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J R Soc Med. 2001 December; 94(12): 660.
PMCID: PMC1282326

Pickles of Wensleydale

Dr Moorhead's article (October 2001, JRSM, pp. 536-540) reminded me of an episode in the mid 1950s. The Department of Medicine in (I think) Sheffield were conducting a survey to correlate bronchial disease and air pollution, and chose Aysgarth as the control site because of the detailed morbidity records available from Will Pickles' practice. They had no doubt that the beautiful high Pennine village of Aysgarth would provide an excellent pollution-free comparison to the environment of industrial Yorkshire. They were extremely discomfited to find that every second person in Aysgarth was coughing his or her lungs out. Was there really air pollution in Aysgarth, or were their standard assumptions wrong?

In those far-off days the most sensitive measure of clean air was not chemical but the proportion of melanic trunk-sitting months. Typical cryptic moths become visible to their bird predators when the lichens disappear from the tree-trunk resting-places, while melanics remain hidden—hence their spread into all polluted areas after the Industrial Revolution. The details of the spread and distribution of the industrial melanics were chronicled and experimentally studied by Bernard Kettlewell, a Surrey general practitioner who in partnership with the pioneer medical geneticist E A Cockayne, developed and published the results of his amateur lepidopterology in the 1930s and moved to E B Ford's group in Oxford after the Second World War. Kettlewell was asked to catch moths in Aysgarth. I went with him as a newly fledged ecological geneticist, and met the venerable Will Pickles.

Over 94% of the species we studied (the peppered moth, Biston betularia) were the melanic form. Aysgarth was in fact highly polluted by air swept up into the hills on the prevailing south-west wind, carrying the atmospheric output from the Lancashire manufacturing towns. The clinical symptoms were not anachronistic; Aysgarth was not a proper control.

Since those days, Kettlewell's research has become a standard example of evolution in action, described in elementary biology textbooks. And a sequel to it was the two-decade-long study conducted by the late Cyril Clarke in his back garden in the Wirral near Liverpool, and barely mentioned in his obituaries, in which he monitored the decline of the black form of the peppered moth as air quality improved following the implementation of the Clean Air Act. Clarke recorded the fall in frequency of the melanics in his locality from over 90% to below 10%. I routinely used this saga to point out to medical students that a little biological knowledge can, on occasion, be very helpful.


Articles from Journal of the Royal Society of Medicine are provided here courtesy of Royal Society of Medicine Press