The introgression of genes carried by a small group of immigrants is s
tudied. The recipient and the donor populations differ at several auto
somal loci subject to weak selection, and two allelic forms of each ge
ne are considered. Fitness variation is determined by additive allelic
effects, by dominance effects, and by two-locus additive-by-additive
epistatic interaction of the effects of the alleles. The fate of the g
roup of immigrants is quantified by the selection barrier that describ
es the cumulative mean fitness of the hybrids and hybrid descendants r
elative to the fitness of the resident population. The monomorphic and
the polymorphic loci of the recipient population contribute different
ly to the selection barrier. If the genetic difference between recipie
nt and donor population is small, then the contribution of the monomor
phic loci is dominated by a positive term dependent on the difference
in gene frequencies. The contribution of the polymorphic loci depends
only on the difference of the leading order in the pairwise linkage di
sequilibria between the two populations. This contribution may be posi
tive or negative; and, thus, polymorphic loci may either contribute to
the barrier or inflate the introgression.