Dr Bernard Kettlewells Hypothesis Statement

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 carbonariaallele (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 Finch.  Like 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.


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This entry was written by whyevolutionistrue and posted on February 10, 2012 at 5:01 am and filed under adaptation, evolution. Bookmark the permalink. Follow any comments here with the RSS feed for this post. Both comments and trackbacks are currently closed.

Dr Woody Benson is a man I admire greatly, and he makes some very good points here. He clearly knows the topic and its literature well. In general I wholeheartedly agree with him that criticisms of Kettlewell have been overblown.

However, I fear that by criticising Luiggi's enjoyable and brief overview of the recently published Majerus work (Cook et al. 2012 "Selective bird predation ...") in such strident terms, he is in effect shooting the messenger. It was the scientific literature, especially Majerus's 1998 book "Melanism" and Jerry Coyne's review of that book in 'Nature' that cast the doubts on the validity of Kettlewell's experiments. These problems led Majerus to design experiments that answered what were, in his view, flaws in earlier experiments, including those of Kettlewell.

If Luiggi is perpetrating myths, they are myths that we scientists ourselves developed and should have debunked long ago. I don't think we can expect a journalist to have encyclopaedic knowledge of all the scientific background papers on a topic (in this case, stretching back nearly 60 years ago); for that she will typically use the recent material with which she is supplied, and also infer what the story is from interviews. As one of the interviewees, to whom Luiggi sent the draft story before publication, I feel myself to blame for any errors in her story. I'll therefore answer each one of Dr. Benson's points.

Did Kettlewell use living and dead moths? In his 1955 and 1956 experiments, Kettlewell used only living moths. So Benson is entirely correct that Luiggi's piece gets this wrong.

However, many other later experimenters did use dead moths in predation experiments. In the Cook et al. 2012 "Selective bird predation ..." article Luiggi was reviewing, is the statement "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 [3,9]." Ref. 3 is to Laurence Cook's 2003 review and meta-analysis of all the many tens of experiments that had been done (in Quarterly Review of Biology), and ref. 9 is to Kettlewell's 1955 paper.

So Luiggi's mistake is entirely understandable, and I should have caught her error when sent the draft article for fact-checking. I apologise for not catching it.

Did Kettlewell place moths on trunks only, or on trunks and boughs? I have to say I was myself surprised to learn that Kettlewell placed the moths on boughs as well as on trunks, even though I'd reread Kettlewell's papers many times, and used his results in teaching for years. But as Benson correctly shows, Kettlewell was a great naturalist, and well aware that peppered moths rested on boughs, as well as trunks. Benson is also correct that Kettlewell did release the moths on boughs, as well as on trunks.

However, although Luiggi says that "Kettlewell released moths on tree trunks", and that this "threatened to put a serious dent in the validity of Kettlewell's setup," she obtained these ideas directly from the literature she had read. Again from Cook et al. 2012 (electronic supplement 1) : "In his experiments Kettlewell often used high densities of reared insects. These were placed on tree trunks and observed, or marked, released and subsequently recaptured ... These procedures all have potential drawbacks".

So it's easy to see why Luiggi, and indeed anyone else might have been misled. Furthermore, Kettlewell (1955, 1956) himself is somewhat ambiguous about whether he released on trunks, or on trunks and boughs; he often seems to use "trunks" as shorthand for "trunks and boughs" in his text. His photographic plates also show moths on trunks, not boughs [see transcribed extracts, below].

Moreover, the idea that Kettlewell only released on trunks, and that this was a "flaw" in his experiments seems to have been mainly promoted by Majerus himself, especially in his 1998 book. Once again, I think that this misunderstanding cannot be blamed on Luiggi.

Other "flaws" in Kettlewell's experiments? I have long been of the opinion that none of the supposed flaws in Kettlewell's pioneering experiments were very grave, and I wrote to Majerus at the time suggesting he should have been more careful with his criticisms.

But others disagree. Jerry Coyne has always stoutly defended his own more extreme attack on Kettlewell's methodology in his book review. However, Dr. Coyne has never specified what he thought the actual flaws in Kettlewell were, and he seems to have ignored all of the other experiments in many different places and by many scientists on the same topic -- a vast weight of data overall, all pointing clearly in a single direction. The bird predation explanation is not dependent on Kettlewell's experiments alone. The totality of data were to myself and Laurence Cook (2003, see above), as well as to Majerus himself, together highly convincing evidence for the bird predation explanation of melanism.

Both boughs and trunks are characterised by having bark and lichens, and are similarly coloured, so that problem (which, as Benson shows, was not in fact even a problem with Kettlewell's experiments), has never seemed a big issue with me.

So was the new Majerus experiment strictly unnecessary? In my view, it was; and this also appears to be Benson's view. It was "proving the bleeding obvious" as we say in the UK.

