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Let’s Mothball the Peppered Myth

Let’s Mothball the Peppered Myth


In his famous 1959 article, Dr. Bernard Kettlewell described his research on the peppered moth as the “consummation and confirmation” (p. 53) of Darwin’s theories. (B. Kettlewell. 1959. Darwin’s Missing Evidence. Scientific American. March pp. 4 8-53) Since then, the peppered moth has become standard fare in biology and genetics texts. Educators really like this example because it is “extremely visual” and thus easily understood and remembered. While all scientists, whether creation model support ers or evolution model supporters, agree that shifts in the proportions of various characteristics do take place in nature, only the evolutionists draw more extensive conclusions from the data. For example, Nelson Canada’s 1993 high school text Biolog y (authored by teachers from Alberta and Newfoundland), suggests that students explain the statement “Evolution and adaptation usually occur by means of small changes” in connection with the case of the peppered moth. (p. 110)

Often students receive the impression from science texts that the peppered moth is an example of “evolution in action” rather than merely an example of shifts within a population of proportions of minor characteristics. It is certainly true that there has been a shift in colour within the peppered moth population. It is equally true that the population is now shifting back towards its original condition. None of this is under dispute. The big news (not really that new either, but newly recognized) is t hat we really have no idea why these shifts have occurred. (Robert Matthews. 1999. Moth test of Darwinism said based on blunders. Edmonton Journal. March 15.)

To study the peppered moth we first must go to England’s industrial heartland. There the energy for the industrial revolution came from the burning of coal. Not surprisingly, in an age when pollution controls were unheard of, industrial centres became shrouded in air pollution and all outdoor surfaces became liberally coated with black soot. In the late 1800s, in the countryside near these horrific mills, more than 70 species of night flying moths were observed to become typically darker coloured. What was going on? Dr. Bernard Kettlewell set out to research these moths. He chose Biston betularia (peppered moth) for his studies. Early in the nineteenth century, the population of this moth was almost exclusively light coloured with pepper coloure d speckles. On very rare occasions much darker forms were noted. The earliest such finding was before 1819. However, by the time Dr. Kettlewell conducted his research in the 1950s, most of the individual moths were dark coloured. Breeding experiments reve aled that the melanic or dark form was controlled by a dominant gene. The relevant question then was why the melanic or dark form became so very numerous by mid twentieth century.

Dr. Kettlewell thought he knew the reason and he set out to test his hypothesis. He placed both forms of moth on trees in unpolluted woodlands. The dark form was obvious on the lichen covered trunks while the lighter form was almost invisible. He watch ed while birds found the darker form and ate it. The lighter form usually escaped detection so that it was not eaten. In polluted woods the opposite occurred. The dark form could scarcely be seen on sooty trunks while the light form became conspicuous. Na turally the birds honed in on the lighter form. Another experiment involved the release of moths early in the day and later recapture of some of them. Depending upon the environment, more light coloured moths or more darker ones were recaptured. In the po lluted woods a greater proportion of dark forms was recaptured while in the unpolluted woods the situation was reversed. Dr. Kettlewell concluded that bird predation in a polluted environment was driving the peppered moth population towards a predominantl y dark population. On the basis of his data, this seemed reasonable enough. What was not reasonable, even then, was the idea that this process had anything to do with evolution.

Since then, forty years have come and gone. We now realize that Dr. Kettlewell’s interpretation concerning bird predation may be all wrong. For a start, these moths fly at night – not during the day when the hungry insect-eating birds are about. Secon dly, in forty years of observations in the wild, only six moths have ever been observed resting on tree trunks. As a matter of fact, we still do not know where the moths rest in the daytime. Suffice it to say that they are well hidden. Hungry birds will not readily find a peppered moth to eat.

Thirdly, in the same area in England, there are marked differences in the proportion of melanics (dark forms) found among populations of other night-flying moth species. Obviously some different factors are operating in these various populations. Oth erwise they would all have the same proportion of dark forms. The question then arises as to what these other factors might be which promote higher or lower proportions of dark forms. On this topic, a paper by J. A. Bishop and Laurence M. Cook (1975. Moth s, Melanism and Clean Air. Scientific American. January pp. 90-99) speculated that the popular explanation might be wrong. Concerning the gap between predictions based on Kettlewell’s model, and actual observations, they reflected: “The discrepancy may indicate we are not correctly assessing the true nature of the resting sites of living moths when we are conducting experiments with dead ones [glued onto trees]. Alternatively, the assumption that natural selection is entirely due to selective preda tion by birds may be mistaken.” (pp. 97-98) That was already twenty four years ago when those suggestions were made. They already understood that there were problems. In the interim nothing changed as far as educational curricula went.

Further indications that the Kettlewell model is wrong have come from data in the United States. The numbers from the Liverpool environs in England, and southern Michigan are almost identical. For example the proportion of melanics in both places in 19 60 was over 90% and just below 20% for both in 1994. However the number of moths caught in England was many times larger than in Michigan. Despite the close similarity in the proportions, examination of the Michigan environment revealed no changes in tree trunk appearance over that time interval. Thus camouflage could not have been a factor in the change in proportions of the moth population. Indeed the scientists could find no explanation which fits both locations. They concluded: “Although the evolution of melanism in this southern Michigan population seems to have paralleled the changes in British populations in both directions [increasing and later decreasing proportions of dark forms], common causes for the changes are not obvious.” (Bruce Grant, Den is F. Owen and Cyril A. Clarke. 1995. Decline in melanic moths. Nature 373 February 16 p. 565).

Thus we find in 1999 that all those pictures in science texts were staged which show moths resting on tree trunks. These pictures do not reflect what happens in nature. Moreover the moths hide so well that people, at least, can’t find them. In addition , the changes in proportion of dark and light moth forms do not necessarily have anything to do with camouflage. The only thing we know for certain is that the moth populations have changed their proportions of dark and light forms at the same time that p ollution has increased and declined. To this point no single explanation has been found. It seems however that one factor which is unlikely is differential bird predation. The actual cause could well be a complex of factors. The text book discussions should be completely rewritten or the moths left out altogether. Indeed, at this point, the traditional peppered moth story is myth, not science.

Margaret Helder
July 1999

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