Peppered moth mutation discovered at last

The story of the peppered moth, Biston betularia, is one of the most famous evolutionary tales known to the public, and is a staple of both popular literature and biology texts. It’s appealing because it’s an example of “evolution in action”: a case in which we could see evolution happening over only one or two human generations, and we now understand the forces of natural selection that caused the evolutionary change.

The change was a simple one: the replacement of an originally light-colored form of the moth (called the “typica” form, with a “peppered” appearance; see below) by a dark-colored form (“carbonaria“) in Britain, all occurring between about 1850 and 1900.  The difference between the forms was due to variants of a single gene, with the carbonaria form being dominant over the typica form, so that having one copy of the “dark” allele makes you fully dark. (There’s also an intermediate “half dark” form called “insularia“.)

Why the evolutionary change? (Remember, evolution can be defined as “genetic change in populations”). The Wikipedia article on “Peppered moth evolution” gives a good summary of the situation.

Early experiments demonstrated that the likely cause was bird predation. Birds are visual predators and pick moths off the trees. Before Britain’s Industrial Revolution, the trees were light in color, often festooned with lichens, and so light-colored moths were camouflaged, less likely to be eaten than were the carbonarias. The first dark-colored moth, a rarity, was described in 1848 in Manchester, and then the frequency of this “morph” (color variant) rose rapidly as soot from factories darkened the trees and killed off the lichens.

This change in tree color, most pronounced in industrial areas like Manchester, now reversed the selective advantage of the carbonaria versus typica forms: the dark moths now were more camouflaged and less likely to be eaten. The frequency of that form then rose strongly in many areas: to nearly 100% in  the grim town of Manchester (apologies to Matthew). All of this change took place over only about 50 years, so natural selection was very strong.

Here are two photos showing the typica and carbonaria forms on darkened versus “normal” trees: you can see how the wrong-colored moths stick out, and so would be obvious to birds looking for a meal:

A soot-darkened tree:


A tree not exposed to pollution. Notice the speckled color of the typica form, which blends in well with the tree bark:

Peppered Moths - melanic & normal (Biston betularia). Essex, UK. The peppered moth has 2 colour forms, a normal light colour & a darker melanistic form. The dark form is an interesting example of industrial melanism. Over a period of time in the industrial areas, where vegetation is affected, the darker individuals have been more successful in avoiding predators. So the melanistic gene has had a survival advantage and now predominates in many industrial regions in the UK, Europe & US. In less polluted areas the light form survives well & the melanisitc gene does not have the advantage so the light form predominates.

After the passage of clean air legislation in Britain in the 1950s, pollution abated and tree color began to return to its normal light appearance. As one might expect, the frequency of the light typica form began to increase, and it’s now well above 70% in most locations.

Experiments by Bernard Kettlewell in the 1950s, involving placing moths on trees and then later recapturing them, suggested the bird predation theory, as dark moths released in light woods were recaptured less often than were light moths, suggesting that the wrong-colored moths had a higher mortality. The opposite was the case for releases in polluted woods. Other research, involving placing moths on trees, suggested the same scenario.

There were, however, problems with the design and execution of these experiments, and I acquired a bit of notoriety in 1998 by pointing these issues out, adding that the demonstration of bird predation as the cause of the evolution was suggestive but not conclusive. I was somewhat demonized by British evolutionists who were wedded to the whole story, and my objections were, of course, picked up by creationists, who misused them to cast doubt on all of evolution. If the textbook case of natural selection was flawed, they claimed, then the whole Darwinian enterprise was wrong.

However, Michael Majerus, a researcher in Cambridge, repeated the release-recapture experiments properly beginning in 2001 (he used only light-colored woods, for the dark, polluted woods were gone), and found the same result as did Kettlewell. In Majerus’s light colored woods, the dark moths that were released along with light ones were recaptured less frequently, and Majerus actually observed birds and bats eating these moths. (Sadly, he died before the work was published, but his colleagues wrote it up and got it published.) Majerus’s findings, along with the observation that the same species in North America experienced the same changes in morph frequency coincident with both the rise and fall of pollution in the U.S., satisfied me that the story of the moths is now pretty solid. There is now very little doubt that we have a case of evolution by natural selection in action, and we pretty much know what kind of selection was operating.

