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 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.

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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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