Mean fitness under different dispersal rules. Mean fitness (±SE) at the steady state in populations homogeneous at the modifier locus as a function of the cost of dispersal c, the dominance coefficient h, and the mean dispersal rate α (A: α = 0.01, B: α = 0.1, C: α = 0.3). The mean fitness of UNI (open markers) decreases with the cost, because the cost is a component of the fitness. This effect becomes stronger as the dispersal rate α increases (from A to C) and the cost is paid more often. In contrast, the mean fitness of FAD (filled markers) increases with the cost of dispersal, because less fit individuals pay the cost more often and deleterious alleles are purged from the population. As the dominance coefficient h increases, the masking of deleterious mutations in a heterozygous state weakens and fitness is slightly reduced in most cases. Note that in most cases the error bars, showing the standard error of the mean, are smaller than the markers.