Appendix 5—figure 1. Process of speciation.

Left: Sketch of a branching event at $t=0$ with two branches 1 and 2. The fitnesses of the most fittest strains (noses) in branch 1 and 2 are $x_1$ and $x_2$. Branch 1 is the fitter one $x_1>x_2$. The antigenic distances from the cross-immune bulk to the noses of the two branches are $d_1$ and $d_2$. The Gaussian profile in fitness is illustrated in blue. Right: The fitness difference between the two branches $x_1-x_2$ is doing a biased random walk in time $t$ of step size $s$ with a reflecting boundary at $x=0$ and an absorbing boundary at $x=x_n$.