NOTE 23P: There is an interesting way to work out the equilibrium number of mutations. When fitness is plotted against number of mutations on a log scale (as in Fig. P23.6), the slope of the fitness curve equals the selection coefficient *s* (i.e., the increase in relative fitness due to one extra mutation). Because the equilibrium number of mutations is *U*/*s*, the vertical distance between the intercept of the tangent, and the point where it touches the curve is just *U* (Fig. P23.6). When the fitness curves downward, as here, this is much greater than the actual mutation load *L*.