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The Scientist, Vol:13, #11, p. 13, May 24, 1999
Note: This is the original published version as printed in The Scientist, Vol:13, May 24, 1999. The longer, unedited version can be viewed by clicking here.
Every student of biological evolution learns about peppered moths. During the Industrial Revolution, dark ("melanic") forms of this moth, Biston betularia, became much more common than light ("typical") forms, though the proportion of melanics declined after the passage of pollution-control legislation. When experiments in the 1950s pointed to cryptic coloration and differential bird predation as its cause, "industrial melanism" became the classical story of evolution by natural selection. Subsequent research, however, has revealed major flaws in the classical story. It's time to take another look.
In 1896, J.W. Tutt noted that typicals were well camouflaged against the light-colored lichens that grow on tree trunks in unpolluted woodlands; but in woodlands where industrial pollution has killed the lichens, exposing the bark and darkening the tree trunks, melanics are better camouflaged. Since conspicuous moths are more likely to be eaten by predatory birds, Tutt attributed the increase in the proportion of melanic forms to natural selection.
In the 1950s, Bernard Kettlewell tested the idea experimentally by marking several hundred peppered moths (typicals as well as melanics) and releasing them onto tree trunks in a polluted woodland near Birmingham, England. Kettlewell observed through binoculars that melanics seemed less conspicuous than typicals, and that birds took conspicuous moths more readily than inconspicuous ones. That night he recaptured 27.5 percent of the melanics, but only 13.0 percent of the typicals, suggesting that a much higher proportion of melanics had survived predation. Kettlewell later repeated this experiment in an unpolluted woodland in Dorset, England, where the recapture percentages were the opposite of those obtained in Birmingham. He concluded that "birds act as selective agents, as postulated by evolutionary theory," and that industrial melanism was "the most striking evolutionary change ever actually witnessed in any organism."1,2
Experiments conducted by other biologists seemed at first to corroborate Kettlewell's conclusions. When industrial melanism began to decline after the passage of antipollution legislation (presumably because pollution was no longer darkening the tree trunks), the decline seemed consistent with the theory that industrial melanism was due to cryptic coloration and differential bird predation. Industrial melanism in peppered moths quickly became the standard textbook example of natural selection in action. Doubts about the classical story, however, began to emerge soon after Kettlewell's experiments, and it is now clear that those experiments were fundamentally flawed.
When biologists looked beyond Birmingham and Dorset, where Kettlewell had conducted his experiments, they found discrepancies in the expected geographical distribution of melanic moths. For example, if melanic moths in polluted woodlands enjoyed as much of a selective advantage as Kettlewell's experiments seemed to indicate, then they should have completely replaced typicals in heavily polluted areas such as Manchester. This never happened, however, indicating that factors other than selective predation must be affecting melanic frequencies.3,4
In rural East Anglia, where there was little industrial pollution and typicals seemed better camouflaged, melanics reached a frequency of 80 percent, prompting D.R. Lees and E.R. Creed to conclude that "either the predation experiments and tests of conspicuousness to humans are misleading, or some factor or factors in addition to selective predation are responsible for maintaining the high melanic frequencies."5 Reviewing the geographical evidence in 1990, R.J. Berry concluded, "it is clear that melanic peppered moth frequencies are determined by much more than differential visual predation by birds."6
One notable discrepancy in the distribution of melanism was its lack of correlation with lichen cover on tree trunks. Even Kettlewell had observed that melanism began declining before lichens returned, and Lees and his colleagues found a lack of correlation with lichen cover, which they considered "surprising in view of the results of Kettlewell's selection experiments."7,8 According to B.S. Grant and R.J. Howlett, if the rise of industrial melanism had originally been due to the demise of lichens on trees, then "the prediction is that lichens should precede the recovery of the typical morph as the common form. That is, the hiding places should recover before the hidden. But, this is clearly not the case."9
In the United States, the frequency of melanics in southeastern Michigan dropped from more than 90 percent to less than 20 percent between 1960 and 1995, thus paralleling the decline of melanism in the United Kingdom. Yet the decline in Michigan "occurred in the absence of perceptible changes in local lichen floras," prompting Grant and his colleagues to conclude that "the role of lichens has been inappropriately emphasized in chronicles about the evolution of melanism in peppered moths."10 Recently, T.D. Sargent and his colleagues noted that "the recent declining frequency of melanism in B. betularia in North America, where the hypothesis of a cryptic advantage of melanism never seemed applicable," is "perplexing" in view of the classical story.11
So the rise and fall of industrial melanism did not depend on lichens. Why, then, did lichens appear to be significant in Kettlewell's experiments?
