EVOLUTION OF DIMORPHIC TRAITS - EFFECT OF DIRECTIONAL SELECTION ON HERITABILITY

Directional selection on a quantitative character is predicted to decrease genetic variance and lead to rapid fixation of alleles. However, many traits vary in a dichotomous manner although the underlying genetic determination is polygenic. This paper presents a theoretical examination of the effects of directional selection on a dimorphic trait. Such selection can change the incidence of the selected morph from 50 per cent to 99 per cent within 20 generations. However, after an initial decline heritability is predicted to rise back to its original value. The consequences of a finite number of alleles and loci on this prediction are examined using a simulation model. These simulations show that, unlike the case of directional selection on a continuously distributed trait, there is little loss of genetic variance even when the trait is controlled by only three loci with two alleles per locus.