We study a hybrid zone between two populations of a diploid organism.
The populations differ at one locus. Homozygotes have equal fitnesses
and the heterozygote fitness is reduced by beta + delta (beta is the b
irth rate deviation and delta is the death rate deviation). The popula
tions extend along a one dimensional continuous habitat, and migration
occurs by diffusion of individuals. The model is formulated as a set
of simple continuous time demographic models without age structure for
the three genotypes, and the system is transformed into three new var
iables, the total population size N, the gene frequency p, and the dev
iation from Hardy-Weinberg proportions F. The gene frequency in a stea
dy state dine always follows a hyperbolic tangent closely. Analysis of
the asymptotic behavior of the dine far from the hybrid zone suggests
a qualitative prediction of the shape of N, p and F over the zone. Fo
r weak selection the shape is determined by a central steepness of roo
t(beta+delta)/4D, as observed by Bazykin in 1969, where D is the diffu
sion coefficient. For strong selection the dine is less steep than the
Bazykin dine, and the form is dominated by the migration process. The
steepness at the center of the dine is close to root b/4D where b is
the birth rate of homozygotes.