The first person ever to watch a bird snack on a peppered moth was Hazel Kettlewell. It was July 1, 1953, and Hazel was peering through a pair of binoculars at a peppered moth resting on a tree trunk in Cadbury Bird Reserve, a piece of woodland under the smoke of Birmingham. Suddenly, a hedge sparrow leaped up from the bracken, snatched the moth from its trunk, and disappeared from sight again.
It was a momentous occasion. Not only because it was the first time that the crucial observation—that birds do forage on peppered moths resting on trees—was made, but also because it happened in the course of one of evolutionary biology’s most famous experiments. Hazel was the wife of Bernard Kettlewell, a medical doctor and self-taught zoologist, who had recently been recruited by Oxford University to do experimental work on natural selection and industrial melanism. It was not a random appointment. The energetic, skilled, and knowledgeable Kettlewell had been a long-time friend of the founder of Oxford’s informal “School of Ecological Genetics,” E.B. (“Henry”) Ford. After many years of trying, Ford had finally succeeded in finding enough money to lure Kettlewell from his self-imposed South African exile. He was convinced that if anybody could find the missing pieces of the peppered moth puzzle (do birds really eat the moths? do they really eat fewer of the moths that better match their background? and is the difference really big enough to be the driving force of the wing color evolution?), it was Kettlewell.
So, Kettlewell and his family spent much of 1952 in a trailer in Wytham Woods, a forest that belonged (and still does) to Oxford University. There, they collected and hand-reared some 3,000 peppered moth caterpillars, carefully nurturing them to the point of pupation, and caring all winter for the resting pupae. In June of the next year, shortly before the moment the pupae were expected to hatch, Kettlewell drove up to Cadbury Bird Reserve, with the pupae, carefully packed in gauze, in the back of his Plymouth.
He had chosen Cadbury Bird Reserve because it lay near enough to Birmingham to be covered in thick industrial grime. Setting up a field lab in his caravan, for eleven consecutive days he would be working round the clock, assisted by Hazel. They put individual paint marks on the wings of the moths as they emerged from their pupae, placed the insects onto tree branches, and then at night ran two kinds of moth traps: mercury vapor lights and gauze sleeves with sexually receptive female peppered moths, both tried and tested ways of attracting moths (the latter of course only works for males). The expectation was that, during those eleven days, birds would eat more of the pale, poorly camouflaged moths than inconspicuous black ones, and that this uneven predation would show up as a difference in how many of each category would survive long enough to get into the traps.
It was during one of these days Hazel spotted that hedge sparrow picking off a moth. And more observations followed. Over the next few days they saw several hedge sparrows and robins feeding on both pale and dark Biston betularia. They walked their rounds of the forest and recorded which of the moths released in the morning were still on their perch in the evening. For the dark-winged moths, 63 percent were still where they had last seen them, but for the pale-winged moths, this was only 46 percent. The difference was pretty much exactly as large as Haldane had predicted. As Kettlewell wrote in his famous 1955 article “Selection Experiments on Industrial Melanism in the Lepidoptera,” this was because “birds act as selective agents, as postulated by evolutionary theory.”
Meanwhile, the night-time traps were also reeling in data. Over the eleven days the traps operated, the Kettlewells released 630 (pale as well as dark) male moths, of which they caught 149 in their traps. The recaptures were not equally divided among pale and dark forms. Of the released pale moths, they recovered 13 percent, while more than twice that proportion (namely, 28 percent) of the released dark moths were caught in the traps. Again, it seemed that something was picking off pale moths at a greater rate than dark ones. That something, as now seemed to be confirmed, were birds.
Two years later, Kettlewell performed the counter-experiment, namely releasing some 800 marked moths in an unpolluted forest under the clean Dorset air, and, as expected, found exactly the reverse. Here, it was the dark moths that stood out on the clean, lichen-festooned birch stems, while the pale moths were much harder to spot. Sure enough, the latter seemed to survive better, as they turned up at a greater frequency (14 percent) in his traps than the former (5 percent). This time, he brought another companion to his field camp: Dutch behavioral biologist, and later Nobel Prize winner, Niko Tinbergen, who was gaining fame with his studies of bird behavior, and pioneering the use of cinematography in field biology. While Kettlewell busied himself with his muslin moth bags and mercury vapor lamps, Tinbergen sat in his hide with his camera and shot stunning footage of spotted flycatchers, nuthatches, and yellowhammers feasting on the rich pickings of pale and dark Biston betularia that had been laid out for them.
