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Claim CB601.2.1:

Melanic moths never completely replaced non-melanic ones in heavily polluted areas such as Manchester. This is inconsistent with the theory that visually selective predation was responsible for their increase in numbers.


Wells, Jonathan, 1999. Second thoughts about peppered moths. or
Wells, Jonathan, 2000. Icons of Evolution, Washington DC: Regnery Publishing Inc., pp. 144-149.


  1. Other factors besides visual selection affect frequencies. These other factors, especially migration, explain the frequency of light-colored moths in heavily polluted industrial areas.

    The claim is based on a calculation of Haldane's (1956). He had used a crude 2-allele model of natural selection, and data collected by Kettlewell (1956), to estimate the length of time it should have taken for the proportion of light moths to fall below 5%. The model assumes that the local relative selection intensity between the light and dark moths is the only factor influencing the changes in their frequencies. With selection of the intensity suggested by Kettlewell's data, Haldane found that the proportion of light moths should have fallen below 5% within about 30-40 years. However, about 10% of the wild moths Kettlewell had collected were of the light form, and the numbers he collected were sufficiently large for this difference to be statistically significant. Moreover, the conditions near Birmingham where Kettlewell collected his data had apparently favoured the melanic moths for much longer than 40 years.

    Haldane concluded that either the local selection intensity was normally much less than Kettlewell's data suggested, or that it was not the only factor influencing the relative frequencies of the light and dark moths. He proposed two possible explanations---gene flow from unpolluted areas, which he dismissed as unlikely to be important, and heterozygous advantage. Evidence in favour of the latter hypothesis is at best equivocal (Creed et al. 1980, 258ff; Bishop et al. 1978, 507ff). However, evidence suggesting that gene flow might be more important than Haldane thought has accumulated steadily since he performed his calculations.

    First, Kettlewell (1958, 60) and Bishop (1972, 222ff) found that male peppered moths can travel quite large distances each day. Cook and Mani (1980) then developed a mathematical model which incorporated the effects of both migration and selection. Although the continued presence of light moths in heavily polluted industrial areas was well accounted for by this model, there were other features of the moth distribution which weren't. Mani (1982), however, found that these features could be accounted for by incorporating the effects of non-visual selection into the model (see the reply to claim CB601.2.2 ).

    None of these developments has cast any doubt on the importance of visually selective predation in determining the relative frequencies of the different forms of peppered moth. In fact, Mani and Cook's partially successful model and Mani's successful one both incorporated strong visual selection, with intensities estimated from field data.

  2. Manchester is a particularly poor choice of location to cite as evidence for the claim. The only published data on the numbers of peppered moths collected in Manchester between the early 20th century and the mid 1960s are those of Michaelis, first published by Kettlewell (1958, 1965, as cited by Askew et al. 1971, 251). Not one of the 760 peppered moths collected by Michaelis between 1952 and 1964 was of the typical light variety.

    It is true that eleven (i.e. 1.4%) of the moths collected by Michaelis were not of the darkest melanic form, carbonaria, but of a lighter one, called insularia. But from the data available, Haldane's model is completely incapable of providing any meaningful estimate of what the frequency of insularia should have been in Manchester in the 1950s. All that could be concluded from it is that if the conditions in Manchester had satisfied its underlying assumptions, then the proportion of the typical light moth there should have been extremely low. But that is exactly what was observed.


  1. Askew, R. R., L. M. Cook and J. A. Bishop, 1971. Atmospheric pollution and melanic moths in Manchester and its environs, J. Appl. Ecol., 8: 247-256.
  2. Bishop, J. A., 1972. An experimental study of the cline of industial melanism in Biston betularia (L.) (Lepidoptera) between urban Liverpool and rural North Wales, J. Anim. Ecol., 41: 209-243.
  3. Cook, L. M. and G.S. Mani, 1980. A migration-selection model for the morph frequency variations in the peppered moth over England and Wales, Biol. J. Linn. Soc., 13, pp.179-198.
  4. Creed, E. R., D. R. Lees and M. G. Bulmer, 1980. Pre-adult viability differences of melanic Biston betularia (L.) (Lepidoptera), Biological Journal of the Linnean Society, 13: 251-262.
  5. Haldane, J. B. S., 1956. The theory of selection for melanism in Lepidoptera, Proc. R. Soc. Lond. (B), 145: 303-306.
  6. Kettlewell, H. B. D., 1956. A resume of investigations of the evolution of melanism in the Lepidoptera, Proc. R. Soc. Lond. (B), 145: 297-303.
  7. Kettlewell, H. B. D., 1958. A survey of the frequencies of Biston betularia (L.) (Lep.) and its melanic forms in Great Britain, Heredity, 12: 51-72.
  8. Kettlewell, H. B. D., 1965. A 12-year survey of the frequencies of Biston betularia (L.) (Lep.) and its melanic forms in Great Britain, Entomologist's Rec. J. Var., 77: 195-218.
  9. Mani, G. S., 1982. A theoretical analysis of the morph frequency variation in the peppered moth over England and Wales. Biological Journal of the Linnean Society 17: 259-267.

Further Reading:

Grant, Bruce S., 1999. Fine tuning the peppered moth paradigm. Evolution 53(3): 980-984. ""
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