  The ability of genetic isolation to block gene flow plays a key role in the
speciation of sexually reproducing organisms. This paper analyses the hybrid
zone dynamics affected by "weak" Haldane's rule, namely the incomplete hybrids
inferiority (sterility/inviability) against the heterogametic (XY or ZW) sex
caused by a Dobzhansky-Muller incompatibility. Different strengths of
incompatibility, dispersal and density-dependent regulation are considered; and
the gene flow and clinal structures of allele frequencies in the presence of
short-range dispersal (the stepping-stone model) are examined. I show that a
weak heterogametic hybrid incompatibility could constitute a substantial
barrier that could reduce gene flow and result in non-coincident and discordant
clines of alleles. It is found that the differential gene flow is more
pronounced under a stronger density-dependent regulation. This study provides a
mechanistic explanation for how an adaptive mutation, which may only have a
marginal fitness effect, could set a gene up as an evolutionary hot-spot.
