天擇的範例： peppered moths 鄭先祐 生態主張者 Ayo 工作室 Ref: Wells, J. (1999) Second thoughts about peppered moths. The Scientist 13:13.
Ecology 2001 2 The classical story: Industrial melanism During the industrial revolution, dark ( melanic ) forms of the peppered moth became much more common than light (typical) forms. The proportion of melanics decline after the passage of pollution-control legislation. 經過 1950s 的研究報告後，這個現象 (industrial melanism) 就成為天擇的古典範例。
Ecology 2001 3 Fig. 10.8 Two forms of Biston betularia, the peppered moth.
Ecology 2001 4 發現與證實的過程 於 1896 年， J. W. Tutt noted typicals were well camouflaged against the light-colored lichens that grow on tree trunks in unpolluted woodlands; but in woodlands where industrial pollution has killed the lichen, exposing the bark and darkening the tree trunks, melanics are better camouflaged. Since conspicuous moths are more likely to be eaten by predatory birds, Tutt attributed the increase in the proportion of melanic forms to natural selection.
Ecology 2001 5 Kettlewell ’ s experiments I In the 1950s, Bernard Kettlewell tested the idea experimentally by making several hundred peppered moths and releasing them onto tree trunks in a polluted woodland near Birmingham, England. Kettlewell observed through binoculars that melanics seemed less conspicuous than typicals, and that birds took conspicuous moths more readily than inconspicuous ones. That night he recaptured 27.5% of the melanics, but only 13.0% of the typicals, suggesting that a much higher proportion of melanics had survived predation.
Ecology 2001 6 Kettlewell ’ s experiments II Kettlewell later repeated this experiment in an unpolluted woodland in Dorset, England, where the recapture percentages were the opposite of those obtained in Birmingham. He concluded that birds act as selective agents, as postulated by evolutionary theory, and that industrial melanism was the most striking evolutionary change ever actually witnessed in any organism.
Ecology 2001 7 隨後的驗證 Experiments conducted by other biologists seemed at first to corroborate Kettlewell ’ s conclusions. When industrial melanism began to decline after the passage of antipollution legislation, the decline seemed consistent with the theory that industrial melanism. However, doubts about this classical story began to emerge soon after Kettlewell’s experiments, and it is now clear that those experiments were fundamentally flawed.
Ecology 2001 8 Problems with the classical story Some biologists found discrepancies in the expected geographical distribution of melanic moths. For example, in rural East Anglia, where there was little industrial pollution, melanics reached a frequency of 80%. One notable discrepancy in the distribution of melanism was its lack of correlation with lichen cover on tree trunks. Melanism began declining before lichens returned.
Ecology 2001 9 Problems continued In the US, the frequency of melanics in southeastern Michigan dropped from more than 90% to less than 20% between 1960 and 1995, thus paralleling the decline of melanism in the UK. Yet the decline in Michigan occurred in the absence of perceptible changes in local lichen flora. (Sargent et al. 1998)
Ecology 2001 10 Industrial melanism on lichens Kettlewell released moths directly onto tree trunks, the moths were not free to take up their own choice of resting site. Most textbook pictures of peppered moths show specimens that have been manually placed on tree trunks. However, since 1980, it has become clear that peppered moths do not normally rest there. The normal resting place of the Peppered moth is beneath small, more or less horizontal branches, probably high up in the canopies.
Ecology 2001 11 Unnatural selection 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. Many moths rest underneath, or on the side of, narrow branches in the canopy. It seems that the classical example of natural selection is actually an example of unnatural selection.
Ecology 2001 12 Fig. 10.8 Two forms of Biston betularia, the peppered moth.
Ecology 2001 13 Conclusion The fact that peppered moths do not normally rest on tree trunks invalidate Kettlewell ’ s experiments and poses a serious problem for the classical explanation of industrial melanism in peppered moths. Yet textbooks continue to present the classical story of industrial melanism as an example of evolution. Clearly, this is misleading. In particular, it is misleading to illustrate the story with photographs showing moths on tree trunks where they do not rest in the wild.
Ecology 2001 14 References Berry, R. J. (1990) Industrial melanism and peppered moths(Biston betularia L.). Biological J. of the Linnean Society 39:301-322. Grant, B. S., et al. (1996) Parallel rise and fall of melanic peppered moths in American and Britain. J. Heredity 87:351-357. Kettlewell, H. B. D. (1955) Selection experiments on industrial melanism in the Lepidoptera. Heredity 9:323-324. Kettlewell, H. B. D. (1956) Further selection experiments on industrial melanism in the Lepidoptera. Heridity 10:287-301.
Ecology 2001 15 References Lee, D.R., and E. R. Creed (1975) Industrial melanism in Biston betularia: the role of selective predation. J. Animal Ecology 44:67-83. Mani, G. S. (1990) Theoretical models of melanism in Biston Betularia – a review. Biological J of the Linnean Society 39:355-371. Sargent, T. D. et al. (1998) The classical explanation of industrial melanism: assessing the evidence. Evolutionary Biology 30:299-322. Wells, J. (1999) Second thoughts about peppered moths. The Scientist 13:13.