directional epistasis
Khan AI, Dinh DM, Schneider D, Lenski RE & Cooper TF 2011 Negative epistasis between beneficial mutations in an evolving bacterial population. Science 332:1193-1196.
- negative epistasis contributes to declining rates of adaptation over time
- sign epistasis was rare in this genome-wide study, in contrast to its prevalence in an earlier study of mutations in a single gene