Sexual Selection in Natural Populations of Seaweed Flies: Variation in the Offspring Fitness of Females Carrying Different Inversion Karyotypes

The effect of indirect sexual selection on offspring fitness with respect to a large chromosomal inversion system was estimated in nine widely separated populations of the seaweed fly, Coelopa frigida. The pattern of female rejection was determined for each population and estimates were made of the relative fitnesses of the inversion karyotypes. Two sets of progeny frequencies were calculated: the first on the basis of the observed pattern of mating, and the second on the hypothetical basis that all females had accepted the male available: in other words, random mating. The relative fitnesses of the offspring produced by these two mating regimes were compared. In seven of the populations the exercise of choice resulted in a modest change in progeny fitness. The change depended on the karyotype of the female: ββ females produced fitter offspring in all populations, αα’s usually produced less fit offspring, and there was little effect on the progeny of αβ's. An association was also found between a physical factor, tidal range, and the offspring fitness of αα females, but no association exists for αβ or ββ females. It is suggested that the relevant genes have been subject to different evolutionary forces on the two forms of the inversion.