Data are presented for the Manchester area, showing the recent change in frequency of the melanic morph carbonaria of the peppered moth Biston betularia (L.). The frequency has fallen from 90– in 1983 to below 10% at present; this decline shows that the phenomenon of industrial melanism, first noted in this species in Manchester, is now almost past.
Data from the Wirral peninsula, to the west of Manchester, published by C. A. Clarke and F. M. M. Clarke, show a slightly less rapid decline starting some ten years earlier from a lower maximum. Records from north–west Kent, published by B. K. West, also show a less intense decline from a lower peak several years in advance of the Manchester decline. The changes observed agree with a migration–selection model, which predicts subsidence of the plateau of high carbonaria frequency, with contraction from the edges. Selection in this model includes a non–visual fitness advantage of carbonaria homozygotes, a fitness difference associated with change in atmospheric sulphur dioxide concentration (which may act through differential crypsis) and frequency–dependent protection of rare forms. When all available data are compared, there is a negative relation between estimated fitness of carbonaria over the period of decline and initial level of atmospheric pollution.