Multi-locus population genetics - How does selection alter haplotype frequencies?
How important is epistatic selection?
In organisms which reproduce sexually there is little deviation from linkage equilibrium in nature. The main evidence comes from gel electrophoretic surveys of protein polymorphisms, to see directly whether genes at different loci are associated.
Research was done with the fruitfly Drosophila . Some evidence of linkage disequilibrium was found, but the results suggested the level is low and most loci are in linkage equilibrium. One conclusion from this result would be that, although epistatic interactions are important in particular cases, like Papilio , they may not be of general importance at least in the evolution of sexually reproducing species.
More holistically minded biologists would disagree. The amount of interaction between loci that must go on during the development of a complex, organic body is so high that they would expect epistatic fitness interactions to be common.