NOTE 18J: The deterministic expectation just described is actually an average over cases where the allele is fixed and where it is lost. The mutation has only a chance of 2*s* = 0.04 of being fixed, and so the expected frequency, *given* that it will be fixed in the population, is actually *p*_{0}(1/2*s*)exp(*st*). (See Fig. 18.1.) This effect can be ignored here.