Peppered moth
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| Biston betularia Linnaeus, 1758 |
The peppered moth (Biston betularia (L.)) is a temperate species of night-flying moth notable for industrial melanism in Europe and North America, but particularly in Britain. Differential rates of bird predation on genetically controlled morphs as a form of natural selection caused evolution.
Introduction
The industrial revolution in Britain, starting in the 1840s, produced a large amount of airborne pollution both in the form of soot and toxic gases. These killed lichens on the bark of trees in, and downwind of, the industrialised areas. Until this time, adult peppered moths came in one pale speckled form, f. typica, which was camouflaged against these lichens. A new dark, melanic, form called f. carbonaria was recorded in Manchester, Britain in 1848. The phenotype frequency of this new morph increased and spread to other areas. Since melanism was controlled genetically, evolution was recorded. The correlation between industrialisation and melanism resulted in the phenomenon which was recorded in many other moth species as industrial melanism. This was due to the melanic morph being better camouflaged from predatory birds whilst resting on trees denuded of lichens; i.e. by natural selection, as shown by the classic experiments of Bernard Kettlewell. From the 1960s onwards the Clean Air Acts reduced the pollution levels, the lichens returned and the frequency of the typica morph increased. Similar changes occurred across Europe and North America.
The peppered moth story has become a staple of school textbooks. The opportunity is taken here to explain the ecology and life cycle of the peppered moth, the classic experiments of Bernard Kettlewell and later workers, and criticisms of them. Suggestions that Kettlewell committed fraud appear to be groundless, and despite assertations to the contrary, particularly from creationists, there is no evidence of any paradigm shift amongst the leading researchers.
Genetics
Main article: peppered moth genetics
The morphs are controlled genetically. At present the biochemistry of the melanism remains unknown, though it should be possible for it to be elucidated.
Ecology and Life Cycle
Main article: peppered moth ecology (including a discussion of resting positions)
In Britain, the peppered moth is univoltine (i.e. it has one generation per year), whilst in south-eastern North America it is bivoltine (two generations per year). The Lepidopteran life cycle consists of four stages; ova (eggs), several larva instars (caterpillars), pupae and imagines (adults). During the day, the moths cryptically rest on trees, where they are predated by birds.
Evolution
Main article: peppered moth evolution
Predation Experiments
Main Article: peppered moth predation experiments
Experiments to show differential bird predation in the wild have been conducted, notably by Bernard Kettlewell.
An experimental design has to be both practical and produce enough data to generate useful statistics. Experiments with the peppered moth have all suffered from some degree of artificiality.
Attacking the Peppered Moth Paradigm
Attacks on the peppered moth paradigm have come from two sources. Firstly, some scientists have proposed alternative explanations, particularly the phenotypic induction hypothesis, which is discredited. Secondly there have been attacks by creationists.
Alternative Selection Mechanisms
Selection is differential
Several alternative selection mechansisms have been proposed. Note again that evolution can be defined as "a change in an allele frequency within a gene pool" (Dobzhansky, 1937). This change, be it caused by natural selection, mutation, migration or genetic drift by definition, is differential (see evolutionary differentiation). However, the magnitude of the changes observed can only be accounted for by natural selection. It can be seen from population genetics that a non-differential change will not cause evolution. If the allele frequencies are denoted by the algebraic terms p and q, and (say) p = 0.6 and q = 0.4, then a non-differential reduction in population size from say 2000 to 100 individuals, will still produce the same values of (approximately) p = 0.6 and q = 0.4.
The Phenotypic Induction Hypotheis
John William Heslop-Harrison (1920) rejected Tutt's differential bird predation hypothesis, on the basis that he did not believe that birds ate moths. Instead he advocated the idea that pollutants could cause changes to the soma and germ plasm of the organism.
The origin of this hypothesis probably has its roots in the 1890s, when it was proposed as a form of Lamarckism. It is important to note its historical context.
The first to try to prove this hypothesis was Hasebroek (1925), who contended that air pollution altered lepidopteran physiology, thus producing an excess of black pigment. He exposed pupae of Lepidoptera to various doses of pollutant gases, namely hydrogen sulphide (H2S), ammonia (NH3) and "pyredin" (presumably his spelling of pyridine). He used eight species in his studies, four of which were species of butterfly that did not exhibit melanism. Ford (1964) contends that Hasebroek's illustrations showed that the abnormal forms that appeared were not melanics, and Hasebroek failed to study their genetics.
Heslop Harrison (Harrison and Garrett 1926; Harrison 1928) suggested that the increase of melanic moths in industrialised regions was due to "mutation pressure", not to selection by predators which he regarded as negligible. Salts of lead and manganese were present in the airborne pollutant particles, and he suggested that these caused the mutation of genes for melanin production but of no others. He used Selenia bilunaria and Tephrosia bistortata as material. The larvae were fed with leaves that had incorporated these salts and melanics subsequently appeared.
Similar experiments by Hughes McKenney (1932) and Thomasen and Lemche (1933) failed to replicate these results. However, the statistician and geneticist Ronald Fisher, showed that Heslop Harrison's controls were inadequate (Fisher, 1933).
This theory however appeared to be falsified by breeding experiments. Further evidence, if it were needed, is likely to come from research into the biochemistry of melanism.
Creationism
Traditionally creationist claims have been limited to pointing out that the "peppered moth story" showed only microevolution rather than the important macroevolutionary trend of speciation (e.g. Watchtower Bible and Tract Society, 1985) but this is not disputed. Other claims fall into more general categories and should be explained elsewhere.
From 1998 onwards however, this changed. A review by evolutionary biologist Jerry Coyne of Majerus's 1998 book Melanism: Evolution in Action was published in Nature. It famously included the following:
- [Coyne compared his reaction to] "the dismay attending my discovery, at age 6, that it was my father and not Santa who brought the presents on Christmas Eve."
- "For the time being we must discard Biston as a well-understood example of natural selection in action…
However, it appears that Coyne was strongly influenced by Sargent et al (1998), and it has been contended by Majerus and others as not reflecting Majerus' original work. Secondly, Coyne later claimed that he is misquoted by creationists.
This was subsequently picked up by Sunday Telegraph journalist Robert Matthews, to who wrote:
- "Evolution experts are quietly admitting that one of their most cherished examples of Darwin's theory, the rise and fall of the peppered moth, is based on a series of scientific blunders. Experiments using the moth in the Fifties and long believed to prove the truth of natural selection are now thought to be worthless, having been designed to come up with the “right” answer."
Majerus claims Matthews hadn't read his book Melanism: Evolution in Action, and that Majerus had tried to explain the principles of the story to him in a 30 minute telephone conversation. Majerus alleges that the report contains misquotations of him, and the report includes several factual errors.
Subsequently, the creationist theologian, Moony and biochemist Jonathan Wells wrote an essay on the subject (2001), a shortened version of which appeared in The Scientist (Wells, 1999). Also, he wrote a book Icons of Evolution (Wells, 2000), which attacks.
In 2002 Hooper's Of Moths and Men was published, which claimed that Bernard Kettlewell had committed scientific fraud (see above). Despite scathing criticism in book reviews in scientific journals (e.g. Coyne in Nature, Bryan Clarke in Heredity, Bruce Grant in Science)