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SummaryThe influence of non-random mating on the melanic polymorphism in Adalia bipunctata was investigated in the Netherlands. Study sites were along clines in melanic frequency. The phenotype frequencies among mating pairs (total N = 3890) and non-mating insects were scored for each sample obtained during the spring mating period. The analysis of individual samples and of the data grouped into frequency classes provided no support for published findings from less homogeneous data of a frequency dependent mating system. Contingency table analyses for individual samples and data combined by site revealed that a mating advantage is gained by melanics. There was some evidence of heterogeneity between populations. No assortative mating was found for melanism or dry weight. An absence of any difference for mating insects in morph frequency between the sexes is not consistent with the operation of a female choice system as found by other workers for an English population. The analysis of frequency data for the offspring of mating populations collected as pupae provided strong evidence that the mating advantage gained by melanics is reflected in an increase in melanic frequency in the following adult generation. Mean estimates of selective advantage (non-melanics = 1) for the mating advantage and for the adult to pupal period are 1·16 and 1·10, respectively. It is argued that the data give strong support for Lusis's (1961) suggestion that increases in melanic frequency observed during the summer in Berlin (and in the Netherlands) can be explained by more frequent mating of melanics as a result of the effects of thermal melanism.

Selection along clines in the ladybird adalia bipunctata in the netherlands: A general mating advantage to melanics and its consequences