Summations Only

Were all the moths found in the non-polluted areas born there? Lived there exclusively? Could the study prove this? Seems like it would be extremely hard - sort of like finding leaves on the ground, and trying to put them back on the tree in place.

Various theoretical models have been proposed to account for the discrepancies. Some include the effects of gene flow due to migration, though according to Jones "gene flow alone cannot explain... why melanics are so common in some unpolluted parts of Britain" (Jones 1982, p. 109).

Most textbook pictures of peppered moths show specimens that have been manually placed on tree trunks.11 Since 1980, however, it has become clear that peppered moths do not normally rest there. K. Mikkola observed that "the normal resting place of the Peppered Moth is beneath small, more or less horizontal branches (but not on narrow twigs), probably high up in the canopies, and the species probably only exceptionally rests on tree trunks." He noted that "night-active moths, released in an illumination bright enough for the human eye, may well choose their resting sites as soon as possible and most probably atypically." Thus "the results of Kettlewell (1955, 1956) fail to demonstrate the qualitative predation of the morphs of the Peppered Moth by birds or other predators in natural conditions."13Mikkola used caged moths, but data on wild moths support his conclusion. In 25 years of fieldwork, C.A. Clarke and his colleagues found only one peppered moth on a tree trunk, and admitted that they knew primarily "where the moths do not spend the day."14 When Howlett and M.E.N. Majerus studied the natural resting sites of peppered moths in various parts of England, they found that Mikkola's observations on caged moths were valid for wild moths as well and concluded: "[I]t seems certain that most B. betularia rest where they are hidden ... [and] that exposed areas of tree trunks are not an important resting site for any form of B. betularia."15 In a separate study, T.G. Liebert and P.M. Brakefield confirmed Mikkola's observations that "the species rests predominantly on branches .... Many moths will rest underneath, or on the side of, narrow branches in the canopy."16In a recent book on melanism, Majerus criticizes the "artificiality" of much previous work in this area, noting that "in most predation experiments peppered moths have been positioned on vertical tree trunks, despite the fact that they rarely chose such surfaces to rest upon in the wild."17 It seems that the classical example of natural selection is actually an example of unnatural selection

Figure 2. Problems with the classical story: The U.K. and The Netherlands. (a) Despite theoretical predictions, the proportion of melanics around heavily-polluted Manchester never reached 100% (Bishop and Cook 1980, Mani 1990). (b) The proportion of melanics in East Anglia reached 80% despite the absence of any apparent pollution (Lees and Creed 1975); after the introduction of pollution control legislation, typicals became predominant before lichens returned to the trees (Grant and Howlett 1988). (c) South of latitude 52*N, the relatively poor correlation of melanism with sulfur dioxide concentration suggested that non-industrial factors were of greater importance than selective predation (Steward 1977a,b); after the introduction of pollution control legislation, the proportion of melanics decreased in the north, as expected, but increased in the south (Bishop and Cook 1980, Jones 1982). (d) The frequency of typicals on the Wirral Peninsula increased dramatically before the return of lichens to tree trunks (Clarke et al. 1985, Grant et al. 1998). (e) The decline of melanism in The Netherlands has been accompanied by an increase not only in typicals, but also in an intermediate form almost as dark as melanics. (Brakefield 1990).

Steward cautioned that "it may not be possible to generalize from the results for one area, to explain geographic variation over the rest of Britain" (Steward 1977a, pp. 239, 242).After the passage of anti-pollution legislation, the proportion of melanics decreased north of London (as expected), but inexplicably increased to the south (Bishop and Cook 1980; Jones 1982). In The Netherlands, the decline of melanism took another twist. As air pollution declined, not only did the frequency of typicals increase, but also the frequency of an intermediate form which was almost as dark as melanics, suggesting a more complex change than was seen in Britain (Brakefield 1990).Various theoretical models have been proposed to account for the discrepancies. Some include the effects of gene flow due to migration, though according to Jones "gene flow alone cannot explain... why melanics are so common in some unpolluted parts of Britain" (Jones 1982, p. 109). Mani (1990), like Steward (1977a), obtained a good fit between melanism and sulfur dioxide concentration, but cautioned that "such a correlation does not define causal connection. It only says that SO2 concentration can be used as an approximate measure of the level of pollution that affects the morphs differentially in some unknown way" (Mani 1990, p. 368; emphasis in original). Whatever the actual causes may be, Berry concluded, "it is clear that melanic peppered moth frequencies are determined by much more than differential visual predation by birds" (Berry 1990, p. 312).

In rural East Anglia, where there was little industrial pollution and typicals seemed better camouflaged, melanics reached a frequency of 80 percent, prompting D.R. Lees and E.R. Creed to conclude that "either the predation experiments and tests of conspicuousness to humans are misleading, or some factor or factors in addition to selective predation are responsible for maintaining the high melanic frequencies."5 Reviewing the geographical evidence in 1990, R.J. Berry concluded, "it is clear that melanic peppered moth frequencies are determined by much more than differential visual predation by birds."6One notable discrepancy in the distribution of melanism was its lack of correlation with lichen cover on tree trunks. Even Kettlewell had observed that melanism began declining before lichens returned, and Lees and his colleagues found a lack of correlation with lichen cover, which they considered "surprising in view of the results of Kettlewell's selection experiments."7,8 According to B.S. Grant and R.J. Howlett, if the rise of industrial melanism had originally been due to the demise of lichens on trees, then "the prediction is that lichens should precede the recovery of the typical morph as the common form. That is, the hiding places should recover before the hidden. But, this is clearly not the case."9In the United States, the frequency of melanics in southeastern Michigan dropped from more than 90 percent to less than 20 percent between 1960 and 1995, thus paralleling the decline of melanism in the United Kingdom. Yet the decline in Michigan "occurred in the absence of perceptible changes in local lichen floras," prompting Grant and his colleagues to conclude that "the role of lichens has been inappropriately emphasized in chronicles about the evolution of melanism in peppered moths."10 Recently, T.D. Sargent and his colleagues noted that "the recent declining frequency of melanism in B. betularia in North America, where the hypothesis of a cryptic advantage of melanism never seemed applicable," is "perplexing" in view of the classical story.11So the rise and fall of industrial melanism did not depend on lichens. Why, then, did lichens appear to be significant in Kettlewell's experiments?