My discussion with Jim Manzi on epistasis generated a lot commentary. It's a complex topic, as I said there are different ways to define epistasis, and evolutionarily its effect on trait value might be different from its effect on fitness. Finally, I think it is important that epistatic and additive genetic variation can convert from one to the other; and over the long term it is this heritable variation which is the "stuff of evolution," so to speak. But a friend recommended that I post a figure from Genome-wide association analysis identifies 20 loci that influence adult height.
One figure does not an argument make, but the cumulative impact of these data will resolve to some extent disputes about the efficacy of interaction effects. I say to some extent because evolutionary biology is predicated on variation, and I will not be surprised if one finds it difficult to fruitfully generalize about the ubiquity of particular processes across the whole of deep time & the tree of life.