Figure 1.

Three different kinds of epistasis possible in fitness graphs: (a) no epistasis, (b) sign epistasis, and (c) reciprocal sign epistasis. Arrows are directed from lower fitness genotypes toward mutationally adjacent higher fitness genotypes. Genes $a,A$ and $b,B$ are labeled such that fitness $w\left(AB\right)>w\left(ab\right)$. In the center of each graph is a marker for the type of epistasis, the marker’s various rotations and reflections cover the cases where $AB$ does not have the highest fitness. For this more exhaustive classification and discussion see SA Figure A.1 and SA A.1.