Forward population genetic simulations are used to explore the evolution of a sequence of nucleotide sites subject to reversible mutation under selection, mutation, and drift. Three selection schemes are studied: synergistic, antagonistic, and multiplicative interactions among sites. Their respective effects on the level of nucleotide diversity, the pattern of linkage disequilibrium, and the allele frequency spectrum are determined. Surprisingly, none of these aspects are affected by directional epistasis when the overall strength of selection is held constant (where the equilibrium allele frequencies are equal). The equilibrium mean fitness does differ with selection regime, and is relatively higher with synergistic interactions while lower with antagonistic epistasis. These findings legitimate the application of many population genetic models assuming multiplicative selection when there are actually epistatic interactions among sites, and have important implications on the evolution of recombination.
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