Figure 3. Multiple-step evolution.

Here multiple evolutionary changes are required for evolution of the pathogen to have an R₀ > 1. a, Jackpot model with µ = 0.1 and n intermediate changes each with R₀ equal to that of the introduced pathogen: increasing the number of steps (n) greatly decreases the probability of evolution, and makes it more sensitive to the R₀ of the introduced pathogen. b, Alternative multi-step models for the one-intermediate (n = 1) case. The jackpot model (solid line), additive model (dashed line) and fitness valley model (dotted line) are shown (see text for details).