The strength of the selection barrier between populations

Genetic differences among populations exposed to selection form barriers ag ainst genetic exchange by mortality among hybrids. The strength of such a s election barrier, with which one (recipient) population reacts against immi gration from another (donor) population, may be measured as the cumulative mean fitness of hybrids and their descendants relative to the fitness of th e recipient population. Previous work analysed a case of weak selection wit h pairwise epistatic interactions by assuming small genetic distance betwee n two populations in contact. The present study allows large genetic differ ence between the donor and recipient populations and considers weak multilo cus selection with arbitrary epistatic interactions between two or more lin ked loci, An approximate analytical expression for the barrier strength is obtained as an expansion in which the strength of selection plays the role of a small parameter. It is shown that allele frequencies and gametic linka ge disequilibria contribute in different ways to the strength of the select ion barrier.