polluted and unpolluted woods in England, finding that he always recaptured more of the camouflaged morph (typica in unpolluted woods, carbonaria in unpolluted woods). This suggested that birds were eating the more conspicuously-colored moths in both types of woods.

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! Judith Hooper, a science journalist, wrote an execrable book claiming that Kettlewell committed deliberate fraud designed to buttress Darwinism, and that evolutionists were complicit in this coverup.  I trashed Hooper’s dreadful book in another review in Nature (if you want a pdf, email me). 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.

Between 2001 and 2007 in his garden near Cambridge, England, Majerus collected both black and white Biston moths in the proportions that were flying in his area (most of these were typica). He put each moth in a mesh sleeve on a tree, allowing it to settle in its preferred resting places at night (which is what they do in the wild), and then removed the sleeves before dawn.  Since moths don’t fly during the day, any moth that disappeared by four hours after dawn was presumed to have been eaten (26% of these moths were actually seen being eaten by birds).  This was supplemented by Majerus climbing up trees and finding out where uncaptured moths normally rest.

Majerus’s experiment was one-sided: that is, he released both types of moths at their naturally-occurring frequencies (a good design) in only unpolluted woods, for polluted woods aren’t around in Britain any longer.  Nevertheless, it’s still a decent test of the bird-predation hypothesis, which under Majerus’s conditions predicted that relatively more of the dark moths than of the light moths would be eaten.

And that is what he found, along with observing that a significant fraction of moths found in their natural daytime resting position (35%, to be exact) were sitting on tree trunks, as the predation hypothesis requires (birds have to see the moths to eat them).

Sadly, Majerus died soon after he did the experiments and didn’t publish his results, except as a Powerpoint presentation that was available on the internet.  Now, however, a group of four biologists headed by L. M. Cook have published Majerus’s data on his Biston releases posthumously.  The paper (reference below, and access is free) is in Biology Letters, and that’s important since it’s passed peer review, giving us extra confidence in the results.

And here are those results, succinctly summarized in a single graph.  It shows the fraction of the two types of  released moths that actually survived predation in a single day. You can easily see that in all but one experiment the typica form survived predation more readily than the carbonaria form, as expected since typica is less conspicuous to sharp-sighted birds in Majerus’s woods. Overall, the survival difference between the forms is highly significant (p = 0.003, which means that the probability of this difference this large arising by chance is only 3 in a thousand). The average survival difference in a day is about 9%.

One can go further and estimate the “selection coefficient” against the dark moths assuming they live several days in the wild. That selective coefficient is between 0.1 and 0.2, which means that, relative to the light moths, the dark moths suffer a survival disadvantage of 10-20% per generation in unpolluted woods. To evolutionists that is very strong natural selection, and it’s easily able to account for the increase in frequency of the light form since the Clean Air laws were passed in the 1950s.

Although it’s unfortunate that Majerus couldn’t do the reciprocal release—releasing and recapturing both forms in polluted woods—these data, along with his observations of live resting moths actually being eaten by birds and the fact that a substantial fraction of moths rest naturally on trees, where they’re exposed to bird predation, show fairly conclusively that the Biston story is sound. It’s great that Majerus repeated Kettlewell’s experiment properly. And kudos to the quartet of scientists who wrote up Majerus’s results and got them published properly.

The authors 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 [5,15–17]. 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 [3,5]. 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 [3]. 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’ [21].

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.

There are dozens of other cases of selection in action: see the two last papers cited below or John Endler’s book Natural Selection in the Wild. And of course there is Peter and Rosemary Grant’s famous work on natural selection on beak size in Galapagos finches, summarized in Jon Weiner’s Pulitzer-Prize-winning book, The Beak of the FinchLike the Biston story, the work of the Grants also demonstrates not only selection but the agent of selection: changing seed size and hardness in the case of finches.

h/t: Bruce Grant, my undergrad advisor (and an author of the new Biston paper), who critiqued the original version of this post and gave it a B+.  Hoping to earn an A, I’ve made some changes.