For decades we’ve known that the difference between the carbonaria and typica forms was due to change at a single gene, for genetic crosses showed a nice Mendelian segregation, with carbonaria behaving as a dominant allele. But we didn’t know the exact gene involved, although in recent years it was narrowed down to a 400,000-base section of the moth’s genome. Although the fact that this is a case of evolution by natural selection doesn’t depend on knowing the exact gene involved, to get the complete story from gene to color to the ecological forces involved (bird predation), it would be nice to know what that gene is and what it does.

A big group of researchers has now reported in Nature that they found the gene. The paper is by Arjen E. van’t Hof et al. (reference below, sadly, no free download), and identifies the gene causing the moth color difference as cortex, a well-known gene that’s been studied in fruit flies (Drosophila). A companion paper in the same issue by Nicola Nadeau et al. (reference below, also behind a paywall) shows that mutations at cortex are involved in patterning and mimicry in many species of Lepidoptera. I won’t describe that paper in detail, as it’s only tangentially relevant here.

These are the salient results of the van’t Hoff et al paper:

  • The association between cortex and color was found by “association mapping” in B. betularia. The researchers took a bunch of moths of both the carbonaria and typica forms (and insularia as well), and sequenced DNA in the region where the mutation was known to reside, looking for a consistent change in the DNA that would distinguish the various forms with near-perfect ability.
  • The change was found not to be a single-base mutation in the DNA, but an insertion of a “transposable element” (or “transposon”)—a bit of DNA that can move around in the genome—into the carbonaria form. The whole inserted region comprises 21,925 nucleotides, and involves a single element present in 2.3 copies that has moved as a unit into the cortex gene.
  • The transposable element actually activated (rather than silenced) the cortex gene, increasing the amount of its product in a manner we don’t understand. Curiously, the increased gene activity in the dark form was far more pronounced in larvae (caterpillars, which the paper calls “crawlers”) rather than pupae or adults, presumably because the precursors for scale development and their color are being formed at this stage of development.
  • The association between the transposon and color was nearly perfect, but not 100% so. Every one of the typica and insularia moths lacked the element, while 105 of 110 black carbonaria forms had the element. This means that other genes besides cortex may influence color—or developmental/environmental effects that aren’t genetic—but that the insertion in cortex is likely the most important one: the one that produced the fuel for natural selection on color.
  • By looking at the DNA around the transposon, the researchers could estimate the age of the mutation. If it arose recently in a single individual, most carriers of the mutation would have similar sequences nearby, as recombination wouldn’t have time to put the transposon next to varied DNA from other individuals. If the mutation was old, the surrounding regions of the carbonaria form would be more diverse, as recombination would have combined the inserted gene with other nearby genes from different individuals. Using simulations, the researchers gave the most likely date of the mutation as 1819, shortly before it was first seen in the wild. The “interquartile range” for the dates, which I take to be the range of dates between the 25% and 75% likelihood that it originated, is 1681-1806. Below is the graph showing the probability density of when the mutation to carbonaria originated (i.e., when the transposon moved). You can see that all the dates are fairly recent, so this mutation did not occur thousands of years ago. The highest probability is at 1819, not far from when it was first seen in the wild (1848; dotted line). A new variant butterfly would be found pretty quickly by the Brits, who are avid butterfly and moth collectors!
Screen Shot 2016-06-03 at 8.23.30 AM

(From the paper): Probability density for the age of the carb-TE mutation inferred from the recombination pattern in the carbonaria haplotypes (maximum density at 1819 shown by dotted line; first record of carbonaria in 1848 shown by dashed line).

