Decline of melanism in two British moths: spatial, temporal and inter-specific variation

The decline in industrial melanism over the last quarter century constitutes an exceptional case of an evolutionary change, varying in both time and space, and between species. In Biston betularia and Odontoptera bidentata, the change in melanic frequency is closely replicated at two sites 0.5 km apart. Between seven sites 50-100 km apart, there is heterogeneity in both the speed and timing of change. At sites that were heavily industrialized, the change is faster, from an initially higher frequency, and starts later than at sites which are more rural. We propose a method for estimating systematic change during sigmoid declines in melanic frequencies. This fails to show any significant change over time in selective coefficients. It is concluded that the overall pattern of change has been driven largely by events in the most polluted and industrialized parts of the country. Although migration may contribute to the estimated selective values, natural selection is the only credible explanation for the overall decline.