  The problem lies in the fact that the concept of fitness plays dual roles that are instructively conflated in this quotation. For when fitnesses are viewed as measures of differential abilities of organisms with different genotypes to leave different numbers of offspring, the language of fitness encourages us to suppose that ‘fitness’ refers to the relative selective advantages of genotypes.  On the other hand, if ‘fitness’ simply refers to the measure of reproductive success, it is a quantitative representation of small scale evolutionary change in a population, and leaves entirely open the question of the causes of the change.  But then the assumed connections among the concepts of fitness, adaptation and natural selection are severed. ‘Selection coefficients’ may have nothing to do with selection; what W represents may have nothing to do with selective advantage.