FINE TUNING THE PEPPERED MOTH PARADIGM

The trouble with classic examples of evolution is that they continue to evolve. Once an example of a major principle becomes widely accepted, it gets put into a museum cabinet for public display. Ineluctably, new information accumulates outside the cabinet, but it can't get in because the cabinet is locked. Ultimately the new information demands attention. Interested parties call for revisions. To follow that route requires understanding the true nature of the problems and the remedies. Curators of museum displays, working with limited resources, may decide that some of the old classics have been around long enough, and rather than adjust to vague controversies, it is simpler to replace a flawed display entirely than to fix it, much like discarding an old car that needs too much work to keep it running. Let's get a new one. Such are the prescriptions, at least from some quarters (e.g., Coyne 1998; Sargent et al. 1998), about the classic textbook example of evolution by natural selection: industrial melanism in peppered moths. Sargent et al. subvert the traditional explanation for industrial melanism by presenting an equivocal analysis of the evidence. Coyne laments that our "prize horse" of examples is in bad shape in his review of a recent book, Melanism: Evolution in Action, by Michael E. N. Majerus (1998). Although Majerus assesses the bulk of the literature and controversy that has accumulated since Kettlewell's (1973) The Evolution of Melanism, he could hardly agree less with Coyne or with Sargent et al. that the basic story is built on a house of cards. Majerus argues convincingly that industrial melanism in the peppered moth remains among the most widely cited examples of evolution by natural selection for two reasons: First, the basic story is easy to understand. Second, the evidence in support of the basic story is overwhelming. Majerus has done considerably more than write a book about industrial melanism. His ambitions were broader, although more modest than the title suggests. The book is divided into 10 chapters with some general features of melanism, genetics, and evolution spread across the first four. The last four chapters catalog examples of melanism in various Lepidoptera, and in ladybird beetles, with a call for future research as the book's climax. Readers interested specifically in industrial melanism in peppered moths, however, can skim or skip all of those chapters and focus on the middle two. Indeed, chapters 5 and 6 should be read carefully by all teachers of evolutionary biology. Certainly not all active