Open AccessEvolution of threshold traits: the balance between directional selection, drift and mutation
Author(s) -
Derek A. Roff
Publication year - 1998
Publication title -
heredity
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.441
H-Index - 118
eISSN - 1365-2540
pISSN - 0018-067X
DOI - 10.1046/j.1365-2540.1998.00262.x
Subject(s) - biology , directional selection , selection (genetic algorithm) , stabilizing selection , fixation (population genetics) , trait , genetic drift , evolutionary biology , mutation , disruptive selection , population , threshold model , mutation rate , genetics , frequency dependent selection , genetic variation , natural selection , statistics , gene , mathematics , demography , artificial intelligence , sociology , computer science , programming language
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.