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 flies (Coelopa frigida) male size is subject to both direct and indirect sexual 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. Virtually all the genetic variation is attributable to the αβ chromosomal inversion which is known to be associated with larval survival. Sexual selection appears to have led to differential expression of relevant loci in the two sexes, and in the focussing of this variation into a single region of the genome. Samples from 25 natural populations provided estimates of CVA's in male size. Although the CVA values suggest male size is, on average, a reliable indicator of offspring fitness, both spatial and temporal differences in CVA’s were observed. We suggest that indirect sexual selection is a very inconstant force of evolution.