FIGURE 1.

Null models for epistasis. The expected fitness of a double mutant can be calculated using either a multiplicative (A) or additive (B) model. A multiplicative model is appropriate under a standard exponential growth model, with population size increasing according to λ^t (where t is time). (A) Shows exponential growth curves for a hypothetical wild-type (black, λ = 2, indicating that population size doubles with each unit time), two mutants (orange, yellow, with λ reduced by a factor of r and s, respectively), and the double mutant (blue). Note that the growth parameter λ of the double mutant is determined by the product of r and s. An additive model is appropriate when population sizes are log-transformed, yielding a linear relationship between ln(population size) and time. (B) Shows log-transformed growth curves for the same mutants as in (A). Note that the fitness of the double mutant is now determined by ln(r) + ln(s), where the log of the population size at time t is given by e^In(λt).