MORAN PROCESS AND WRIGHT-FISHER PROCESS FAVOR LOW VARIABILITY

We study evolutionary dynamics in finite populations. We assume the individuals are one of two competing genotypes, A or B. The genotypes have the same average fitness but different variances and/or third central moments. We focus on two frequency-independent stochastic processes: (1) Wright-Fisher process and (2) Moran process. Both processes have two absorbing states corresponding to homogeneous populations of all A or all B. Despite the fact that types A and B have the same average fitness, both stochastic dynamics differ from a random drift. In both processes, the selection favors A replacing B and opposes B replacing A if the fitness variance for A is smaller than the fitness variance for B. In the case the variances are equal, the selection favors A replacing B and opposes B replacing A if the third central moment of A is larger than the third central moment of B. We show that these results extend to structured populations and other dynamics where the selection acts at birth. We also demonstrate that the selection favors a larger variance in fitness if the selection acts at death.