As we have seen in earlier chapters, Darwin did not provide direct evidence for natural selection in his Origin of Species. In the post-Darwinian period, many patterns of variation in plants explicable in terms of selection were described, but the demonstration of natural selection in wild populations remained elusive. Then, in the 1950s, detailed studies of air pollution and sites contaminated with heavy metals began in earnest, providing a number of thoroughly investigated and convincing cases of natural selection in action in animals and plants (Antonovics, Bradshaw & Turner, 1971; Taylor, Pitelka & Clegg, 1991; Macnair, 1981, 1990, 1997; Shaw, 2001).

Bell and Treshow (2002) discuss the long history of concern about the effects of pollution. For example, the diarist John Evelyn published the celebrated account Fumifugium: Or the Inconvenience of the Aer and Smoake of London Dissipated in 1661. As the industrial revolution gathered pace in the nineteenth century, there was a marked deterioration in air quality in towns and cities. As urban growth accelerated and industrialisation developed in Europe and North America, trees and other vegetation were often killed or damaged, especially in towns and cities, and near smelters, factories and industrial installations. Lichens proved to be particularly sensitive indicators of the growing problem of air pollution (Bates, 2002).