Fig. 2.

A computational time step in fitness wave models consists of two substeps. The first substep (Upper) is reproduction and mutations. Growth favors the most fit individuals as indicated by the red arrows. The resulting population density will generally violate the constrained of constant population size. The second substep (Lower) restores the prescribed population size by a random elimination of individuals from the population (this step may also be interpreted as Darwinian selection). The result of the whole time step is a constrained branching random walk of “genotypes,” which shifts the fitness distribution toward higher fitness.