Evolution of threshold traits: the balance between directional selection, drift and mutation

Abstract

Threshold traits are characterized by showing discrete phenotypes (typically two) but by being controlled by many loci of small additive effect, the expression of the phenotype being a consequence of a threshold of sensitivity. In the case of a dimorphic threshold trait, individuals above the threshold display one morph and individuals below the threshold display the alternate. Many threshold traits, such as sex ratio, cyclomorphosis, paedomorphosis and wing dimorphism, are closely connected to fitness but have high heritabilities. The present study investigates the hypothesis that these large heritabilities can be maintained even in the face of directional selection by the countervailing force of mutation. This hypothesis is based on the observation that as selection proceeds to shift the frequency of one morph towards fixation, the selection intensity necessarily declines permitting mutation to restore genetic variation. The hypothesis is tested using a simulation model and a theoretical analysis, the latter assuming no genetic drift. It is shown that over 80 per cent of the original genetic variance can be maintained at equilibrium provided the population (N) and number of loci (n) are reasonably large (N>5000, n=50). However, unless the selection coefficient is very small (<0.001) the equilibrium frequency of the phenotypes (<2 per cent) is considerably below that generally observed. I conclude that mutation could play a significant role in the maintenance of genetic variation in threshold traits but that some form of selection, such as frequency-dependent selection, is required to maintain the phenotypic variation.