Open almost any introductory biology textbook published between 1960 and 2000 and youll see pictures of peppered moths resting on tree trunks, put there to illustrate the classic story of natural selection in action. Since the 1980s, however, biologists have known that the story is seriously flawed. In a new book, Of Moths and Men, Judith Hooper documents the rise and fall of what some scientists are now calling the peppered myth.
Yet the peppered moth story still has its defenders. One of them, College of William and Mary biologist Bruce S. Grant, writes in the August 9, 2002 issue of Science that Hoopers book is “marred by numerous factual errors and by misrepresentations of concepts and controversies.” Ironically, however, it is Grants review that has conceptual problems, misrepresents the controversy, and ignores what is probably the single most important fact about peppered moths.
The Rise and Fall of the Classic Story
Although natural selection is supposed to be the principal mechanism of Darwinian evolution, Darwin himself had no direct evidence for it. The best he could do in The Origin of Species was give one or two imaginary illustrations.1 During the industrial revolution, however, peppered moths changed from being almost all light-colored to being predominantly dark-colored (melanic). Oxford geneticist E. B. Ford believed that industrial melanism, as the phenomenon became known, occurred because melanic moths were better camouflaged on pollution-darkened tree trunks and thereby escaped being eaten by predatory birds — in other words, because of natural selection.
In the 1950s, under Fords guidance, British physician and amateur moth-collector Bernard Kettlewell released light and dark peppered moths onto nearby tree trunks and watched as birds ate the more conspicuous ones. When Kettlewell released moths marked with a tiny spot of paint and later recaptured some of them, the proportion of recaptured moths matching the color of nearby tree trunks had increased. These results were consistent with the camouflage-predation explanation for industrial melanism, and Kettlewell called them Darwins missing evidence for natural selection.2 When pollution was reduced a few years later the proportion of melanic moths decreased, and this was interpreted as still more evidence for the camouflage-predation explanation.
A skeptic of Darwinian evolution might reasonably ask So what? Even if the camouflage-predation explanation were true, it would merely show that natural selection temporarily altered the proportions of light and dark peppered moths. It does not show how a new species of moth originates — much less how new orders (such as beetles or termites) or new classes (such as centipedes or horseshoe crabs) evolve. If the rise and fall of industrial melanism in peppered moths shows anything, it is how little natural selection actually accomplishes.
It now turns out, however, that Kettlewells experiments may not have even demonstrated natural selection. In the mid-1980s, biologists discovered that peppered moths only rarely rest on tree trunks in the wild. These night-flying moths are now thought to rest during the day beneath small branches high up in the trees, where they cant be seen. Since Kettlewell released moths during the day onto exposed tree trunks, where the dazed insects froze in place and became easy targets for birds, his results may have had little bearing on what happens under natural conditions.
Hoopers book lists many other flaws in the classic story, as well. For example, the major predators of peppered moths are probably not day-flying birds, but night-flying bats. Furthermore, Kettlewell measured camouflage by his own eye, even though research has shown that bird vision is quite different from human vision. He and Ford also disregarded the possibility that selection might have operated — not on adult moths — but on caterpillars, through differences in their ability to withstand pollution.3
For these reasons (among others), a growing number of biologists have become critics of the classic peppered moth story. In 1998, University of Massachusetts biologist Theodore D. Sargent and two colleagues wrote in the journal Evolutionary Biology that although the camouflage-predation explanation may be true, in whole or in part, there is little persuasive evidence, in the form of rigorous and replicated observations and experiments, to support this explanation at the present time. The same year, University of Chicago evolutionary biologist Jerry A. Coyne wrote in Nature that the fact that peppered moths do not rest on tree trunks alone invalidates Kettlewells release-and-recapture experiments, as moths were released by placing them directly onto tree trunks. Coyne concluded that this prize horse in our stable of examples of evolution is in bad shape, and, while not yet ready for the glue factory, needs serious attention.4
Grant’s Review: Conceptual Problems
According to Grant, Hooper and other critics exaggerate the flaws in the peppered moth story. Hoopers fundamental problem, he writes, is her “failure to clearly distinguish the evidence for natural selection and the mechanism of selection.5
Grants distinction is confused, however, if not meaningless. Selection IS the mechanism (at least, the principal mechanism) of Darwinian evolution. If we have evidence for natural selection, then we know the mechanism; if we dont know the mechanism, then how can we say we have evidence for natural selection?
Apparently Grant sees evidence for natural selection in the correlation between the rise and fall of melanism and the rise and fall of industrial pollution. He writes: What is the mechanism of selection? Even the answer we have no clue would not invalidate the conclusion that selection has occurred. Fortunately, the circumstances have left clues. Geographic and temporal variations in the incidence of melanism correlate with atmospheric levels of SO2 [sulfur dioxide, a pollutant] and suspended particles.
Yet scientists know that correlation does not mean causation. Indeed, it is part of the scientists job to find the causes for correlations. When Jerry Coyne criticized Kettlewells experiments in 1998, he wrote that it is necessary to unravel the forces changing a character. We must stop pretending that we understand the course of natural selection as soon as we have calculated the relative fitness of different traits. A decade earlier, University of California biologist John Endler had put it even more pointedly: A strong demonstration of natural selection combined with a lack of knowledge of its reasons and mechanisms is equivalent to demonstrating a chemical reaction, and then not investigating its causes and mechanisms–an enterprise Endler considered no better than alchemy.6
Grant wants to call industrial melanism a demonstration of natural selection without clearly establishing the mechanism. According to his fellow evolutionary biologists, however, this pretense of understanding is no better than alchemy. The fundamental problem is not Hoopers failure to make a meaningless distinction, but Grants willingness to confuse correlation with causation.
