Figure 1

Probability of increasing or decreasing the average fitness in the new equilibrium after one mutation event in an initial homogenous population. θ is the probability that a random payoff entry of the mutant, a, b or c is larger than wild type initial fitness d. Blue symbols and lines are simulation and analytical results under frequency dependent selection (average over 10⁶ runs). Red lines are analytical results under constant selection. For constant selection, the average fitness either increases or is unchanged by a new mutation, where the fraction of mutants that increases fitness is determined by θ. However, under frequency dependent selection, the average fitness of the population in the new equilibrium after a mutation can also decrease. The probability to increase, decrease the average fitness or maintain the same average fitness, depends on θ, for $\theta >\frac{\sqrt{2}-1}{\sqrt{2}}$ the probability to increase the average fitness in the new equlibrium is larger than the probability to decrease it.