Melanism in the two spot ladybird: the nature and intensity of selection

Abstract

Intraseasonal changes in the melanic frequency ofAdalia bipunctatahave been recorded in and around Berlin and Birmingham; in the latter place a steady decrease in frequency has also occurred over several years. Estimates of the intensity of natural selection have been made from the observed changes in frequency. Selection against black individuals in winter, and against red ones in summer, appears to be higher in Berlin (52% against black heterozygotes in winter and 33% against red homozygotes in summer) than in Birmingham (24 and 9% respectively); there are indications that mean maximum summer temperature and mean minimum winter temperature influence the intensity of selection, as do local smoke pollution levels, but not sulphur dioxide (as withBiston betularia). In some parts of the ladybird’s geographical range, increased mating activity of the black individuals may also be important.