Grant’s Review: Misrepresenting the Controversy
According to Grant, Hooper’s book turns bizarre when she showcases American biologist T. D. Sargent as a wounded iconoclast whose career was stultified because Kettlewell dismissed his work. She argues that Sargent is now under attack because he questions the classical explanation for industrial melanism.
But Sargent really is under attack — in large part from Grant himself. For example, in his review Grant faults Sargent for protracted speculation about phenotypic induction–the hypothesis that industrial pollution produced melanism directly, by inducing it in embryos, rather than indirectly, by discoloring tree trunks and thereby providing conditions for natural selection.
It is true that Sargent has maintained that until a selective agent has been clearly identified, the induction hypothesis cannot be ruled out; but it is also true that he considers some form of natural selection to be the most likely explanation. He and his colleagues wrote in 1998: We feel certain that this phenomenon is a product of selection, though it is important to consider the role that other selective factors might be playing in the melanism story.7 Grant, it seems, would rather not consider any factors–selective or otherwise–except for camouflage and predation.
Grant continues: But most egregious is Sargents assertion that studies in North America falsify the classical explanation. Part of the classical explanation was that industrial pollution darkened tree trunks by killing the light-colored lichens that covered them. This part of the explanation, however, never made sense in North America, where melanism rose and fell without perceptible changes in lichen cover. Indeed, Grant himself was one of those who reported this fact. In 1996 he and his colleagues concluded that the role of lichens has been inappropriately emphasized in chronicles about the evolution of melanism in peppered moths.8
Nevertheless, Grant claims in his review of Hoopers book: The American studies corroborate rather than contradict the classical explanation. He continues: The history of melanism in American peppered moths closely parallels what has occurred in Britain, and melanism is correlated in like manner with levels of atmospheric pollution. In other words, Grant is merely reasserting the fact (which nobody disputes) that the rise and fall of melanism was correlated with the rise and fall of pollution. As we have seen, however, that hardly qualifies as scientific evidence for natural selection, much less as evidence for the classic explanation of camouflage and bird predation.
So Grant misrepresents the controversy by making it sound as though Sargent rejects selection altogether. He also misrepresents the controversy by omitting the fact that North American studies (some of which he himself conducted) falsified that part of the classic story that emphasized the role of lichens.
Grant’s Review: Ignoring the Most Important Fact
In 1985, Cyril Clarke and his colleagues noted that in 25 years of field work they had found only one peppered moth naturally perched on a tree trunk; they concluded that they knew primarily where the moths do not spend the day. In 1987, Rory Howlett and Michael Majerus reported that exposed areas of tree trunks are not an important resting site for peppered moths. A decade later, Majerus wrote a book summarizing the evidence and concluded that peppered moths do not naturally rest in exposed positions on tree trunks.9
Since peppered moths dont normally rest on tree trunks, the textbook photographs had to be staged. Some were staged using live moths, which are torpid in bright light and stay where they are placed; but many textbook photographs used dead moths glued or pinned in place. Yet textbooks dont tell students they are being shown a completely artificial situation.
Obviously, the natural resting-place of peppered moths is a crucial part of the classic story. As Coyne pointed out in 1998, the fact that they dont rest on tree trunks invalidates Kettlewells experiments; and textbooks that use staged photos without identifying them as such are misleading students about this important fact.
Yet there is no mention of any of this in Grants review. Perhaps he thinks that readers of Science will fail to notice the omission. Yet the fact that peppered moths dont normally rest on tree trunks is no secret. It has been publicized repeatedly since 1998 most recently in The New York Times (June 18, 2002).10 The moth, so to speak, is out of the bag.
Sour Grapes of Wrath
Grant entitles his review Sour Grapes of Wrath. In one of Aesops fables, a fox who is unable to obtain some grapes says they were probably sour anyway. In Grants review the fox is obviously meant to be Sargent, who is allegedly resentful because his work has been ignored.
Whether this was ever true of Sargent I dont know, but under the present circumstances it seems more applicable to Grant himself. If the peppered moth story has a hero, it is Sargent. Hooper points out that he and other critics of the classic story have long been demonized by the industrial melanism establishment. Now (thanks to Hoopers book) Sargent is finally being recognized for his courage in daring to challenge a powerful and dogmatic oligarchy. As a prominent and unrepentant member of that oligarchy, however, Grant cannot share in the glory. Who is really crying sour grapes here?
In John Steinbecks 1939 novel about the Depression (and Julia Ward Howes Battle Hymn of the Republic), the grapes of wrath signify an unjust situation that elicits righteous anger. In Steinbecks story, the injustice was suffered by farmers who were driven from their land. In the peppered moth story, injustice has been suffered not only by demonized critics of the camouflage-predation explanation, but also by generations of biology students who have been systematically misled about the evidence for evolution — and they are not happy about it.
I have been told by college students who took biology courses featuring the peppered myth — complete with staged photographs of dead moths on tree trunks — that they feel like suing for a refund of their tuition fees. I have been told by businessmen that if they were to misrepresent their products or services the way defenders of the peppered myth have misrepresented the scientific evidence, they would probably go to jail.
Defenders of the peppered myth have been sowing seeds of righteous anger for decades, because most people dont like being demonized or misled. Is Grant suggesting that it’s time to trample out the vintage where the grapes of wrath are stored?
Grant concludes his review with: The case for natural selection in the evolution of melanism in peppered moths is actually much stronger today than it was during Kettlewell’s time. A daring claim, but stunningly false. Grant can desperately defend the peppered myth until his dying day; but like the moths in most textbook photographs, the myth is already dead.