SEXUAL SELECTION IN SEAWEED FLIES - GENETIC-VARIATION IN MALE SIZE AND ITS RELIABILITY AS AN INDICATOR IN NATURAL-POPULATIONS

Characters that are evolving in response to indirect sexual selection operating through the production either of attractive sons (the Fisher process) or of progeny with high viability (good genes), are expected to exhibit high levels of additive genetic variation. In seaweed flie s (Coelopa frigida) male size is subject to both direct and indirect s exual selection, whereas female size has probably evolved in response to direct selection alone. Coefficients of additive genetic variance ( CVA's) in adult size were shown to be larger in males than females. Vi rtually all the genetic variation is attributable to the alpha beta ch romosomal inversion which is known to be associated with larval surviv al. Sexual selection appears to have led to differential expression of relevant loci in the two sexes, and in the focussing of this variatio n into a single region of the genome. Samples from 25 natural populati ons provided estimates of CVA's in male size. Although the CVA values suggest male size is, on average, a reliable indicator of offspring fi tness, both spatial and temporal differences in CVA's were observed. W e suggest that indirect sexual selection is a very inconstant force of evolution.