Prevalence of Epistasis in the Evolution of Influenza A Surface Proteins
The epistasis statistic is defined in terms of the total time elapsed between all pairs of consecutive substitutions at sites and (see “Materials and Methods”). In this schematic figure, substitutions at sites and are denoted by red and blue circles, respectively. Substitutions and form consecutive pairs. Substitutions are not consecutive because they occur on different lineages. Substitutions are not consecutive because substitution at site occurs between them.