Tinbergen’s films and photographs, Kettlewell’s papers (he wrote up his Dorset data, as well as a rerun of his Birmingham study, in a second paper in Heredity in 1956), and the repeated wheeling out of Kettlewell and his moths by his mentor, Henry Ford, did much to propel the peppered moth as the celebrated case study of evolution in progress. By the mid-1960s, Biston betularia began to make standard appearances in lectures, documentaries, and articles about evolution, and for the remaining decades of the twentieth century, no biology textbook was complete without photographs of pale and dark moths on ditto tree bark. In fact, this early example of urban evolution is so familiar and worn out that I would not have brought it up here, were it not for a twist in the story that began to unfold in the late 1990s. Chances are that you are vaguely aware of this—that, all the while you were reading the preceding pages, there was this gnawing feeling in the back of your mind that you had heard or read somewhere about something fishy with this paradigm of rapid evolution.
The fishiness started with Melanism: Evolution in Action, a 1998 book by Cambridge evolutionary biologist Michael Majerus. The centerpiece of the book is the painting of a much richer picture of the peppered moth case than had ever been done before. Majerus points out some unanswered questions, several of which had been asked by previous authors as well. Do moths always rest on tree trunks or also in places where their wing color does not afford any protection? Since the moths fly by night, couldn’t bats, rather than birds, actually be their chief natural enemies? And were the artificially high densities of moths released by Kettlewell in his experimental woodlands really a proper way to study real natural selection? Majerus had intended this critique as a way to urge his colleagues not to lie back and consider the peppered moth case closed, but to take up the gauntlet and start up new, more detailed studies to fill gaps and remove uncertainties. In fact, he himself was working on this.
Rather than stimulating new research on industrial melanism in Biston betularia, and much to Majerus’s dismay, his book had the unintended effect of casting doubt on the whole story. A book review in Nature by geneticist Jerry Coyne had it that “[for] the time being we must discard Biston as a well-understood example of natural selection in action,” and: “Depressingly, Majerus shows that this classic example is in bad shape.” Some of Coyne’s colleagues, who knew what Majerus’s true intention had been, were surprised by this interpretation of the book. One wrote, “If I hadn’t known differently, I would have thought the review was of some other book.”
But the damage had been done. Articles began appearing in newspapers bearing headlines like “Scientists Pick Holes in Darwin Moth Theory” and “Goodbye, Peppered Moth.” But the worst was still to come. In 2002, journalist Judith Hooper dropped a bombshell titled Of Moths and Men: Intrigue, Tragedy, and the Peppered Moth, a well-written and well-researched history of the peppered moth case, in which she dissected the complicated relationships among England’s moth men, and insinuated that Kettlewell’s experiments had been soiled by his subservient relationship with the Oxford intellectual giants for whom he worked. Basically, she accused him of fraud, aimed at
pleasing his superiors. Although she offered no concrete evidence of any wrongdoing, she managed to mount a hatchet job by word choice and guilt by association, and her book was immediately picked up by the creationist community in the US, probably her main intended market. The Institute of Creation Research wrote, “What a wonderful time to be a creationist, when even the supposed best proof of evolution in action is so flimsy that it cannot stand the test of truth.”
Hooper’s book, together with the brouhaha surrounding his own book, at least served the purpose of spurning Majerus into action. He set out to do a series of massive experiments of the kind Kettlewell had done, but avoiding the pitfalls, and settle the issue once and for all. Place of action: Majerus’s own, two-and-a-half-acre garden near Cambridge. When: the years 2002 to 2007. The key players: 4,864 peppered moths (nearly ten times as many as Kettlewell had used for any of his experiments), all resident, of their own accord, in Majerus’s garden. (Kettlewell had always mass-reared the moths and then transported them to field sites hundreds of miles away—a procedure that had been criticized, because it flooded the field site with more moths than would naturally occur there, and ones possibly not suited to that particular site at that.)
