Science Fair Project Encyclopedia
parva The peppered moth (Biston betularia) is a temperate species of night-flying moth often used by educators as an example of natural selection (see theory of evolution, industrial melanism ).
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.
Main article: peppered moth genetics
The are several melanic and non-melanic morphs. In Britain, the typical white speckled morph is known as f. typica, the melanic morph is f. carbonaria and the intermediate phenotype is f. insularia. These are controlled genetically. At present the biochemistry of the melanism remains unknown, though it should be possible for it to be elucidated.
Main article: peppered moth evolution
The first carbonaria morph was recorded by Edleston in Manchester in 1848, and over the subsequent years it increased in frequency. This evolution was attributed to natural selection (i.e., too fast to be due to genetic drift), though the increase was not monitored very effectively.
Predation experiments (see below) particularly by Bernard Kettlewell established that the agent of selection was birds who predated on the f. carbonaria morph.
The Clean Air Acts reduced levels of pollution, and typica morph frequency has increased, again generally seen as an example of natural selection.
A similar process was reported in North America.
Main Article: peppered moth predation experiments
Experiments to show differential bird predation in the wild have been conducted, notably by Bernard Kettlewell, who in 1953 and 1955 conducted classic experiments mark-release-recapture experiments. Later experiments have shown that was qualitatively correct.
Main article: peppered moth alternative theories
Alternative theories to explain industrial melanism were proposed during the 1920s and 1930s. Particularly notable was the phenotypic induction hypothesis of J.W. Heslop-Harrison, who proposed that industrial pollutants could induce mutations in the peppered moth. The quality of the original science was poor.
Later some scientific dissenters have criticised the acceptance of the peppered moth story. In particular, Sargent et al (1998) argued for a developmental induction hypothesis, Cook, Grant and Majerus have all argued that Sargent et al's hypothesis does not fit the evidence.
Main article: creationism and the peppered moth
Beginning in 1998, the traditional peppered moth story has been criticised by several people, but most prominently by creationists. "All the peppered-moth pictures were staged," said biologist Jonathan Wells, a senior fellow at the Discovery Institute. "Scientists have known since the 1980s that the moths do not normally rest on tree trunks."
The scientific community remains unimpressed.
The definitive reference on this work is Majerus' 1998 book Melanism: Evolution in Action, which provides an introduction to evolutionary theory as well as describing the peppered moth case study. There have been subsequent summaries by Cook [erm...], and Bruce Grant's can be downloaded here: PDF format
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