  • Finally, the paper by Nadeau et al. immediately following this one in the journal showed that in the butterfly genus Heliconius, which is full of mimics as well as “warningly colored” species, cortex is involved in scale development, and thus probably scale color. The gene has been recruited to a new function in Heliconius, although it may still play some role in cell division during the formation of the butterfly egg.

The upshot.  As I said, we can regard this paper as lagniappe: we already knew the main points of the story—that mutation in a single gene was the basis for an adaptive change in moth color based on bird predation. Now we know that the single gene is cortex.

We also know that the change in cortex, causing a change in moth color from light to dark, was due to the insertion of a mobile transposable element, not the “conventional” mutation we think of: a change in a single base of a DNA sequence. This is still a “mutation”, but a large change in the gene that altered how much product it produced. There may be other adaptations in other species known to rest on insertions or removals of transposons, but I don’t know of any. (Readers with that knowledge should weigh in.)

The mutation occurred only shortly before the environmental change—pollution—that caused the evolution of the color difference. That’s interesting, but it wasn’t necessary, for mutations like this occur continuously, and can hang around permanently. That’s because, although natural selection weeds such genes out of populations (dark moths would be at a disadvantage before the Industrial Revolution), mutation keeps putting them back in, so there is a reservoir of low-frequency mutations hanging around that could be the basis for a new adaptation should the environment change. (This is called a “mutation/selection equilibrium.”) Remember, though, that those mutations aren’t hanging around for the purpose of providing future adaptive evolution.  Errors in DNA happen randomly, cannot arise to anticipate the organisms’s future needs, and sometimes, but not usually, turn out to be useful.

Finally, this paper, and the adjoining one on Heliconius, show that while fruit flies are a good “model organism”—a species that has taught us a lot about development and genetics (after all, most of what we know about Mendelian genetics was worked out in Drosophila)—it didn’t tell us much about evolution in Biston betularia. This is for a simple reason: flies don’t have scales, and so scale color couldn’t be studied using Drosophila. What happened, as we see so often in evolution, is that a gene that does something in one species can be co-opted for a different function in another species.  This twist on evolution was only realized after we developed new genetic and developmental tools over the last 30 years.

h/t: Matthew Cobb, Jonathan


van’t Hof, A. et al. 2016. The industrial melanism mutation in British peppered moths is a transposable element. Nature 534:102-105.

Nadeau, N. J. et al. 2016.  The gene cortex controls mimicry and crypsis in butterflies and moths. Nature 534:106-110.


  1. merilee
    Posted June 3, 2016 at 9:25 am | Permalink


    • GBJames
      Posted June 3, 2016 at 10:20 am | Permalink

      Also sub

      • rickflick
        Posted June 3, 2016 at 10:36 am | Permalink

        me too.

  2. ChrisKG
    Posted June 3, 2016 at 9:35 am | Permalink

    Now the textbooks can be updated again (at great expense I’m sure), but rightly so. I always wondered about this one. My college biology book used it as an example like whales from earlier mammals, birds, and even dogs, but then it seemed to waffle on whether the moth was truly evolution. It makes me wonder how AIG will spin this.

    • Posted June 3, 2016 at 10:31 am | Permalink

      I predict the same spin as always on this one. The peppered moths are still peppered moths, still the same kind, so there hasn’t been any evolution. Sigh.

    • Mark Sturtevant
      Posted June 3, 2016 at 4:01 pm | Permalink

      Their attacks on this story have been:
      1. Kettlewell released the moths in the daytime and the moths fly at night. As I understand it, that is a problem but he recaptured them over several nights afterward and most importantly the Majerus paper dealt with the issue.
      2. The moths rest on the forks of tree branches, but the pictures show them on tree trunks. And my favorite…
      3. The pictures of the moths are usually of dead ones, pinned to the tree. Gasp.

  3. docbill1351
    Posted June 3, 2016 at 9:55 am | Permalink

    This is probably going to awaken Jonathan Wells who will claim the genes were “glued” there, thus fake! Ha, ha, another “Icon of Evolution” shot down!

