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.
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.
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.
References:
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.
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.
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.
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.
Haldane, J. B. S., 1956. The theory of selection for melanism in
Lepidoptera, Proc. R. Soc. Lond. (B), 145: 303-306.
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.
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.
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.
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.