NOTE 23L: This argument was first made by Crow and Kimura (1965), although they omitted the factor of 2s that accounts for the probability of fixation of each mutation. Maynard Smith (1968) criticizes their assumption that each mutation is unique and later (1974) reviews the various different arguments on the relative rates of evolution of sexual and asexual populations. See also Crow and Kimura’s (1969) reply.