  In subdivided populations, migration acts together with selection and genetic
drift and determines their evolution. Building up on a recently proposed
method, which hinges on the emergence of a time scale separation between local
and global dynamics, we study the fixation properties of subdivided populations
in the presence of balancing selection. The approximation implied by the method
is accurate when the effective selection strength is small and the number of
subpopulations is large. In particular, it predicts a phase transition between
species coexistence and biodiversity loss in the infinite-size limit and, in
finite populations, a nonmonotonic dependence of the mean fixation time on the
migration rate. In order to investigate the fixation properties of the
subdivided population for stronger selection, we introduce an effective coarser
description of the dynamics in terms of a voter model with intermediate states,
which highlights the basic mechanisms driving the evolutionary process.
