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Claim CB601.2.4:

In the Wirral Peninsula, East Anglia and Michigan, the proportions of light moths increased before any lichens had reappeared on the trees. This is a serious problem for the theory that industrial melanism is due to cryptic coloration and selective predation.


Wells, Jonathan, 1999. Second thoughts about peppered moths.,
Wells, Jonathan, 2000. Icons of Evolution, Washington DC: Regnery Publishing Inc., pp. 146-148.


  1. The conclusion of the claim is a non sequitur for at least two reasons.

    1. The dark moths were better camouflaged in industrial areas not merely because the surfaces of the trees were lichen-free but also because they had been darkened by soot and smoke. When pollution control was introduced, the surfaces of the trees in heavily polluted industrial areas soon became lighter (Bishop and Cook 1975, 92-93). Even though the black moths would still have enjoyed a selective advantage over the light ones in these areas, it would nevertheless have been smaller than it had been earlier, so the proportion of black moths would be expected to decrease, because their now smaller advantage would no longer be sufficient to counterbalance the effects of migration of the light moths from neighboring areas where they were more common.

      Thus, even if the disappearance of lichens does turn out to have been an important factor in the original spread of the dark moths, there seems to be no good reason why their their reappearance should have preceded the recovery in the numbers of the light moths.

    2. After pollution controls were introduced, lichens would be expected to first recolonize those areas straddling the borders between the countryside where they had remained abundant and industrial centers where they were absent. The proportion of light moths would therefore be expected to increase in these areas first, and then in areas closer to the industrial centers a little later, as the influence of migration began to take effect.

    Thus, it is quite possible for the recovery of lichens in one area to have been indirectly responsible for an increase in the proportion of light moths in another area where lichens had remained absent. Indeed, there is observational evidence to indicate that this may well have been a contributing factor to the increase in the proportion of the light moths in the Wirral during the late 1960s and early 1970s (see below).

  2. Claims that the proportion of light moths began to increase before lichens recovered are based on anecdotal evidence which is either largely irrelevant because it refers to only a single location (Clarke et al. 1985, 191-193), or is so lacking in detail that its significance is difficult to assess (Kettlewell 1973, 151; Clarke et al. 1985, 193; Grant and Howlett 1988, 231; Grant et al. 1995, 1996, 355-356). Moreover, for the Wirral and East Anglia, there appears to be direct observational evidence that contradicts it.

    The first indication of an increase in the proportion of light peppered moths in Britain were obtained by Clarke and Sheppard (1966) at Caldy on the Wirral peninsula. The increase appears to have started between 1962 and 1963, some six or seven years after Britain had passed the Clean Air Act of 1956.

    Henderson-Sellers and Seaward (1979) recorded the recolonization of a large urban area in West Yorkshire by the crustose lichen Lecanora muralis over the period 1969 to 1977. They found that the recolonization of a given location occurred about 5 years after the atmospheric concentration of sulfur dioxide had fallen below 120 μg m-3 there, and progressed at an average rate of about 23 km2 per year over the period of the study. At the start of their study, the lichen was already well established over an area of about 100 km2, indicating that the recolonization could well have been occurring since the early 1960s. While L. muralis is a species of little importance for the protection of the peppered moth (because it inhabits surfaces of stone), closely related species of pollution-resistant lichens, such as L. conizaeoides, which may well be of such importance, are likely to have behaved similarly. Indeed there is some corroborating evidence for this.

    The areas where various bryophytes and lichens appeared on trees in southern Britain in the late 1960s are recorded on a map of Hawksworth and Rose (1970). This map shows that over the Wirral peninsula, and a strip of land about 10km wide on the other side (i.e. the south-western bank) of the river Dee delta, the lichen cover scored 0 to 2 on their classification scale. Roughly speaking, a higher score on the scale corresponds to a greater abundance of the more pollution-sensitive lichens. The categories 0 to 2 correspond to the absence of any lichens except for the grey-green L. conizaeoides occurring at the bases of trees in zones of classification 2. However, measurements by Bishop et al. (1975), showed that by 1975 the land south-west of the Dee delta had been sufficiently recolonized by lichens to be reclassified into category 3 in the south-east and 4 in the north-west. Zones of category 3 are characterized by the trees having L. conizaeoides extending up their trunks, and another crustose species, Lepraria incana, frequent at their bases. Zones of category 4 are characterized by the occurrence of some of the more hardy foliose species of lichen at the bases of trees.

    While these observations were only made after the numbers of light moths had started to increase on the Wirral peninsula, they nevertheless seem to contradict the impression of Wynne (Clarke et al. 1985, 193) that "there has been little change in the lichen situation in N Wales in the last 30 years". Thus, there seems to be no good reason for supposing that the changes in lichen cover recorded by Bishop et al. (1975) had only started to occur some time after Hawksworth and Rose had drawn up their map.

    Kettlewell (1973, 124) stated that in 1954 he had examined large areas of countryside over the eastern half of England, including parts of East Anglia, without finding any lichened woods south of Yorkshire. Yet Hawksworth and Rose's (1970) map shows that by 1970 most of East Anglia scored 6 to 7 on their scale, and a substantial area on the north-eastern coast scored 8. Zones in these categories are characterized by trees bearing a wide range of both foliose and crustose lichens on their trunks. This would appear to contradict Howlett's observation that the recovery of the light moths in East Anglia occurred "in the virtual absence of these lichens". (Grant and Howlett 1988, 231).

    Until the apparent contradictions between these accounts can be resolved by further investigations, categorical declarations that that the proportion of light peppered moths started to increase before the reappearance of any lichens would seem to be unwarranted.


  1. Bishop, J. A. and L. M. Cook, 1975. Moths, melanism and clean air. Scientific American 232: 90-99.
  2. Bishop, J. A., L. M. Cook, J. Muggleton and M. R. D. Seaward, 1975. Moths Lichens and Air Pollution along a Transect from Manchester to North Wales. J. App. Ecol. 12: 83-98.
  3. Clarke, C. A., G. S. Mani and G. Wynne, 1985. Evolution in reverse: clean air and the peppered moth. Biol. J. Linn. Soc. 26: 189-199.
  4. Clarke, C. A. and P. M. Sheppard, 1966. A local survey of the industrial melanic forms in the moth Biston betularia and estimates of the selective values of these in an industrial environment. Proc. R. Soc. Lond.(B) 165: 424-439.
  5. Grant, Bruce and Rory J. Howlett, 1988. Background selection by the peppered moth (Biston betularia Linn.): Individual differences. Biol. J. Linn. Soc. 33: 217-232.
  6. Grant, Bruce, Denis F. Owen and Cyril A. Clarke, 1995. Decline of melanic moths. Nature 373: 565.
  7. Grant, B.S., D.F. Owen and C. A. Clarke, 1996. Parallel rise and fall of melanic peppered moths in America and Britain. J. Hered. 87: 351-357.
  8. Hawksworth, D. L. and F. Rose, 1970. Qualitative scale for estimating sulphur dioxide air pollution in England and Wales using epiphytic lichens. Nature 227: 145-148.
  9. Henderson-Sellers, A. and M. R. D. Seaward, 1979. Monitoring lichen reinvasion of ameliorating environments. Environ. Pollut. 19: 207-213.
  10. Kettlewell, Bernard, 1973. The Evolution of Melanism. Oxford University Press, London.

Further Reading:

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