Cook, L. M., B. S. Grant, I. J. Saccheri and J. Mallet. 2012. Selective bird predation on the peppered moth: the last experiment of Michael Majerus. Biology Letters online,:doi: 10.1098/rsbl.2011.1136.

Hoekstra, H. E., J. M. Hoekstra, D. Berrigan, S. N. Vignieri, A. Hoang, C. E. Hill, P. Beerli, and J. G. Kingsolver. 2001. Strength and tempo of directional selection in the wild. Proceedings of the National Academy of Sciences of the United States of America 98:9157-9160.

Kingsolver, J. G., H. E. Hoekstra, J. M. Hoekstra, D. Berrigan, S. N. Vignieri, C. E. Hill, A. Hoang, P. Gibert, and P. Beerli. 2001. The strength of phenotypic selection in natural populations. American Naturalist 157:245-261.


73 thoughts on “The peppered moth story is solid

  1. This is the kind of report/critique/reevaluation that forms the backbone of the scientific process. Too bad there is little chance that it will be absorbed by the evolution-deniers who jumped in response to the critique.

  2. Great article. I’ve never read the book by Judith Hooper, but now I am intrigued in a perverse way – being called ‘dreadful’ by Jerry Coyne and ‘atrocious’ by Richard Dawkins gives me a proleptic shudder as to how awful it will be.

  3. Good post. I always wanted this classic example explained more to me.

    Of course the creationists would just dismiss this as just “micro-evolution” instead of macro, or whatever the argument is saying they didn’t see the moth turn into an elephant. I’m guessing biologists never even use those terms, right?

    I always thought that sort of thinking would be like saying a math induction proof only works up to some smaller number.

    1. Yes, that is what they will now say, just ‘micro-evolution’ (the biologists who invented the term macroevolution as though it were anything other than long-term microevolution have a lot to answer for). But the point is that, even though the creationists pretend to allow what they call microevolution, as long as Bernard Kettlewell’s work was in doubt they trumpeted it as an embarrassment to science, almost as loudly as they still shout about Piltdown Man.

      1. Apparently Yuri Filipchenko, Dobzhansky’s old mentor, coined the term macroevolution.

        Is there a consensus among evolutionary biologists that macroevolution is “anything other than long-term microevolution”? I don’t think so.

        1. A lot of respected paleontologists will suggest the possibility of such things. They often invoke selection of units of biology higher than individuals in populations. Not too long ago, “species selection” was quite the thing in paleo circles, though I’ve lost touch lately, and I’m not sure how in vogue it is now.

      2. Piltdown man, coincidentally also in the news:

        “[There] the bones and teeth, which have rested in the museum for most of the last century, will be put through a sequence of highly sensitive tests using infra-red scanners, lasers and powerful spectroscopes to reveal each relic’s precise chemical make-up.

        The aim of the study, which will take weeks to complete, is simple. It has been set up to solve a mystery that has baffled researchers for 100 years: the identities of the perpetrators of the world’s greatest scientific fraud, the Piltdown Hoax.”

        Even if the study is successful, creationists will shout. But they may have less to shout about. (Say, if Doyle or de Chardin was the prankster.)

    2. I view the micro vs macro argument as claiming that “yes, we admit that you can take 5 steps but no matter how many time you take 5 steps over and over you will never be able to walk the length of a marathon”.

      1. I believe in microaddition, but not macroaddition — 2 + 2 clearly equals 4, but no one has ever observed 2 million + 2 million equaling 4 million.

        I refuse to fill out tax returns as they require the use of this mythic “macroaddition”, which is against my religion.

        1. I refuse to fill out tax returns as they require the use of this mythic “macroaddition”, which is against my religion.I suspect that this will fly like a maniraptorian theropod dinosaur with the tax authorities, at least until it’s undergone an extended period of microevolution.
          If you feel the touch of the Noodly Appendage, your problem with believing in macroaddition will disappear, as you will realise that macroaddition works perfectly to regulate the output of the heavenly Beer Volcano “sufficient quantities” and “excellent beer” do always add up to “sufficient quantities of excellent beer”. If you’re a believer.