Another thing Majerus did differently was that, rather than placing the moths on a tree, he allowed them to find their preferred resting site by themselves. Each moth was given a mark and then released, at night, into a large cage wrapped around the trunk and branches of a tree. In the morning, before dawn, Majerus removed the cage, noted where the moth was sitting and checked four hours later whether the moth was still there. If it had gone, he assumed that a robin, hedge sparrow, blackbird, or any of the other insect-eating bird species present in the garden had gobbled it up—something which Majerus, scanning the trees from his garden shed through his field glasses, actually saw happening on no fewer than 276 occasions.
Imagine, just for a minute, the dedication! Majerus possessed twelve cages, so each night he could not release more than twelve moths. That means that, over the six years his experiment ran, he spent more than 400 nights rigging the cages, taking notes, setting his alarm to go off before the early mid-summer dawn, removing the cages, sitting with a cup of coffee and his binoculars behind the window and watching out for birds to swoop in. Let’s remember that this work would probably have come on top of his regular teaching and administrative duties at the university—a persistent Herculean effort intended to prove beyond any doubt that the dark-winged peppered moths evolved by natural selection, applied by the conspired action of pollution and predatory birds.
And prove it he did. Or rather, he proved that the peppered moth was, by now, evolving back to its original state. After legislation was put in place in the 1950s and 1960s to curb air pollution, the blackened trees of industrial England had slowly become a thing of the past. The air became cleaner, lichens returned, and the tables were turned on the dark-winged peppered moths. Over much of England, their dusky wings no longer afforded any protection—the reverse, in fact—and bit by bit their advantage waned. As a result, between 1965 and 2005, at more or less the same rate as they had increased a century before, their proportions declined. Today, the dark form is as rare as it was in 1848.
So Majerus’s experiments fell in the tail end of this evolutionary decline. In fact, over the six years that he conducted his studies, the proportions of dark-winged moths in his garden dropped from 10 percent in 2001 to 1 percent in 2007. And the outcome of his experiments matched this: each day, around 30 percent of the dark moths were snatched up by birds against 20 percent of the pale ones.
Majerus presented the results of his six-year experiment at a conference in Sweden in 2007, but, sadly, he was not given the time to publish them in the scientific literature. At the end of 2008, he was struck down by a particularly aggressive mesothelioma, to which he succumbed, aged only 54, in January 2009. Soon after he died, his family permitted four of his friends to work up his notes and the slides he had used for his lecture in Sweden and publish them as a paper in Biology Letters, which appeared in 2012 under the title “Selective bird predation on the peppered moth: the last experiment of Michael Majerus.” The final sentence of the article reads: “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.”
And so it really is. As a final note of triumph, in 2016 the peppered moth saga was crowned with a paper in Nature by a large team of geneticists led by Ilik Saccheri of the University of Liverpool. They showed that the mutation responsible for the black wings was actually a 22,000-letters-long runt of “jumping” DNA that had cut-and-pasted itself into the cortex gene that controls wing coloration in butterflies and moths. Detailed dissection of the gene’s structure and the adjoining parts of the chromosome showed that industrial melanism in the peppered moth dated back to a single DNA-jumping event that, the researchers calculated, must have taken place in the north of England around the year 1819, “which is kind of smack in the middle of the early part of the industrial revolution,” says Saccheri.
The accumulated new evidence of these past few years appears to have vindicated Bernard Kettlewell and reinstated the case of the peppered moth as a genuine textbook example of evolution by natural selection. But in addition, it is also the first recorded case of urban evolution, or more generally, of what has been called HIREC: Human-Induced Rapid Evolutionary Change. It has shown that humans, especially dense urban conglomerations of humans, have the power to exert, on wild animals and plants, novel and unusually strong selection pressures of the order of 10 percent or more. The steep evolutionary rise and fall of the dark-wing mutation in the cortex gene of Biston betularia is nothing less than a writing on the wall that heralds the coming systemic change of urban nature. With the peppered moth, it was a reversible evolutionary see-saw: a single gene that rose and then fell and that has become a celebrated case because of its simplicity, clarity and, despite the controversy, easy interpretation.
