Schematic illustration of epistasis. Two mutations A and B can interact epistatically in different ways with varying effects on fitness. The fitness of the wild-type is represented by the black baselines, and the heights of arrows represent the fitness after one mutation (*W*_{A} or *W*_{B}) and after both mutations (*W*_{AB}). Green, positive epistasis, red, negative epistasis, black, no epistasis. In (*a*), two independently beneficial mutations may have their joint effect increased or diminished (*W*_{AB} larger or smaller), while in (*b*) the independent effect of the two mutations is deleterious and beneficial, respectively, and the combined expected effect on fitness is deleterious. In (*c*), each mutation by itself is deleterious, but when they interact, the result can be reciprocal sign epistasis (green arrow). These sketches illustrate an additive model, where the sum of *W*_{A} and *W*_{B} is equal to *W*_{AB} without epistasis. In our model, using the geometric mean this corresponds to taking the logarithms of the fitness.

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