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.