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Dr Musgrave's reply
February 16, 2002
Dear Dr Musgrave,
I have read your Peppered Moths comments (http://www.nmsr.org/text.htm#moth) with interest. I would be grateful if you would kindly answer the following questions:-
1. Can you give me references to justify your claim that many photos of moths on tree trunks are not staged?
Majerus MEN, "Melanism", 1998, Oxford University Press, see plates 3a-d (page 146 on).
Mikkola K, (1984), Biol. J Linn Soc 21, 409-421
2. Can you give me references to justify your claim that moths rest on tree trunks about 25% of the time?
Majerus MEN, "Melanism", 1998, Oxford University Press, page 123, Table 6.1 Howlett JJ and Majerus MEN (1987), Biol. J Linn Soc, 30, 31-44.
Actually, the figure for trunks is higher because in the main category of resting sites, trunk-branch joints, the moths are actually resting on the trunk.
3. Are these moths in the wild or are they subject to Kettlewell or other special conditions?
These are moths in the wild.
4. Would you still hold the above opinions after reading the more extensive discussions presented on my website:- http://members.tripod.com/aslodge/id62.htm ?
The above are not opinions, these are experimental and field observational results (unless you are talking about the brief online article, where there was not space to go into detailed discussion). I have read all of Kettlewell's original papers, plus the original papers of the groups that have replicated Kettlewell's results (yes, these results have been replicated by 5 separate groups using alternate experimental designs), a significant proportion of the other original work in this field and the key reviews, I have re-analyzed Kettlewell's data. Overall, I feel I my understanding of the data is not completely superficial.
On the topic of the material at the above web site, Wells claims that Thomas manipulates data. In Wells's book he writes in the section titled "Peppered moths don't rest on tree trunks" "...evidence has accumulated showing that peppered moths do not normally rest on tree trunks.". In his reply at http://members.tripod.com/aslodge/id67.htm, Wells writes "The clear conclusion that emerges from all of them is that peppered moths do not normally rest on tree trunks in the wild". Yet when Mr Thomas correctly reports that Majerus found 25% of wild peppered moths on trunks Wells claims that this is "data manipulation" as he does not break them up into exposed and unexposed categories. It is not clear why this is data manipulation when Wells makes no such distinction himself in his statements. As noted above, the largest category of moths resting places is trunk-branch junctions, and here the moths are actually on the trunks, 2-3 inches below the branch, in the shade, so the proportion of trunk resting moths is actually under-represented by the 25% figure, and the figure is closer to 75%. The exposed-unexposed figures are important, as will be discussed later, but if Wells makes no distinction between these sites in his statements, why fault others for doing the same thing in reply to his own statements? Even if we take Wells's distinction, and Howlett and Majerus's concerns into account, then at least one in ten moths will be found resting on exposed tree trunks, a not insignificant proportion, and at variance with Well's statements.
Wells's also takes exception to quoting Mikkola's finding that 40% of moths resting on his tree segments rested on the trunks, claiming this is data manipulation. Yet the question was "where on the tree segments do the moths rest" (and by implication, where on trees do moths rest), and this is the figure that Mikkola himself uses. Even if we take into account the moths that did not move from the transparent walls and floors of the cage (not a natural resting place for moths) into account 27% rested on trunks and 40% rested on branches. Again, a not insignificant proportion, at variance with Wells's claims, and comparable with the proportion found by Majerus in the wild. Mikkola suggests that the numbers on trunks are over represented, however, one can equally make the case that they were underrepresented, as the moths moved to the source of illumination, (Grant and Howlett, 1988), rather than remain on the trunks. The best that we can conclude directly from Mikkola's experiments is that substantial numbers of moths will rest on both trunks and branches.
Wells has stated "In twenty-five years of field work, Clarke (1985) and his colleagues found only one peppered moth on a tree trunk." Compare this with what Clarke et al. actually said: "....all we have observed is where the moths do NOT spend the day. In 25 years we have found only two betularia on the tree trunks or wall adjacent to our traps (one on an appropriate background and one not), and none elsewhere." The way Wells quotes Clarke et al. implies significant numbers of moths were found in other locations, whereas the actual state of affairs is that they had found too few moths in any location to determine where they rested (which was Clarke et al.'s point). In Wells's reply, he refers to an enormous number of moths in studies summarized by Steward, this is irrelevant as in these studies the resting places of the moths was not determined. Only in Majerus's study has there been any significant, systematic attempt been made to determine the resting locations of peppered moths.