  4. William Bateson was also sceptical and, with Royal Society support, in 1900 solicited the help of amateur field naturalists for a “Collective Inquiry as to Progressive Melanism in Moths.” In a later monograph (1913) he summed up: “Taking the evidence as a whole, we may say that it fairly suggests the existence of some connection between modern urban development and the appearance and rise of the melanic varieties. More than this we cannot affirm.”

    1. Long before Kettlewell, of course. And Bateson was sceptical of Darwinian natural selection generally: “We go to Darwin for his incomparable collection of facts [but] . . . for us he speaks no more with philosophical authority. We read his scheme of Evolution as we would those of Lucretius or Lamarck. . . . The transformation of masses of populations by imperceptible steps guided by selection is, as most of us now see, so inapplicable to the fact that we can only marvel . . . at the want of penetration displayed by the advocates of such a proposition.”

      It is rare to find a distinguished scientist so catastrophically and comprehensive and clearly wrong.

          1. I had a similar reaction. My interpretation was that Donald Forsdyke considers himself distinguished and clearly wrong.

              1. Hence my narrowing. If he had expanded his trolling with reasons why he thinks the unnamed distinguished scientist is wrong and with his name then he would be a very distinguished troll (maybe even not a troll if his reasons are good). Reading his website told me that I don’t know enough to be able to judge his work so I stand by my comment but narrowed to only his trolling ways.

                I wish he would be open about his objections though as regardless of whether I agreed or disagreed with them I probably would learn a lot in the process.

        1. As well as William Bateson, Forsdyke has always been very keen on George Romanes, who argued after Darwin’s death for “Physiological Selection”, a kind of group selectionist argument for reproductive isolation between species.

          Darwin (with the help of his mathematician son at Cambridge) convinced Alfred Russel Wallace in an exchange of letters that the latter’s similar ideas were wrong back in the late 1860s.

          Wallace later rebutted the theory in the journal Nature after Romanes’s ideas were published, and returned to the theme in his book “Darwinism” on the 40th anniversary of “The Origin”, although clearly producing a somewhat similar theory himself.

          Both Bateson and Romanes were driven by their belief that the incompleteness of Darwinism was the lack of explanation of reproductive isolation — a belief that was taken up byo Dobzhansky, Mayr, and indeed persists among many evolutionists to the present day.

      1. But, Richard … 

        “When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.” — Clarke’s First Law


  5. Though I recall Weiner used the word “God” a little to liberally for my taste “The Beak of the Finch” reamins one of my *favorite* science books.

  6. I work for a publisher that produces science books. We used to get letters regarding a book of our that used the peppered moth as an example of natural selection. The letters complained that this was a hoax. I took great pleasure in replying with the great amount of reputable science behind the peppered moth story.

  7. Nice of you to publish this. Especially as you got a TON of heat over your initial criticisms of the original.

    Now, if we could just see a bible-thumper like William Lane Craig admit to any one of his thousands of errors. Well, I won’t hold my breath…

    1. Yes, I had much the same thought.

      This is how science works specifically, and skepticism works generally.

      When new data come along, one is compelled to change one’s mind.

    2. I don’t think that this is an admission of an error at all, and nor should it be, because none were committed.

      All Jerry did originally was critique the results of a study that he felt was poorly designed. Then, someone comes along and performs a superior study addressing many of the design flaws that Jerry had originally identified. The improved study confirmed the conclusions of the first study, and now Jerry is more comfortable with those conclusions because they now have much better support.

  8. You darned scientists, always basing things on evidence. Can’t you just stick with the one true story forever so we don’t have to think? I’m tired.

  9. Interesting tale.

    I’m not a scientist at all. I remember the moth story from my high school biology course. And I remember the creationist claims that it had all been disproved by scientists. And since I didn’t look into it any further than my local small town paper’s coverage, I fell for those lies.

    And those lies have been repeated ad nauseum in my current small town.

    I didn’t realize until just now that Dr. Coyne was the scientist who had “disproved” the moth story.

