A Population Genetics Model for Multiple Quantitative Traits Exhibiting Pleiotropy and Epistasis

We study a population genetics model of an organism with a genome ofLtot loci that determine the values of T quantitative traits. Each trait is controlled by a subset of L loci assigned randomly from the genome. There is an optimum value for each trait, and stabilizing selection acts on the phenotype as a whole to maintain actual trait values close to their optima. The model contains pleiotropic effects (loci can affect more than one trait) and epistasis in fitness. We use adaptive walk simulations to find high-fitness genotypes and to study the way these genotypes are distributed in sequence space. We then simulate the evolution of haploid and diploid populations on these fitness landscapes and show that the genotypes of populations are able to drift through sequence space despite stabilizing selection on the phenotype. We study the way the rate of drift and the extent of the accessible region of sequence space is affected by mutation rate, selection strength, population size, recombination rate, and the parameters L andT that control the landscape shape. There are three regimes of the model. IfLT ⪡Ltot, there are many high fitness genotypes and the population may evolve neutrally on high-fitness plateaux. If LT∼Ltot, there are a few high-fitness genotypes which tend to be close together. The population is confined to a small region of sequence space if selection is strong, but can explore more widely if selection is weaker. If LT⪢Ltot, there are many small peaks that can be spread over a wide region of sequence space. Compensatory neutral mutations are important in the population dynamics in this case.