9
SO IT REALLY IS
Despite some recent rumblings, the peppered moth has regained its rightful place as a textbook example of urban evolution in action—by turning from pale to black during the height of the industrial revolution’s air pollution, and back again from black to pale when the worst was over. It has shown that a single change in an organism’s DNA, propelled by strong, human-induced natural selection, can kick-start an explosive evolutionary changeover. In this chapter, I will showcase a few more animals and plants that have recently evolved rapid and dramatic changes in their appearance because of adaptation to the urban environment. But first, there is just one more thing I need to point out about industrial melanism in the peppered moth: namely, that it is an urban version of an otherwise common and natural kind of evolution in moths.
In fact, Bernard Kettlewell wrote a whole book about melanic moths. In the UK alone, dozens of species have evolved two or more shades of gray. Not because of soot-covered tree bark, but because each is better suited to different habitats or different parts of the country. In his letter to Darwin, Albert Farn had already alluded to this in the Annulet: “almost black on the New Forest peat; gray on limestone; almost white on the chalk near Lewes; and brown on clay, and on the red soil of Herefordshire.” Affording the best camouflage against a particular soil type, natural selection would have pushed for different wing-color genes in each area. Where two areas abutted, the habit of the moths to fly around and mate some distance from where they were born would have spread some pale-wing genes into the New Forest, or some black-wing genes into Herefordshire, but not enough to muddle the geographical color pattern, since the inappropriately decked out moths would always be easy pickings for the local avifauna.
Kettlewell also devoted himself to such “natural” melanism (which he called rural, rather than industrial, melanism). One of his former students, Stephen Sutton, recalls how he join
ed Kettlewell on a 1960 expedition to the Shetland Isles, where they investigated rural melanism in a moth named the Autumnal Rustic (Eugnorisma glareosa): “I was stationed on sand dunes (white background) and other assistants were at intervals up the length of the Shetlands to the blasted heaths of Unst. [The moths were] very pale on my dunes, and dead dark on the open peat of Unst. In the simmer dim of June, gulls would forage through the night and matching your background was a key survival factor.”
So, in a sense, the ink blotch of melanic peppered moths that oozed onto England’s map in the late nineteenth, early twentieth century, was no different than the age-old areas with black Annulets on the dark soils of the New Forest, or the “dead dark” Autumnal Rustics on the Shetland heathland. All had probably evolved due to mutations that changed wing coloration, and birds that hunted by eye—although the industrial melanics in the peppered moth had probably evolved much faster, because the landscape changed so quickly and drastically. So fast, in fact, that we saw it happen before our eyes.
In the peppered moth system, urban pollution caused the moths to evolve, in essentially the same way—only faster—that moths had been evolving under natural conditions for millennia, with birds acting as evolutionary middle-men. But birds themselves may also be affected by urban evolution. To see that happening, we should dip into Shakespeare.
In Henry IV Part 1, Hotspur is planning to drive King Henry crazy by letting a starling endlessly repeat the name of Hotspur’s brother-in-law Mortimer: “Nay, I’ll have a starling shall be taught to speak nothing but ‘Mortimer,’ and give it him, to keep his anger still in motion,” muses Hotspur. In 1877, this obscure Shakespearean reference to Sturnus vulgaris, the European starling, landed the bird a place on the list of animals and plants that were to join the human colonizers in the US. For in that year, drug manufacturer Eugene Schieffelin became president of the American Acclimatization Society, a group of idealists who saw it as their calling to “improve” North America by releasing “such foreign varieties of the animal and vegetable kingdom as may be useful or interesting.” And for some unfathomable reason, Schieffelin’s particular brand of acclimatization included the bringing into the USA of every bird ever mentioned in Shakespeare’s works.
Darwin Comes to Town Page 8