    We just can’t repeat the truth too many times, can we?

    Thank you, Dr. Coyne, for this posting (and lots of others too!).

  10. I doubt creationists will be as eager to latch onto Jerry’s opinion this time around.Though I recall Weiner used the word “God” a little to liberally for my taste.

  11. I don’t understand how Jerry could have been so critical to the original work — doesn’t he understand that all biologists have to unthinkingly support evolution, no matter the quality of the evidence? What does he think he’s doing, anyway – science?

  12. Majerus did publish an article just before he died in Evolution, Education and Outreach where he referred to some of his experiments. Glad to see someone put the actual data to peer review.

  13. Well, thanks for posting that and distracting me from the work I was supposed to be doing.

    Instead I downloaded the paper and then ended up re-analysing the results, as the appendix describing the analysis isn’t as clear as I would like.

    I can’t work out exactly what they’ve done but a “standard” analysis using R (binomial response, logit link) reaches the same main conclusion, the model with both Variant and Year works best, and the Variant effect has Melanics surviving less often.

    Note that the picture Jerry reproduces above does not seem to be based on the model, but is simply the observed proportions plus one standard error (using the usual normal approximation), as noted on the y axis. Recall that error bars, i.e. confidence intervals, are plus or minus *two* such standard errors. Hence I would argue that a better picture would double the lengths of the bars on the plot.

    However, confidence intervals for the predicted means based on (my version of) the model give error bars of roughly +/- 3% for the Typicals and +/- 5 or 6% for the Melanics,
    with the lower limit of the Typical just overlapping the upper limit of the Melanic for each year. (The error bars are much narrower using the model as the model allows the data points to “borrow strength” from each other).

    1. I was just about to do this re-analysis, as the eye-ball doubling of the plotted SEs suggested that few of the differences were significant. Thanks. I wish more people would plot the Fisher LSD_.05 vales as error bars (as is the default in Mike Lawrence’s wonderful ez-package for R)—then the interpretation is dead simple to read off the graph.

      1. As the person responsible for the stats on the Majerus data, I did think about using “95% confidence intervals” on the graph; however, these are based on a normal distribution approximation and the distribution is binomial. In the end, I used the binomial standard error because for those who want to think 95% confidence limits, as one of you said, doubling the standard error is a good approximation to the “true”t.05 for even quite small sample sizes, and it’s quite easy to do like that.

        Especially having worked with much more extremely asymmetric distributions (such as gamma, beta), I tend prefer to use the standard error, which has a clear meaning under what ever statistical philosophy you labour (e.g. frequentist, likelihood, Bayesian).

        As has been said, there are quite a few individual years which are not significant because of small sample sizes and variation in the numbers eaten per year. Yet Majerus’ overall disappearance information shows a highly significant reduction in melanic survival over the whole experiment.

        1. This is interesting – I admit I assumed it was Gaussian. I was delighted to see the entire article plus supplements freely available. Could you explain why Majerus’ melanics had such a smaller N compared to typicals – the “availables” in Table 2, which if I understand is the basis of the analysis?

          1. Ah! That’s because Majerus wanted to release the moths at more or less natural frequencies. His idea was to make the experiment as naturalistic as possible. He in fact used frequencies he estimated from his trapping records over the entire previous year, and melanics were getting rarer. By the final year, he had virtually no power to test anything with the small numbers of carbonaria he released, and so he was going to stop the experiment anyway.

  14. Fascinating. Clearly there is still value in the melanic colouring or the genes would vanish – unless they have other things connected with them?