However, accepting Majerus's figures that around one in 10 moths are found on exposed trunks, does this matter? After all, unexposed trunks and branches, even leaves, were also coated with pollution so moths would still be differentially camouflaged in these positions. Also, birds do not only look at exposed trunks, but also search unexposed trunks and branches. Yes, it may make a difference, being in the shadow may reduce the camouflage difference between light and dark moths, the simple increase in areas to be searched may reduce any camouflage advantage. So what are the implications for Kettlewell's experiments? Lets look first at Kettlewell's actual experiments, and then experiments that have directly compared predation on exposed and unexposed trunks.
Kettlewell did 5 different experiments :
1) Cage experiments with moths on trees
2) Direct field observations of predation on released moths
3) Filmed observations of predation on released moths
4) Indirect observations of predation
5) Mark recapture experiments.
In only experiment 3, the filmed experiments, did Kettlewell use moths placed on exposed trunks. This was due to the limitations required by filming. The main reason of the filming was to demonstrate that birds did find and consume moths, as many ornithologists did not believe that birds would search for and find cryptic moths. While the exclusive use of exposed tree trunks was not ideal, as Kettlewell himself observed, it was made necessary by the limitations of cinematography at that time.
In contrast, the direct and indirect predation experiments, and the mark release experiments, moths were released onto trunks AND branches (Kettlewell, 1955, pages 327, 332 and Kettlewell 1956, page 287). Note that, the moths were not only released on trunks. Furthermore, they were released onto shaded or shadowed areas of the trees (cf. moths on trunks in the shadow of branches in Majerus's Trunk-branch joint category). Thus in these experiments the moths were released in to areas typically occupied by about 50% of moths, and the experimental results will be more representative of the moth population in general than the filmed experiments. In these experiments differential predation was demonstrated.
Furthermore, Holwett and Majerus have done a pilot experiment comparing predation of moths on exposed trunks with predation of moths in shadow at trunk-branch junctions, the major resting site in their field observations. Differential predation was still observed for moths placed at shadowed trunk-branch junctions. For example, in polluted woods, more pale moths were taken by birds than dark moths, regardless of whether they were on exposed trunks or shadowed trunk-branch junctions. There was no statistically significant difference between the ratio of light to dark moths taken at trunk-branch junctions and the ratios taken on exposed trunks. The study was small, and a small difference may have been missed. However, the main conclusion we can draw is that Kettlewell's experiments are not invalid.
This must be emphasized. The majority of researchers, including critics such as Majerus (see for example Howlett & Majerus, 1987, Majerus 1998), agree that Kettlewell's results are qualitatively correct. Bird predation is a major, if not the major, selective force in industrial melanism. What these researchers are trying to establish is a correct quantitative estimate of the selective advantage of the various moth morphs in various environments. Kettlewell's original estimates are almost certainly overestimates, but not big overestimates.
Summary: if our question is "Do peppered moths, in the wild, often settle on tree trunks", then the data from Majerus, the largest and best study of moth resting places in the wild, is a resounding "yes". If the question is "Do peppered moths, in the wild, often settle on exposed tree trunks" then the answer is a qualified yes, depending on whether one feels at least one moth in 10 resting on exposed trunks is "often" (it is certainly non-trivial). This later question is of less relevance as the majority of Kettlewell's experiments were not performed with moths resting exclusively on exposed trunks, but on shaded trunks and branches more representative of the moths natural resting places.
Wells recommends that readers consult the original papers to see for themselves, I agree most strongly, readers should go to the original papers and make up their own minds.
H.B.D. Kettlewell, Heredity 9 (1955): 323-342.
H.B.D. Kettlewell, Heredity 10 (1956): 287-301.
R. C. Steward, Ecological Entomology 2 (1977): 231-243.
K. Mikkola, Biological Journal of the Linnean Society 21 (1984): 409-421.
C. A. Clarke, G. S. Mani & G. Wynne, Biological Journal of the Linnean Society 26 (1985): 189-199.
R. J. Howlett & M. E. N. Majerus, Biological Journal of the Linnean Society 30 (1987): 31-44.
B. Grant & R.J. Howlett, Biological Journal of the Linnean Society 31 (1988): 217-232.
M. E. N. Majerus, Melanism: Evolution in Action (Oxford: Oxford University Press, 1998)
Parenthetically, it is alleged that I ignore Spetner's claim, that there is no evidence that random mutations have played any role in industrial melanism ("Not by Chance", Judaica Press, 1998, pp. 67, 177). As I was addressing Wells's claims this issue did not arise. However, Spetner's claim is actually a misunderstanding of a paper by Bishop and Cook (1975). These authors do not claim, nor produce any evidence that random mutations are not involved. Indeed they specifically refer to mutations being involved. The role of mutation is well established in industrial melanism (see Majerus 1998, and references therein)
J.A. Bishop & L.M. Cook, Scientific American 232 (1975): 90-99.
I hope this goes some way to clearing up these issues.
Ian F Musgrave
Wells' third reply contains Dr Well's comments on the above remarks by Dr Musgrave.