Figure A.1.

Three different kinds of epistasis possible in fitness graphs: no epistasis ($\uparrow$), sign epistasis (, ), and reciprocal sign epistasis (). Arrows in the fitness graph are directed from lower fitness genotypes toward mutationally adjacent higher fitness genotypes. In the middle of each fitness graph is a symbol showing the kind (and orientation) of epistasis. Note that the bottom left ($\mathrm{\circlearrowleft }$) and top right ($\mathrm{\circlearrowright }$) fitness graphs violate transitivity.