    1. Not all trees have light (lichen-covered) bark.
      There’s also a non-trivial point that the moths don’t *only* rest on trees. The only time that I’ve seen one (TTBOMK ; I’m a rock person not a bug person) it was resting on a megalithic pillar of a granitic rock – a fairly leucocratic rock, with some light coloured lichens too. However in the same area (not Kettlewell’s or Majerus’ area, it should be said) there are also abundant melanocratic (dark) boulders. So, I would suspect (without putting it any stronger than that) that the natural lifestyle of Biston in different parts of the country/ continent/ world (see above comment about rock person not bug person) involves some of them, some of the time, coming to rest on dark surfaces, be it of rocks or trees. So that some areas provide a “reservoir” where the carbonaria form is more adaptive than the typica form, which for the larger, mixed, population maintains the allele in the gene pool.
      I’ve got photos of the bug somewhere, on it’s 2500 to 4000 year-old megalith. I’ll have to hunt them out. (I like the fact that our uncertainty in the age of the megalithic monument is comparable to the whole of post-Roman history.)

  15. I had never understood this story, which I think appeared in both the High School and College Biology texts I had. What I took away was that the moths actually changed color under changed enivronment, not that there were two types, one of which was now selected more heavily for. (I won’t say that the texts explained it badly, but they might have.) I’ve actually thought about this a few times in recent years. As original understod, this sounded too much like Lamarckism. Now it makes much more sense.

    Wow. I almost feel like sending UC an Alumni check for continuing my education. Almost.

    1. Goodness. It would be hard to imagine a more comprehensive misunderstanding. If that is really what your textbooks told you it is a scandal. Do you still possess those books, or do you recall their names, preferably with dates?

    2. your comment made me get my Campbell’s Biology book ca. 1990 (the series with Ansel Adams photos on it). There is no mention of what you said, but I found this :

      pp. 432-433

      “… But industrial pollution darkened the landscape of much of the countryside in the late 1800s, mainly by killing lichens that covered rocks and the dark bark of trees. Against this darkened background, light moths stood out, and dark moths were concealed from light birds”

      … using the word “mainly” leads one to wonder if soot was covering the trees also. I think a few of my neurons kept some idea that something was being colored by soot until I just checked now. Then there’s a clear picture of the two moths against lichens “covering” a tree and then just a plain tree. One might think the tree isn’t even there. If it were my book, I would rewrite that paragraph and maybe clarify the photo – but that was 1990, perhaps its different now. or perhaps that’s why I don’t write biology textbooks.

      … anyways, that was interesting, thanks for reminding me.

      P.S: I bought the book was used.

  16. This reminds me of honey bee dancing. It just seems obvious that they’re indicating the location of food, so it’s easy to dismiss a skeptic. But until a rigorous experiment is done to confirm what seems obvious, we’re all just pontificating from armchairs.

    1. I think the difference is that von Frisch’s honey bee dancing story seemed too good to be true, too amazing to be plausible. That is why it was important that it was beautifully confirmed by James Gould, using a more rigorous and more ingenious experimental design than von Frisch’s original.

      But Kettlewell’s peppered moth results, though also beautiful, were never anything but plausible. Nobody should have been surprised that birds choose whatever prey are conspicuous on the backgrounds where they are hunting, and that when the background changed due to industrial pollution this should change gene frequencies in subsequent populations. Admittedly some people were surprised that the resulting evolutionary change was so rapid, although that rapidity was all-of-a-piece with other ecological genetic work going on at the time.

      It was still important that Majerus repeated Kettlewell’s findings and verified them, because they had been criticised on methodological (and ad hominem) grounds, and because they had become ubiquitously quoted in textbooks. But they were never implausible on the scale of the bee dance.


      1. Presumably there are no moths of ‘in between’ form, and there is no gradation dark to light because of the type of genes concerned (either ‘on’ or ‘off’)? If predation were not an issue, would one form be dominant and one recessive? In which case is that why the two variants just see-saw in population percentage?

  17. Regarding the plot:

    Why are error bars overlapping for both groups in some years? Are the error bars at 68%? Is this the standard error *of the mean*?

  18. I’m pretty impressed (given the stories on sometimes reads) at the good science of Majerus. Too often a researcher who is criticised becomes defensive and sulks.
    Dr Coyne criticised the research Majerus had based a part of his book upon and instead of sulking or blindly defending his position Majerus set out to rectify the problems. This in turn gave the evidence for Jerry to change his position.

    SCIENCE. It works, bitches!

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