    In three, two, one …

  4. Frank Bath
    Posted June 3, 2016 at 10:09 am | Permalink

    I’m so pleased to read about this because I well remember reading many years ago about the moth demonstrating evolution in action, only to later find the fact of the new form had disappeared from my popular reading and even dismissed. Much chagrin.

    I suppose creationists will say something like god anticipated industrial pollution and put a new moth out there.

  5. TJR
    Posted June 3, 2016 at 10:13 am | Permalink

    Interesting that a gene “for” (phenotypic) character X in fact only appears in 105/110 of the individuals with X, though none of those without it.

    Is this the usual sort of occurrence rate?

  6. Posted June 3, 2016 at 10:30 am | Permalink

    Very interesting, thanks for compiling and explaining these data!

    I think I see one typo: “A new variant butterfly would be found pretty quickly by the Brits, who are avid butterfly collectors!”

    I think you may mean moth in the first instance in that sentence.

    • Posted June 3, 2016 at 10:40 am | Permalink

      Yes, I changed it to “butterfly and moth collectors” (one could also use “lepidopterists”).

      • Jonathan Wallace
        Posted June 3, 2016 at 11:29 am | Permalink

        Lepidopterists sounds grander but I should perhaps point out that collecting of butterflies and moths certainly was a popular hobby but nowadays most enthusiasts for this group of insects (myself included) are content to record and release(and sometimes photograph them) without the need to assemble cases and cases of pinned specimens!
        It is still quite a regular event to catch melanic forms of the peppered moth as well as a variety of other moth species (for whom I assume the genetics are more or less the same)though in most locations the ‘typical’ forms predominate.

  7. Torbjörn Larsson
    Posted June 3, 2016 at 10:33 am | Permalink

    That was an easy read and a fascinating context. When I heard that transposons had inserted without ill effects I jumped too. Not surprising in retrospect perhaps, but a surprising event in itself.

    But it was so close in time to the first observation, so that too may be my sense of likelihood meter needing a head wrenching adjustment.

    Speaking of timing:

    my objections were, of course, picked up by creationists, who misused them to cast doubt on all of evolution. If the textbook case of natural selection was flawed, they claimed, then the whole Darwinian enterprise was wrong.

    Too bad creationists will want to have it both ways.

    Else the jumping nature of transposons and near in time presence of this transposon jump would reject creationist front loading. This time in a ‘textbook’ [well] case of creationist claims.

    If the ‘textbook’ case of creationism claims is flawed, I can now morally claim, then the whole ‘Christian’ enterprise is wrong.

    • Posted June 3, 2016 at 1:03 pm | Permalink

      When judging the likelihood, it’s important to recognize that this is one moth species out of very many (most of which did not acquire an important degree of industrial melanism). The likelihood that any particular species would get this insertion just as the Industrial Revolution was cranking up is very low. But the likelihood is much higher that one species out of a hundred, or a thousand species, or whatever the number of macro-moth species in the UK, would get it during that period. This is a case of post-hoc selection of outcomes, so we have to be careful when thinking about likelihoods.

  8. Posted June 3, 2016 at 10:37 am | Permalink

    Recessive alleles can stick around in a population for a very long time. I find it satisfying that this dominant allele, which couldn’t “hide” behind other alleles, apparently didn’t occur a long time before the pollution. I hypothesize that if the pollution hadn’t happened, that allele would have been gone by 1900. Of course, we’ll never know.

    • Posted June 3, 2016 at 10:41 am | Permalink

      But remember, deleterious dominant genes will also attain an equilibrium frequency, which will be u/s, where u is the mutation rate from the recessive to the deleterious form, and s is the selection coefficient against that bad dominant allele. This will be a very low frequency unless the mutation rate is high or the selection against the dominant is very weak.

  9. mecwordpress
    Posted June 3, 2016 at 10:41 am | Permalink

    “There were, however, problems with the design and execution of these experiments, and I acquired a bit of notoriety in 1998 by pointing these issues out, adding that the demonstration of bird predation as the cause of the evolution was suggestive but not conclusive.”