However, by addressing so many of the supposed or real "flaws" of earlier work that he himself and others had raised, and also by performing the biggest ever experiment on the topic, Majerus has done us all a great service.

Transcriptions of some of Kettlewell's actual text from the original online PDF documents.

Kettlewell 1955:

p. 324:
It was noticed (i) that the species affected by this phenomenon [industrial melanism] rest fully exposed on tree trunks,walls or fences, apparently protected from predators by a concealing pattern.

p. 325:
By day the [peppered] moth rests on tree trunks and boughs, on which it takes up its position at dawn.

p. 326:
[in: Method of Scoring the Moths on their Backgrounds; this was for his experiment, validated by an independent score from noted animal behaviour professor Robert A. Hinde]

After making the initial decision at two yards, if the insect was judged "conspicuous" one walked away from it until a point was reached where it became inseparable from its background, be it carbonaria on lichen covered oak trunk in Devon (50 yards), or a typical betularia on a Birmingham oak trunk (40 yards).

p. 327:
[in: Results of Scoring Moths on Their Backgrounds]

The same method of scoring was used for both aviary and field experiments, the insects being released on trunks of different species of trees relative to their proportions within the wood.

[but later on on the same page he says:] Thus 651 male and female betularia were released in a circumscribed wood in the Birmingham district, where the melanic form comprised about 90 per cent, of the population. These consisted of 171 typical, 416 carbonaria, and 64 insularia. There were 33 release points, being the trunks and boughs of three birch trees and thirty oaks.

p. 328
[in: Aviary Experiments (i) General Features of the Aviary]

The supports of the cage were made of dark larch and spruce trunks with the bark still present. There were 13 of these and in addition 20 other resting sites were introduced, making 33 in all (15 light and 18 dark). There were also four horizontal poles. All the original construction trunks could be referred to as being lichen-free and with dark coloured bark, but among the introduced "furniture" birch and lichened trunks were included, and as variable an assortment of natural backgrounds as possible. The three forms of betularia were released on these.

p. 332:
[in: Experimental Releases in an Industrial Area: (ii) Method of Release]

In all the experiments, whether in the field or the aviary, boughs and trees selected for release-sites, were each given a number. In the wood, the proportion of such trees belonging to different species bore direct relationship to their estimated frequency in the locality.

Kettlewell (1956)

p. 287:
[in: Previous Experiments]

(a) When released on to available trunks and boughs, their normal resting places, over 97 per cent, of carbonaria (the black form) appeared to the human eye to be inconspicuous....

p. 293
[in: Direct Observation]
It became increasingly obvious that one was passing over the typical form on the lichened tree trunks, and they are practically impossible to see. ... The cryptic efficiency of the typical on a lichened background is, in fact, greater than that of carbonaria on the blackened Birmingham tree trunks.

p. 296:
The result was that I produced a high concentration of moths on comparatively few tree trunks (an average of 4 per tree), nor were the two forms on every occasion released in equality per tree.

p. 298:
It [the act of predation] involves, apart from insect cryptic efficiency, such other considerations as insect density, bird conditioning, and searching intensity per trunk, stimulated by an immediate previous experience of finding a conspicuous insect.

p. 298:
The "other method" of release referred to was used successfully on 18th June. This took place just before sunrise, between 4 and 4.30 a.m. Forty-two carbonaria, 65 typical (and 3 insularia) were allowed to fly out of their separate boxes which had been previously warmed on the engine of my car. The majority flew and took up positions on the boughs and trunks of nearby trees.

Plate I
FIG. 1. --Typical belularia (left) and its melanic carbonaria (right) at rest on lichened tree trunk, Deanend Wood, Dorset.
FIG. 2. --Typical betularia and its melanic carbonaria at rest on lichen-free tree trunk near Birmingham.
FIG. 3. --Song Thrush, Turdus ericetorum L., examining tree trunks from the ground with a carbonaria in its beak.

Plate II
FIG. 1. --Nuthatch, Sitta europera L., in the act of taking typical betularia from lichened tree trunk, Deanend Wood, Dorset. This species took 40 carbonaria to 11 typical while under observation.<
FIG. 2. --Spotted Flycatcher, Muscicapa striata L., about to take carbonaria from oak trunk, Deanend Wood, Dorset. This species was seen to take 81 carbonaria to 9 typical.
FIG. 3. --Robin, Erithacus rubecula L., with carbonaria in its beak taken from lichened tree trunk,Deanend Wood, Dorset. There were 3 typicals on this trunk at the moment of this photograph being taken. This species took 52 carbonaria to 2 typical whilst being watched.
FIG. 4. --Yellowhammer, Emberiza citrinella L., searching tree trunk, Deanend Wood, Dorset. A pair took altogether 20 carbonaria and on no occasion whilst under observation did they discover the typical form which, on every occasion, was offered in equal numbers to the black.

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