Figure 5. The substitution rate at the front of an advancing population compared to the neutral substitution rate is described by the function , which is displayed here as a function of the selective advantage of the mutants.

Notice that the axis is logarithmic. Blue, red and yellow symbols correspond to the carrying capacity , and . All curves approach in the neutral case, , in which the substitution and mutation rates are equal. Notice the rather slow increase of substitution rates with increasing selection coefficient, for small values of : even for and the highest carrying capacity, the substitution rates are merely 4 times higher than the neutral baseline, illustrating the ineffectiveness of selection at expanding fronts. For larger selection coefficients, however, the substitution rate grows roughly exponentially with .