Transcript 天擇的範例：peppered moths

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.
The classical story:
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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。
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Fig. 10.8 Two forms of Biston betularia, the peppered moth.
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發現與證實的過程
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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.
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Kettlewell’s experiments I
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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.
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Kettlewell’s experiments II
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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.
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隨後的驗證
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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.
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Problems with the classical story
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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.
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Problems continued
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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)
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Industrial melanism on lichens
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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.
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Unnatural selection
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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.
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Fig. 10.8 Two forms of Biston betularia, the peppered moth.
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Conclusion
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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.
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References
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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.
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References
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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.
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Study questions
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What points favor Kettlewell’s
interpretations and conclusions?
Are they valid?
What do you conclude about peppered
moths as an example of natural selection?
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