Relationship between the distribution of high-fitness sequences and directional epistasis, according to Wilke and Adami . The drawing on the left visualizes genotype space, with the small filled circles representing high-fitness genotypes. A and B are two particular reference sequences, and the concentric rings around A and B indicate the mutants that are a fixed Hamming distance away from either A or B. In the case of A, the average fitness w(d) of the sequences at Hamming distance d from A decays faster at higher d than at lower d, and therefore A shows synergistic epistasis. In the case of B, the decay of w(d) slows down as d increases, and hence B shows antagonistic epistasis.