THE SELECTION BARRIER BETWEEN POPULATIONS SUBJECT TO STABILIZING SELECTION

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.