Kettlewell's experiment

Peppered moth, carbonaria type on the left, and typica on the right

Kettlewell's experiment was a biological experiment in the mid-1950s to study the evolutionary mechanism of industrial melanism in the peppered moth (Biston betularia).^[1][2] It was executed by Bernard Kettlewell, who was working as research fellow in the Department of Zoology, University of Oxford. His aim was to investigate the scientific reason for the appearance of dark-coloured moth since Industrial Revolution in England in the 19th century. He conducted his first experiment in 1953 in the polluted woodland of Birmingham, and his second experiment in 1955 in Birmingham as well as in the clean woods of Dorset.

The experiment found that birds selectively prey on peppered moths depending on their body colour in relation to their environmental background. Thus, the evolution of a dark-coloured body provided a survival advantage in a polluted locality. The study concluded that "industrial melanism in moths is the most striking evolutionary phenomenon ever actually witnessed in any organism, animal or plant."^[3] It is now regarded as the classic demonstration of Charles Darwin's natural selection in action and one of the most beautiful experiments in evolutionary biology.^[4][5]

Background

The Industrial Revolution in Great Britain caused extensive pollution, and industrial cities such as Manchester and Birmingham were covered with black soot. R.S. Edleston was the first to identify the unusual black peppered moth in 1848 in Manchester. By the end of the century, it was recorded that the black moth, the carbonaria type, outnumbered (90% in some regions) the natural white ones, named typica.^[6][7] There were conflicting ideas as to the biological basis of this so-called industrial melanism. Humidity, environment, heredity, disease, temperature and protection (such as camouflage) were the factors put forward. J. W. Tutt was the first to come up with natural selection as an explanation, and stated in 1894 that the phenomenon was due to selective predation by birds.^[1] With the rise of evolutionary statistics, the theoretical background was set. For example, J.B.S. Haldane estimated in 1924 the rate of evolution by natural selection in the peppered moth in his first series of A Mathematical Theory of Natural and Artificial Selection. He estimated that for the peppered moth having reproductive cycle in a year, it would take 48 generations to produce the dominant (melanic or black) forms, and the melanic population could dominate the entire moth population after 13 generations. He concluded that "the only probable explanation is the not very intense degree of natural selection".^[8] University of Oxford zoologist E. B. Ford supported the bird-predation hypothesis. To experimentally investigate the issue he recruited Bernard Kettlewell in 1952 under a grant from Nuffield Foundation.^[9]

Biology of the peppered moth

Peppered moth insularia on the bark of a lichen-covered birch

By the time of Kettlewell, it was known in England that there were three varieties of peppered moth. The normal, typica, is whitish-grey in colour with dark speckles on the wings. The colour was a perfect camouflage on light-coloured trees covered with lichens. The new form, carbonaria, was completely black. There was an intermediate form, called insularia, which was light-coloured with speckled wings, but distinct from typica in that it was not whitish. The moths were active at night, and rested on tree trunks and boughs during the day.^[10][11]

The experiment

The main experiment, called mark-release-recapture, started in the summer of 1953 and lasted for three years. It consists of two continuous phases.

Preparation

Kettlewell first devised a standard procedure for scoring the moths. It was necessary to determine how far apart the moths should be placed so they were indistinguishable from their backgrounds. Correct separation would result in effective and selective predation by birds, because if the moths were all too close then birds would be able to differentiate even well camouflaged individuals.^[12] He tested his scoring method in the woodlands near Birmingham by releasing 651 peppered moths (consisting of typica, carbonaria and insularia), and then at an aviary at the Research Station in Madingley in Cambridge. In the aviary he released 69 moths, which he allowed two great tits (Parus major) to prey upon. He found that the initial procedure failed, as the birds actively looked for any moth, regardless of their colour or background. He succeeded only by using freshly captured moths consisting of 9 black and 8 white types, which he released separately. He found that the birds preferentially caught the moths according to the background colour on which the moths were present.^[3][4]

First phase

For his first experimental site, Kettlewell chose Christopher Cadbury Bird Reserve, near Rubery, Birmingham, because it was heavily polluted, but still inhabited by a number of bird species. He caught all three types of peppered moth and marked them underneath their wings with cellulose paint, so that he would be able to identify them later from non-experimental individuals after recapture. He started capturing the moths on the night of 26 June 1953, and lasted till 5 July. Out of his total capture, he selected 630 (447 carbonaria, 137 typica, and 46 insularia) male moths and released them into the woods. Within two days, 149 moths were recaptured, out of which carbonaria was 27.5%, typica 13%, and insularia 17%. (The total capture after release was 770, but 621 of them were non-experimental moths, i.e. not bearing the paint marks.) Thus their survival values were 5.72%, 1.48%, and 4.32% respectively. This shows that black moths had the best survival advantage in a darkened and polluted environment.^[3][4]

Second phase

To compare the conditions of predation in different environments Kettlewell planned to perform similar experiment in both polluted and clean environments. In 1954 he surveyed several woodlands including Devon and Cornwall, but found them unsuitable because of presence of some carbonaria forms, which indicated unclean environment. Finally he went to Deanend Wood in Dorset, which was well covered with lichens. He found only the white type, including 1 insularia form, indicating clean environment. In mid-June 1955 he started the experiment. He brought along 3,000 carbonaria type. After several days, he found that out the total 190 moths eaten by birds in the wild, 86% were the black type while only 14% were the white type. In his release-and-recapture experiment for 11 days, he used 799 moths, and his recapture rate was 13.7% for the white type, but only 4.68% for the black type.^[13]

Immediately after completion, he headed back to Birmingham, now accompanied by the renowned ethologist Niko Tinbergen. Tinbergen was responsible for video recording of the experiment, particularly to verify whether or not birds were the main predators. Out of 227 moths they released, 154 were carbonaria, 64 typica, and 9 insularia. The recapture rate within two days was 82%, 16%, and 2% respectively. It was another success, with Tinbergen capturing live video of birds eating the moths.^[2][4]

Conclusion

The conclusion of Kettlewell's experiment can be summarised as follows:^[10][13]

1. Birds were the main predators of moth.
2. Moth were eaten by birds selectively in both polluted and clean forests, indicating camouflage efficiency of the different varieties of moths.
3. The more conspicuous form of moth was always less in number after recapture; i.e. the white type in Birmingham, and the black type in Dorset.
4. In clean and lichened area, dark moths remained scarce and were rapidly eliminated because of their conspicuousness even when experimentally introduced.

See also

• Peppered moth evolution for historical background, criticism of Kettlewell's work, and restoration of its importance
• The Evolution of Melanism, a book by Kettlewell
• Melanism: Evolution in Action, a book by Michael Majerus, the most important defender of Kettlewell's research
• Of Moths and Men, a book by Judith Hooper, which severely attack Kettlewell and his works

References