Kettlewell did not use the normal resting places of peppered moths. In his experiments, Kettlewell released moths directly onto tree trunks, and acknowledged that they "were not free to take up their own choice of resting site.... I admit that, under their own choice, many would have taken up position higher in the trees."1 He assumed, however, that he could disregard this observation. Before the 1980s, most investigators shared Kettlewell's assumption, and many of them found it convenient to conduct predation experiments using dead specimens glued or pinned to tree trunks. Some biologists who used dead moths suspected, however, that the technique was unsatisfactory. For example, J.A. Bishop and L.M. Cook noted discrepancies in their results that "may indicate that we are not correctly assessing the true nature of the resting sites of living moths when we are conducting experiments with dead ones."12
Most textbook pictures of peppered moths show specimens that have been manually placed on tree trunks.11 Since 1980, however, it has become clear that peppered moths do not normally rest there. K. Mikkola observed that "the normal resting place of the Peppered Moth is beneath small, more or less horizontal branches (but not on narrow twigs), probably high up in the canopies, and the species probably only exceptionally rests on tree trunks." He noted that "night-active moths, released in an illumination bright enough for the human eye, may well choose their resting sites as soon as possible and most probably atypically." Thus "the results of Kettlewell (1955, 1956) fail to demonstrate the qualitative predation of the morphs of the Peppered Moth by birds or other predators in natural conditions."13
Mikkola used caged moths, but data on wild moths support his conclusion. In 25 years of fieldwork, C.A. Clarke and his colleagues found only one peppered moth on a tree trunk, and admitted that they knew primarily "where the moths do not spend the day."14 When Howlett and M.E.N. Majerus studied the natural resting sites of peppered moths in various parts of England, they found that Mikkola's observations on caged moths were valid for wild moths as well and concluded: "[I]t seems certain that most B. betularia rest where they are hidden ... [and] that exposed areas of tree trunks are not an important resting site for any form of B. betularia."15 In a separate study, T.G. Liebert and P.M. Brakefield confirmed Mikkola's observations that "the species rests predominantly on branches .... Many moths will rest underneath, or on the side of, narrow branches in the canopy."16
In a recent book on melanism, Majerus criticizes the "artificiality" of much previous work in this area, noting that "in most predation experiments peppered moths have been positioned on vertical tree trunks, despite the fact that they rarely chose such surfaces to rest upon in the wild."17 It seems that the classical example of natural selection is actually an example of unnatural selection.
The fact that peppered moths do not normally rest on tree trunks invalidates Kettlewell's experiments and poses a serious problem for the classical explanation of industrial melanism in peppered moths. Although cryptic coloration and selective predation have not been ruled out, one recent review concludes that "there is little persuasive evidence, in the form of rigorous and replicated observations and experiments, to support this explanation at the present time."11
Yet textbooks continue to present the classical story of industrial melanism in peppered moths as an example of evolution in action. Clearly, this is misleading. In particular, it is misleading to illustrate the story with photographs showing moths on tree trunks where they do not rest in the wild. Our students deserve better.
Jonathan Wells (JonWells1@compuserve.com) is a postdoctoral biologist at the University of California, Berkeley, and a fellow of the Discovery Institute, Seattle.
Copyright © 1999 by Jonathan Wells. All rights reserved.
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