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Claim CB601.2.3:

South of latitude 52 degrees north, melanism in the peppered moth showed no correlation with sulphur dioxide concentration, and the proportion of dark moths increased there after pollution control was introduced. These observations are inconsistent with Kettlewell's explanation that the spread of the dark moths was caused by natural selection resulting from selective predation.


Wells, Jonathan, 1999. Second thoughts about peppered moths. ,
Wells, Jonathan, 2000. Icons of Evolution, Washington DC: Regnery Publishing Inc., pp. 144-146.


  1. Wells (2000, 146) wrote:
    R.C. Steward found a correlation between melanism and the concentration of sulfur dioxide (an airborne pollutant) north -- but not south -- of latitude 52 degrees north.
    The assertion that Steward found no correlation between melanism and the concentration of sulphur dioxide south of latitude 52 degrees north is simply false. The correlation which Steward (1977) found between the proportion of dark moths and the concentration of sulphur dioxide (or, more accurately, its square-root) was highly significant both over England and Wales as a whole, and south of latitude 52 degrees north.

    What Steward in fact observed was that for 165 sites scattered over the whole of England and Wales, the square-root of the concentration of sulphur dioxide was the most significantly correlated with the proportion of dark moths out of the thirteen variables he tested. South of latitude 52 degrees north, however, the most significantly correlated variable was east-west location, rather than the square-root of sulphur dioxide concentration.

    It was nevertheless true that the correlations of both east-west location and square-root of sulphur dioxide concentration with the proportion of dark moths were highly significant both when all sites were included in his analysis as well as when only those south of the given latitude were included.

    According to Steward these observations supported an inference he had drawn from other results that
    in the south of Britain non-industrial factors may be of greater importance in determining carbonaria frequency than in the rest of Britain (1977, 239).
    But his reasoning here is hard to follow. In the midlands and north of England, most of the major industrial centres are located towards the west, whereas in the south they are located in the east, near London. In the south-west, the counties of Somerset, Dorset, Cornwall and Devon are among the least polluted in the whole of England and Wales. Moreover, in the north and midlands the prevailing south-westerly winds carry airborne pollution long distances towards the east. It is therefore hardly surprising, if Kettlewell's explanation is valid, that the proportion of dark moths correlates more strongly with east-west location and less strongly with sulphur dioxide concentration in the south than it does in the north.

    Not content with merely taking Steward's shaky argument at face value, however, Wells (2000, 146) badly misquotes him when he writes:
    Steward concluded that "in the south of Britain non-industrial factors may be of greater importance" than camouflage and bird predation.
    This clearly misrepresents the quoted statment of Steward's as referring to a comparison of non-industrial factors with camouflage and bird predation, whereas his actual comparison was between non-industrial factors in the south and those same factors in the north.

    Had Wells omitted the quotation marks, the resulting indirect quotation could perhaps have been justified as a paraphrase of something which Steward did say earlier in his article (1977, 238):
    The results suggest that, although selective predation may have an important secondary effect on carbonaria frequency, it is not the major factor determining frequencies at these sites.
    "These sites" here refers to 52 sites, of which 48 were located south of latitude 52 degrees north, where Steward had studied the effects of camouflage on rates of predation. Also, "the results" being referred to are those from this study of the effects of camouflage, not those from the study of correlation between melanism and other variables.

    Nevertheless, even this tentative suggestion of Steward's seems to have been unwarranted. Mani (1990) showed that the observed relative proportions of the three varieties of peppered moth could be well accounted for using no more than experimentally determined rates of visual predation and migration, and strengths of non-visual selection estimated from field data (see the response to claim CB601.2.2).

  2. The increase in the proportion of dark moths mentioned in the claim appears to be a phenomenon for which no explanation has yet been presented in the scientific literature. Bishop and Cook (1980, 398) merely say "The reason is not obvious". Nevertheless, there is a quite plausible (but as yet speculative) explanation in terms of cline dynamics, which is outlined below.

    The increase in question was first pointed out by Lees and Creed (1975, 71, 78). They noted that at a large number of locations south-west of a line running north-west from roughly just south of London to north-east Wales, the proportion of dark moths increased over the period from the mid 1950s to about 1970, while at most locations north-east of this line it decreased. Note that the line separating these regions of increase and decrease was not, as claimed by Wells (2000, 145), the same line of latitude 50 degrees north that was associated with the phenomenon discussed above in relation to the first part of the claim. Most of the increases and decreases were quite small---Lees and Creed noted that they were not statistically significant at most of the individual locations considered in isolation. However, when the figures from individual locations were combined to form a total net increase south-west of the line, and a total net decrease north-east of it, the results were highly significant.

    As noted in the response to claim CB601.2.2, peppered moths over most of England and Wales form a series of clines -- namely, populations over extended areas whose compositions varied from place to place. It is very possible that the cline extending from south-west England and Wales to north-east England and East Anglia was not in equilibrium when pollution controls were introduced there starting from the late 1950s. If this were the case, the proportion of dark peppered moths would have then been increasing over large portions of the cline. The first crucial point here is that while this disequilibrium towards increasing dark moth proportions would start to decrease not long after the introduction of pollution controls, it would not have disappeared immediately. Thus, the proportion of dark moths would have continued to increase for a short time until the disequilibrium was reduced to zero. The population would then pass through a transient equilibrium into a disequilibrium in the opposite direction. The proportion of dark moths in the population would only then start to decrease.

    The second crucial point is that there is no reason why the various parts of the population should have all passed through the stage of transient equilibrium at the same time. Lees and Creed's observation (op. cit.) would seem to indicate that the population to the north-east of their dividing line had already passed through this equilibrium by the end of the period covered by their data, while that to the south-west of it had not.

    While this explanation remains as yet speculative, it is by no means contrived, and is, at least partially, testable. The mathematical models of Mani (1982, 1990), referred to in the responses to CB601.2, CB601.2.1, and CB601.2.2, are not sensitive enough in their current form to reproduce the frequency variations seen in the phenomenon noted by Lees and Creed. However, if the explanation proposed above is correct, it should be possible to make plausible minor modifications to one of Mani's models which would cause it to do so. Thus, if it were to be found that no such modified version of any of Mani's models could be made to do this, then the proposed explanation would be disconfirmed. Conversely, if the observed frequency variations could be reproduced by such a modified model, then that would provide some (admittedly fairly weak) support for the explanation.


  1. Lees, D. R. and E. R. Creed, 1975. Industrial melanism in Biston betularia: the role of selective predation. J. Anim. Ecol. 44: 67-83.
  2. Mani, G. S., 1982. A theoretical analysis of the morph frequency variation in the peppered moth over England and Wales. Biol. J. Linn. Soc. 17: 259-267.
  3. Mani, G. S., 1990. Theoretical models of melanism in Biston betularia -- a review. Biol. J. Linn. Soc. 39: 355-371.
  4. Steward, R. C., 1977. Industrial and non-industrial melanism in the peppered moth, Biston betularia (L.), Ecological Entomology 2: 231-243.

Further Reading:

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