WHY IS THERE SO MUCH GENETIC-VARIATION FOR WING DIMORPHISM

Wing dimorphism appears in general to be determined either by a single locus, 2 allele system in which brachyptery is dominant, or by the ad ditive action of numerous loci. In the latter case studies indicate th at the heritability is typically quite large. It is generally postulat ed that wing dimorphism is under strong selection: why then is genetic variation not eroded? In this paper I consider three possible explana tions. First, genetic variation may not be exposed to selection becaus e environmental heterogeneity effectively makes heritability zero. Bec ause wing dimorphisms are known to evolve it seems unlikely that this is the primary factor. Second, directional selection on a threshold tr ait may push the population almost to monomorphism but erodes genetic variance at a very slow rate. This mechanism cannot preserve variation but makes it possible for other factors to more easily maintain varia bility. Finally, I demonstrate that in a heterogeneous environment spa tio-temporal variation in fitness will itself maintain a genetic polym orphism for wing dimorphism.