Melanism: Evolution in Action
- Michael E. N. Majerus
From time to time, evolutionists re-examine a classic experimental study and find, to their horror, that it is flawed or downright wrong. We no longer use chromosomal polymorphism in Drosophila pseudoobscura to demonstrate heterozygous advantage, flower-colour variation in Linanthus parryae to illustrate random genetic drift, or the viceroy and monarch butterflies to exemplify Batesian mimicry. Until now, however, the prize horse in our stable of examples has been the evolution of ‘industrial melanism’ in the peppered moth, Biston betularia, presented by most teachers and textbooks as the paradigm of natural selection and evolution occurring within a human lifetime. The re-examination of this tale is the centrepiece of Michael Majerus's book, Melanism: Evolution in Action. Depressingly, Majerus shows that this classic example is in bad shape, and, while not yet ready for the glue factory, needs serious attention.
According to the standard textbook litany, before the mid-nineteenth century, all B. betularia in England were white moths peppered with black spots, a form called typica. Between 1850 and 1920, typica was largely replaced by a pure black form (carbonaria) produced by a single dominant allele, the frequency of which rose to nearly 100% in some areas. After 1950, this trend reversed, making carbonaria rare and typica again common. These persistent and directional changes implied natural selection. In a series of studies, this conclusion was verified by several investigators, most prominently Bernard Kettlewell of Oxford.
According to these workers, the evolution of colour was caused by birds eating the moths most conspicuous on their normal resting site — tree trunks. The increase in black moths was attributed to pollution accompanying the rise of heavy industry. A combination of soot and acid rain darkened trees by first killing the lichens that festooned them and then blackening the naked trunks. The typica form, previously camouflaged on lichens, thus became conspicuous and heavily predated, while the less visible carbonaria enjoyed protection and increased in frequency. After the passage of the Clean Air Acts in the 1950s, trees regained their former appearance, reversing the selective advantage of the morphs. This conclusion was bolstered by a geographical correlation between pollution levels and morph frequencies (carbonaria was most common in industrial areas), and most prominently by Kettlewell's famous experiments which showed that, after releasing typica and carbonaria in both polluted and unpolluted woods, researchers recaptured many more of the cryptic than of the conspicuous form. The differential predation was supported by direct observation of birds eating moths placed on trees. Finally, Kettlewell demonstrated in the laboratory that each form had a behavioural preference to settle on backgrounds that matched its colour.
Criticisms of this story have circulated in samizdat for several years, but Majerus summarizes them for the first time in print in an absorbing two-chapter critique (coincidentally, a similar analysis [Sargent et al., Evol. Biol. 30, 299-322; 1998] has just appeared). Majerus notes that the most serious problem is that B. betularia probably does not rest on tree trunks — exactly two moths have been seen in such a position in more than 40 years of intensive search. The natural resting spots are, in fact, a mystery. This alone invalidates Kettlewell's release-recapture experiments, as moths were released by placing them directly onto tree trunks, where they are highly visible to bird predators. (Kettlewell also released his moths during the day, while they normally choose resting places at night.) The story is further eroded by noting that the resurgence of typica occurred well before lichens recolonized the polluted trees, and that a parallel increase and decrease of the melanic form also occurred in industrial areas of the United States, where there was no change in the abundance of the lichens that supposedly play such an important role.
Finally, the results of Kettlewell's behavioural experiments were not replicated in later studies: moths have no tendency to choose matching backgrounds. Majerus finds many other flaws in the work, but they are too numerous to list here. I unearthed additional problems when, embarrassed at having taught the standard Biston story for years, I read Kettlewell's papers for the first time.
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.
Occupying a quarter of the book, the Biston analysis is necessary reading for all evolutionists, as are the introductory chapters on the nature of melanism, its distribution among animals, and its proposed causes. Majerus, however, designed his book for both professional and lay readers, and this causes some unevenness in the material. The Biston story is sandwiched between less compelling chapters, including long sections on the basic principles of genetics and evolution, which can be skipped by evolutionists. Other discussions, involving melanism in ladybirds and other Lepidoptera, as well as the author's unpublished work on habitat selection, are full of technical details that will overwhelm the lay reader. Unfortunately, most of the work described is inconclusive; despite the widespread occurrence of melanism, its evolutionary significance is nearly always unknown.
What can one make of all this? Majerus concludes with the usual call for more research, but several lessons are already at hand. First, for the time being we must discard Biston as a well-understood example of natural selection in action, although it is clearly a case of evolution. There are many studies more appropriate for use in the classroom, including the classic work of Peter and Rosemary Grant on beak-size evolution in Galapagos finches. It is also worth pondering why there has been general and unquestioned acceptance of Kettlewell's work. Perhaps such powerful stories discourage close scrutiny. Moreover, in evolutionary biology there is little payoff in repeating other people's experiments, and, unlike molecular biology, our field is not self-correcting because few studies depend on the accuracy of earlier ones. Finally, teachers such as myself often neglect original papers in favour of shorter textbook summaries, which bleach the blemishes from complicated experiments.
It is clear that, as with most other work in evolutionary biology, understanding selection in Biston will require much more information about the animal's habits. Evolutionists may bridle at such a conclusion, because ecological data are very hard to gather. Nevertheless, there is no other way 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.
About this article
Proceedings of the National Academy of Sciences (2019)
Communications Biology (2018)
PLoS ONE (2014)
Biology Letters (2012)