Epistasis correlates with relative fitness loss. For each standard test (filled black circles), for example w·ar - a·r, and each double-double test (open red circles), for example ar·bs - as·br, anchored with the fittest type on the left-hand side of the equation, the value of the test (epistasis) is plotted versus the average fitness decrement (Δ) associated with the two deleterious two-point mutations that define the genotypes on the right hand side of the equation. Epistasis tends toward more positive values in the context of more deleterious mutations. The significance of this correlation was robust to different assumptions about the independence of the data, as described in the text.
Beerenwinkel et al. BMC Evolutionary Biology 2007 7:60 doi:10.1186/1471-2148-7-60