 Although recurrent mutation can maintain a polymorphism indefinitely at mutation-selection balance, here Ford is clearly interested in a more active role for natural selection in the maintenance of polymorphism. The first task was facilitated by early developments in population genetic theory, particularly the findings of Fisher (1930). Ford interpreted these to mean that naturally occurring, discontinuous phenotypic variation is “nearly always genetic”. The reasoning stems from the theoretical findings that, in large populations, it is unlikely that the positive and negative fitness effects of an allele (or chromosomal inversion) will be exactly balanced. As a result, the number of individuals with a rare neutral mutation is proportional to the number of generations since its origin. Furthermore, if truly neutral, such alleles would spread so slowly through a large population by random genetic drift that the 