A colleague suggested I post in several installments my past response to critics of Icons of Evolution. The series begins here, which includes the full reviewer citations.
Part 4: Peppered Moths
Peppered moths come in a light variety and a dark variety. Before 1800 the light variety was ubiquitous, but during the industrial revolution the dark variety became much more common. According to evolutionary theory, the shift occurred because of natural selection: Dark moths were better camouflaged against pollution-darkened tree trunks, and thus more likely to survive bird predation.
In the 1950s, British physician Bernard Kettlewell released captive moths onto nearby tree trunks and observed as birds ate the more visible ones. He then released moths that had been marked on the underside with a tiny spot of paint. When he later recaptured some of the marked moths, the proportion matching the color of nearby tree trunks was significantly higher than in the batch he had released, consistent with the camouflage-predation theory. The peppered moth story soon became the classic textbook case of natural selection in action.
In the 1980s, however, scientists discovered that peppered moths rarely rest on tree trunks in the wild, and a growing number of biologists now question the classic story.
For example, University of Chicago evolutionary biologist Jerry Coyne (yep, the same Coyne cited above) wrote in 1998: “From time to time, evolutionists re-examine a classic experimental study and find, to their horror, that it is flawed or downright wrong.” According to Coyne, the fact that peppered moths do not rest on tree trunks “alone invalidates Kettlewell’s release-and-recapture experiments, as moths were released by placing them directly onto tree trunks.” [12]
Several reviewers of Icons of Evolution (not including Coyne, of course) fault me for getting the moths’ resting-places wrong. According to Scott, “Wells argues that moths don’t rest on tree trunks,” but “he ignores research showing that moths rest on all parts of trees (including the trunks).” (Scott, p. 2258) And Padian and Gishlick write: “Wells erroneously claims that moths do not rest on tree trunks, although research has shown that moths rest on trunks 26% of the time, and on trunk/branch junctions 43% of the time (Majerus 1998, p 123).” (Padian & Gishlick, p. 36)
But Scott doesn’t cite any research, and the research she says I ignore shows clearly that exposed tree trunks are not the natural resting-places of peppered moths. For example, in 1984 Kauri Mikkola reported that “the species probably only exceptionally rests on tree trunks;” and in 1987 Rory Howlett and Michael Majerus wrote that they were “convinced that exposed areas of tree trunks are not an important resting site” for peppered moths. [13]
What about the statistics Padian and Gishlick attribute to Majerus? Majerus’s 1998 book lists a total of 47 moths found in the wild from 1964 to 1996. Of these, 6 were found on exposed tree trunks, 6 on unexposed trunks, 20 in trunk/branch joints, and 15 on branches. Padian and Gishlick obtain their percentages from the first two categories (13% plus 13%) and the third category (43%). But Majerus’s 47 moths are not--and are not claimed to be--an unbiased sample representing peppered moths in general. In the decades since Kettlewell’s experiments, scientists have counted tens of thousands of peppered moths; one 1977 paper alone listed data for 8,426 moths in southern Britain between 1952 and 1974. [14] These thousands, however, were found in artificial traps, not in normal resting positions. Researchers suspect that the moths normally spend the day hidden under horizontal branches high in the trees, where they cannot be seen.
So even if all 47 of Majerus’s moths had been found on tree trunks, they would still represent less than 1% of all peppered moths studied during the same period. Trying to determine the normal resting-places of peppered moths by doing statistics on Majerus’s sample is a bit like trying to determine the normal habitats of ocean fish by doing statistics only on those spotted from a boat. But of course Majerus knows this, which is why he (unlike Padian and Gishlick) concludes that “peppered moths do not naturally rest in exposed positions on tree trunks.” [15]
Before biologists discovered that peppered moths don’t normally rest on tree trunks, many experiments were conducted by pinning or gluing dead moths to tree trunks. This practice should have been abandoned, however, once biologists knew that it fails to test the camouflage-predation theory under natural conditions. In Icons of Evolution, I criticized textbooks that continue to use staged photos of moths on tree trunks to illustrate natural selection--though I stopped short of calling them “fraudulent.”
Yet according to Scott: “Researchers glued moths to trees to test whether birds differentially prey upon moths that contrasted against their surface, an experiment necessary to test the hypothesis of bird predation. This is not fraud, it’s research.” (Scott, p. 2258) And Padian and Gishlick write: “Wells then pretends righteous indignation about ‘fraudulent,’ ‘staged’ textbook photographs of light and dark moths against light and dark backgrounds. But these photographs merely illustrate the differential camouflage that field experiments tested--a reasonable and expected part of science. Can Wells be so ignorant of this investigative tradition or the purpose of an illustration?” (Padian & Gishlick, p. 36)
In the investigative tradition that I was taught, however, field research is supposed to approximate natural conditions as closely as possible. Since the surface on which peppered moths rest is a key factor in the camouflage-predation theory, the tradition I learned would require that experiments be conducted using the moths’ normal resting-places--and that textbook illustrations portray those resting-places accurately.
Apparently, evolutionary biology relies on a different investigative tradition--one in which understanding nature is less important than finding ways to prop up Darwin’s theory. Maybe Padian and Gishlick are right, and I was ignorant of that tradition. But I’m learning.
NOTES:
[12] Jerry Coyne, “Not black and white,” a review of Michael Majerus’s Melanism: Evolution in Action, Nature 396 (1998): 35-36. See also Jonathan Wells, “Second Thoughts about Peppered Moths,” The Scientist (May 24, 1999): 13.
[13] Kauri Mikkola, “On the selective forces acting in the industrial melanism of Biston and Oligia moths (Lepidoptera: Geometridae and Noctuidae),” Biological Journal of the Linnean Society 21 (1984): 409-421, p. 416; Rory J. Howlett and Michael E. N. Majerus, “The understanding of industrial melanism in the peppered moth Biston betularia (Lepidoptera: Geometridae),” Biological Journal of the Linnean Society 30 (1987): 31-44, p. 40. See also Tony G. Liebert and Paul M. Brakefield, “Behavioural studies on the peppered moth Biston betularia and a discussion of the role of pollution and lichens in industrial melanism,” Biological Journal of the Linnean Society 31 (1987): 129-150, p. 145.
[14] M. E. N. Majerus, Melanism: Evolution in Action (Oxford: Oxford University Press, 1998), Table 6.1, p. 123.
[15] R. C. Steward, “Industrial and non-industrial melanism in the peppered moth, Biston betularia,” Ecological Entomology 2 (1977): 231-243, p. 236; Majerus, Melanism: Evolution in Action, p. 121. For an excellent new book on this subject see Judith Hooper, Of Moths and Men: Intrigue, Tragedy & the Peppered Moth (London: Fourth Estate, 2002).
