A simple model of migration between two populations, each in a balance between mutation and stabilizing selection on a polygenic trait, is explored. Below a critical migration rate, genetic differences between the two populations can be maintained, even if the populations are selected towards the same phenotypic optimum. Gene flow then maintains genetic variance within each population. For this process to account for heritable variation, there must be some mechanism that causes divergence. The possibility that fluctuating selection could lead to the initial differentiation of the populations is explored.