  We consider a population subdivided into two demes connected by migration in
which selection acts in opposite direction. We explore the effects of
recombination and migration on the maintenance of multilocus polymorphism, on
local adaptation, and on differentiation by employing a deterministic model
with genic selection on two linked diallelic loci (i.e., no dominance or
epistasis). For the following cases, we characterize explicitly the possible
equilibrium configurations: weak, strong, highly asymmetric, and
super-symmetric migration, no or weak recombination, and independent or
strongly recombining loci. For independent loci (linkage equilibrium) and for
completely linked loci, we derive the possible bifurcation patterns as
functions of the total migration rate, assuming all other parameters are fixed
but arbitrary. For these and other cases, we determine analytically the maximum
migration rate below which a stable fully polymorphic equilibrium exists. In
this case, differentiation and local adaptation are maintained. Their degree is
quantified by a new multilocus version of $\Fst$ and by the migration load,
respectively. In addition, we investigate the invasion conditions of locally
beneficial mutants and show that linkage to a locus that is already in
migration-selection balance facilitates invasion. Hence, loci of much smaller
effect can invade than predicted by one-locus theory if linkage is sufficiently
tight. We study how this minimum amount of linkage admitting invasion depends
on the migration pattern. This suggests the emergence of clusters of locally
beneficial mutations, which may form `genomic islands of divergence'. Finally,
the influence of linkage and two-way migration on the effective migration rate
at a linked neutral locus is explored. Numerical work complements our
analytical results.
