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 resp ect to a large chromosomal inversion system was estimated in nine wide ly separated populations of the seaweed fly, Coelopa frigida. The patt ern of female rejection was determined for each population and estimat es were made of the relative fitnesses of the inversion karyotypes. Tw o 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 t hese two mating regimes were compared. In seven of the populations the exercise of choice resulted in a modest change in progeny fitness. Th e change depended on the karyotype of the female: beta beta females pr oduced fitter offspring in all populations, alpha alpha's usually prod uced less fit offspring, and there was little effect on the progeny of alpha beta's. An association was also found between a physical factor , tidal range, and the offspring fitness of alpha alpha females, but n o association exists for alpha beta or beta beta females. It is sugges ted that the relevant genes have been subject to different evolutionar y forces on the two forms of the inversion.