We consider a simple model of a one-locus two-allele population inhibiting a two-patch system and experiencing spatially heterogeneous viability selection. The populaton size is finite. We use a diffusion approximation and singular perturbation techniques to find the probability of fixation of a mutant allele. We focus on situations when each allele is advantageous in one patch and deleterious in the other patch. Our theoretical results supports previous conclusions of Ohta (1972) and Eldridge (1995, 2002) that under certain conditions small populations respond faster to selection than large populations. We emphasize that the knowledge of the dependence of migration rates on population size is crucial in evaluating the effects of the population size on the rates of evolution.