What's all the fuss about epistasis?

A worthy (so I believe) repost from my other blog.... [begin repost] Several years back David wrote about Sewall Wright's Shifting Balance Theory. If you know much about the history of mathematical genetics you know that R.A. Fisher and Wright's disputes over the importance of population substructure, genetic drift and the adaptive landscape was a simmering pot looming in the background of the emergence of the Modern Synthesis. One of the points that Fisher and Wright clashed over was the relative evolutionary importance of epistasis. I want to emphasize the evolutionary importance, because of course R.A. Fisher did not reject mechanistic epistasis as a background feature of a specie's genetic architecture, rather he was skeptical of its relevance as a driver of evolution. It was the average effect of a single allele against a genetic background which resulted in phenotypic adaptation according to Fisher. Each beneficial mutation would be driven ...