Claim CB601.2.2:

In rural East Anglia, where there were lichen-covered trees, and typical light moths seemed better camouflaged than melanics, the latter reached a frequency of 80%. In rural areas of northern Wales the proportion of melanic moths was higher than expected, and in southern Wales, where melanics were better camouflaged, they comprised only 20% of the population. This is inconsistent with Ford and Kettlewell's explanation for the spread of the dark moths in terms of selective visual predation.

Source:

Response:

1. The relative proportions of light and dark moths in East Anglia and northern Wales have been very well accounted for by a combination of visual selective predation, non-visual selection, and gene flow. This explanation merely fleshes out some of the details of Ford and Kettlewell's, and is perfectly consistent with it.
   
   Indeed, both visual selective predation and non-visual selection formed part of Ford and Kettlewell's original explanation for the spread of the dark peppered moth. Ford (1937, 487) was aware that the melanic forms of many species of moth and butterfly seemed to be more hardy than the non-melanic ones. Creed et al. (1980) found strong evidence that this was true for the peppered moth. Thus, the occurrence of non-visual selection in favour of the dark peppered moth is not merely a just-so story invented to account for problems with observed data. There is in fact strong experimental evidence for it.
   
   While the effects of gene flow did not feature prominently in Ford and Kettlewell's explanation for the spread of the dark peppered moth, it is evident from their accounts that they were nevertheless well aware of its influence.
   
   In the tracts of land between unpolluted areas of Britain, such as East Anglia and central Wales, and the highly polluted industrial centres around the midlands and north-west of England, light moths from the unpolluted areas and dark ones from the polluted areas, will clearly intermingle as they migrate from one area to another. Such areas, where the proportions of two or more different forms of the same species change from place to place are called clines. As one moves along such a cline, away from the polluted industrial centres of England towards the less polluted countryside, say, the proportion of dark moths in the population will obviously decrease to some value intermediate between those which it assumes at the extreme ends of the cline.
   
   There is no reason at all, however, to expect that the places in these clines where the light and dark moths are equally well camouflaged will coincide precisely with those where their proportions are equal. It is therefore to be expected that there will be areas within the clines where the worse camouflaged form will be more numerous than the better camouflaged. Indeed, since there is good evidence that non-visual selection favours the melanic form, it is natural to expect that this form will be more numerous in many areas of these clines where it is not as well camouflaged as the non-melanic form. This is exacly what has been observed in North Wales and East Anglia.
   
   The exact proportions of the two forms of moth at any location within a cline will depend on the strength of the selection favouring one form over the other at the given location, the current proportions of the two forms there and at nearby locations, and the rates of migration of the moths from one location to another. Using experimentally determined values for migration rates and the strengths of visually selective predation, and various assumptions about the strength of non-visual selection, Mani (1982, 1990) developed mathematical models for predicting the proportions of the three varieties of peppered moth throughout England and Wales. When he used field data to estimate the strength of non-visual selection (assumed constant for each of the several forms of moth), the predictions of his model fitted the observed moth frequencies along the various clines in England and Wales very well.
   
   It is true that Mani's estimate of the non-visual selection in favour of the dark peppered moth was significantly less than that of Creed et. al. (op. cit.). However, the results obtained by the latter investigators were for specimens bred in the laboratory, and there is every reason to believe that non-visual selection on moths in the wild would differ substantially from that on ones bred in a laboratory.
   
   Thus, when all the factors affecting the relative frequencies of the various forms of peppered moth in northern Wales and East Anglia are given their proper weight, the predictions which follow from Kettlewell and Ford's account are found to be in excellent agreement with the observed data.
   
   
2. The assertion by Wells (2000, 146) that "melanics in south Wales seemed better camouflaged than typicals, yet they comprised only about 20% of the population" is false. Wells appears to have misread a statement in one of the articles he cited in support of this assertion in his earlier essay of 1999. In the referenced article, Steward (1977a, 232) wrote

   . . . experiments in 1973 and 1974 at Tongwynlais in south Wales (Steward 1977b) showed that carbonaria was at an advantage to typical in this locality, and yet was only present at a low frequency (c.20%).

   Wells seems to have taken the words "this locality" in this sentence as referring to south Wales instead of to the rather more obvious and more limited vicinity of Tongwynlais. However, it is clear from the table appearing just two pages further on in Steward's article that the conditions he reported for Tongwylnais did not generalise to the whole of south Wales.
   
   Even more destructive of Wells's argument is the fact (which he conveniently fails to mention) that it is the intermediate, less intensely melanic form of peppered moth, insularia, rather than carbonaria which was both the best camouflaged and the most numerous of the three forms of moth in the vicinity of Tongwynlais. Indeed, this form constituted 44%, and the two melanic varieties together, 62%, of the population there. Given the selective advantage of insularia over both carbonaria and typica, there is nothing at all surprising about the frequency of carbonaria's being "only about 20%".
   
   What is mildly surprising at first sight is that the frequency of typica was about twice that of carbonaria. However, the relatively high frequency of typica can be easily explained as resulting from the migration of that form into the area from the surrounding countryside. In fact, the best of Mani's mathematical models (Mani 1982), which takes the effects of such migration into account, predicts the relative proportions of the three varieties of peppered moth over southern Wales quite well. So Wells's insinuation that these proportions are inconsistent with the classical explanations for the distribution of the varieties of peppered moth in southern Wales simply does not stand up to scrutiny.

References:

Further Reading:

Grant, Bruce S., 1999. Fine tuning the peppered moth paradigm. Evolution 53(3): 980-984. http://www.wm.edu/biology/melanism.pdf

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