    I remember that! I remember that some creationists ran with it but I also remember some consternation from some biologists that you were (rightfully) critical of a story that I (too) learned in school.

    In fact, Dr Coyne, it was that 1999 piece in Nature that put you on my “scientist to pay attention to” list, with a few year later “Speciation” and of course now WEIT. The moth story was something of an introduction to your writing for me. I knew of your previous work (esp on Haldane’s rule…I think from the late 80s, when I was Gradual school (sic)) and your book reports in scientific publications from the time. So when I saw your discussion of Majerus’ work and the subsequent though relatively mild (and in the end eminently satisfying) kerfluffle from the critical review of Kettlewell I made a note to PAY ATTENTION when he writes something.

    Been doing it since. Thanks, Dr. Coyne.

    Nice to see the story is tied up with a nice bow now too. I might have missed this if it wasn’t for WEIT.

    • Jonathan Wallace
      Posted June 3, 2016 at 11:39 am | Permalink

      It was quite fair and reasonable to point out shortcomings in Kettlewell’s experimental design and I believe that Majerus also pointed these out. What was not fair was the book Judith Hooper produced on the subject which was widely considered to have unfairly traduced Kettlewell’s reputation. It was great that Majerus was able to carry out his own set of experiments which addressed the specific criticisms of Kettlewell’s work but nevertheless broadly supported his original conclusions. The Peppered Moth remains a great case study for natural selection in action and the new paper on the underlying genetics gives a very satisfying conclusion to it.

      • Posted June 3, 2016 at 8:51 pm | Permalink

        I gave Hooper’s book a very negative review in Nature: Coyne, J. (2002). “Evolution under pressure. Review of Judith Hooper: “Of Moths and Men: Intrigue, Tragedy and the Peppered Moth””. Nature 418 (6893): 19–20. doi:10.1038/418019a.

  10. Mark Sturtevant
    Posted June 3, 2016 at 10:43 am | Permalink

    This is a fascinating story! I used to use the Kettlewell and the Majerus papers in my intro biology class when we covered evolution. I always wondered about the basis of the mutation.

  11. Vaal
    Posted June 3, 2016 at 10:54 am | Permalink

    That was an absolutely compelling read, thank you very much Prof CC!

  12. DrBrydon
    Posted June 3, 2016 at 11:28 am | Permalink

    I encountered the story of the peppered moth in high school and several times since. I don’t believe in any of the presentations was the fact that the switch from light to dark moths reversed after pollution abated.

  13. Richard Bond
    Posted June 3, 2016 at 11:28 am | Permalink

    A beautiful example of science as a process: a good hypothesis, early partial confirmation, sceptical evaluation, further corroboration over years, and finally pretty well conclusive support from an independent line of research.

  14. Posted June 3, 2016 at 11:36 am | Permalink

    Another piece of the puzzle of the universe filled in. 🙂

  15. Posted June 3, 2016 at 11:50 am | Permalink

    Neat – but…there is something screwy on the date estimation – the most likely date (1819) is outside the IQR – is that a typo and that range should end at 1909? Or something…..

    • Posted June 3, 2016 at 8:51 pm | Permalink

      Yes, I noticed that and didn’t know what to make of it!

    • Posted June 4, 2016 at 4:38 am | Permalink

      It’s a very skewed distribution, so I think the peak date could lie outside the middle 50% of the probability distribution. It just means that less than 25% of the area under the curve is above 1806. If I understand the plot correctly, the peak probability of 1809 is still less than 1%. (The IQR is over 100 years!)

  16. Posted June 3, 2016 at 12:17 pm | Permalink

    Great news and excellent post as always. As another example of a famous phenotype caused by a transposable element my favorite one is certainly the “wrinkled” trait studied by Mendel! In this case, the mutation causes a loss of function for one isoform of starch-branching enzyme (SBEI).

    see abstract at

    The wrinkled-seed character of pea described by Mendel is caused by a transposon-like insertion in a gene encoding starch-branching enzyme. Bhattacharyya MK1, Smith AM, Ellis TH, Hedley C, Martin C. Cell. 1990 Jan 12;60(1):115-22.

