EVOLUTION OF THRESHOLD TRAITS - THE BALANCE BETWEEN DIRECTIONAL SELECTION, DRIFT AND MUTATION

Threshold traits are characterized by showing discrete phenotypes (typ ically two) but by being controlled by many loci of small additive eff ect, the expression of the phenotype being a consequence of a threshol d of sensitivity. In the case of a dimorphic threshold trait, individu als above the threshold display one morph and individuals below the th reshold display the alternate. Many threshold traits, such as sex rati o, cyclomorphosis, paedomorphosis and wing dimorphism, are closely con nected to fitness but have high heritabilities. The present study inve stigates the hypothesis that these large heritabilities can be maintai ned even in the face of directional selection by the countervailing fo rce of mutation. This hypothesis is based on the observation that as s election 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 drif t. It is shown that over 80 per cent of the original genetic variance can be maintained at equilibrium provided the population (N) and numbe r of loci (n) are reasonably large (N>5000, n = 50). However, unless t he selection coefficient is very small (<0.001) the equilibrium freque ncy of the phenotypes (<2 per cent) is considerably below that general ly observed. I conclude that mutation could play a significant role in the maintenance of genetic variation in threshold traits but that som e form of selection, such as frequency-dependent selection, is require d to maintain the phenotypic variation.