Figure 3: Distribution of the number of types.

The probability of observing a certain number of coexisting types is shown for different selection intensities. As expected, our simulations (filled symbols) agree with Ewens' sampling formula under weak selection (lines). The top panels show a low mutation rate, μ=10⁻⁶ per time step. For constant selection (a), diversity decreases slightly with increasing intensity of selection. For frequency-dependent selection (b), diversity increases substantially with increasing intensity of selection. For strong selection, we can alternatively compute the stationary distribution from the transitions between the different polymorphisms (Fig. 3 (open symbols)). Although the number of types is not limited in our model, there are typically 4 or less types coexisting in our simulations at the same time. The bottom panels show higher mutation rates, μ=10⁻⁴ per time step, where the diversity under neutral selection is already high. Under frequency-independent selection (c) diversity increases compared with (a), owing to the increasing mutation rate. But frequency-dependent selection (d) increases diversity further compared with constant fitness (c) or lower mutation rates (b) (population size N=1,000, averages obtained over 500 independent realizations and 10⁷ generations per realization. All simulations begin in a monomorphic state, averages start after 25,000 generations).