Sexual selection and the evolution of preferential mating in ladybirds i. Selection for high and low lines of female preference

Abstract

Fisher’s theory of the evolution of female mating preference depends on the crucial premise that the preferences for specific male phenotypes are genetic. In natural populations of the Two-spot Ladybird, Adalia bipunctata, females prefer to mate with melanic males. We have created high and low lines of this preference: in the high line, we selected females that mated with melanic males; in the low line, we selected those that mated with non-melanic males. Two replicates of each line have been maintained. In the high line, the preference rose from 20 per cent of females preferring melanic males in the base population to 65 per cent after nine generations of selection. In the low line, the preference has been virtually eliminated. The rapid response to selection shows that the female preference is highly heritable. Its heritability can be estimated from the response. We derive formulae for the estimation of the heritability and its variance according to two different genetic models of preference. In one model, a preferential mating tendency is assumed to be normally distributed and hence determined by many genes. In the other model, the preference is an all-or-nothing character such as might be determined by a single gene. All our experiments have shown a large initial response to selection. The normally distributed polygenic model gives heritabilities that are consistently too high when calculated from the initial response. The single gene model gives more realistic estimates. As a sociobiological theory of the evolution of sexual behaviour and sexual differences, sexual selection now rests on a secure foundation: females do prefer some males to others; their preferences are genetic and evolve along with the preferred characters of the males. © 1985 The Genetical Society of Great Britain.