    • Posted June 3, 2016 at 12:35 pm | Permalink

      Indeed, but I don’t know of any transposon that’s been involved in an adaptive mutation in nature. “Wrinkled” isn’t one of these: it’s in an artificially selected species and I doubt that humans would like to eat wrinkled peas!

      • Posted June 3, 2016 at 4:46 pm | Permalink

        Just find a good advertising agent to spread the word that they are “healthy”.

      • Posted June 4, 2016 at 4:58 am | Permalink

        I don’t know about eukaryotes (or Archaea) but there are examples of adaptive transposable element insertions from bacteria going back to the ’90s (and maybe before). I studied advantageous insertions in the lab for my PhD, developing a model of how occasional (transient/reversible) beneficial insertions can maintain an otherwise harmful transposable element in a clonal population.

  17. Carl W
    Posted June 3, 2016 at 12:19 pm | Permalink

    Very neat. I’m particularly impressed (and slightly surprised) that they can estimate the time of the initial mutation so precisely (even if it’s not terribly precise).

  18. Stephen P
    Posted June 3, 2016 at 2:55 pm | Permalink

    It may just be me not understanding things, but isn’t it rather weird to refer to the “interquartile range” of a modelled distribution which is so strongly skewed that the point of maximum likelihood doesn’t even fall within the central two quartiles? It looks like an irrelevant quantity here.

  19. Militant Scientist
    Posted June 3, 2016 at 4:08 pm | Permalink

    Marvelous post and it is nice to see the genetic chapter of the Peppered Moth story.

  20. kelskye
    Posted June 3, 2016 at 6:49 pm | Permalink

    It would be nice to know how frequent such mutations occur. I’d imagine in a population like moths (given the propensity of insects to reproduce in large number) such a mutation might occur quite regularly, though this seems it won’t be true of adaptation in mammals or birds. What lessons can we draw from the peppered moths that have a wider applicability to evolution as a whole?

  21. Posted June 3, 2016 at 8:51 pm | Permalink

    Are you sure Majerus observed bats eating the moths? I remember Hooper speculating about it but I don’t recall Majerus observing it.

    • Posted June 3, 2016 at 8:57 pm | Permalink

      No, I’m not sure about the bats. I’m going from memory here–Majerus’s online lecture put up before he died–and I may be wrong about the bat observations.

      • Posted June 3, 2016 at 9:11 pm | Permalink

        I checked Majerus’s lecture and it turns out that you were correct. He did do an experiment with bats:

        “Despite the extreme logical gymnastics and unrealistic assumptions one would have to perform if bat predation were to be responsible for industrial melanism in the peppered moth, I decided to do an experiment to test whether bats do prey on typica and carbonaria differentially.
        The design was simply to release equal numbers of the forms near moth-traps where pipistrelle bats were feeding and watch which were eaten”

        “Across the four runs, 208 carbonaria were eaten, while 211 typica were eaten, with no significant difference between sites or runs.
        There is no evidence of differential bat predation of the tyipca and carbonaria forms of the peppered moth.”

  22. ThyroidPlanet
    Posted June 6, 2016 at 6:24 am | Permalink

    This comment can be considered a vote for this kind of WEIT post.

    Some thoughts FWIW about science posts on WEIT:

    It sat in my email inbox until I knew o I had a chance to pay attention a bit more than usual

    I am not asking some questions because I want to read more first ( not an expert)…

    … except : is the purple corn story in the biology textbook with Ansel Adams covers then related to this one by the transposons? I think it has a picture of B. McClintock?…

  23. ThyroidPlanet
    Posted June 6, 2016 at 6:30 am | Permalink

    Another note:

    An important metric for WEIT would be – perhaps this is already mentioned – not just email subscriptions, but notifications/subscriptions to individual posts. I guess this could be a ratio… I have tended to not subscribe to posts *that I comment on* unless I really am ready for it.

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