Equation (11) is a general equation for the change in geometric mean fitness at a male haploid or sex-linked locus when the selection intensity is small. The rate of change in fitness depends on the additive genetic variance in fitness, which is defined as the variance of the additive genotypic values when the additive model of gene action assumes no dominance in females and complete dosage compensation in males. If the relative fitnesses of genotypes are constant, then under slow selection the increment in fitness is equal to the additive variance if either (1) the population is in random mating or (2) there is no dominance in females and complete dosage compensation in males. If both conditions hold, then a male haploid population evolves one-third faster than the corresponding diploid population and one-third slower than the corresponding haploid population. In computer-simulated populations undergoing random mating, the additive variance is a good approximation to actual fitness changes over a wide ra...