We consider an age-structured population that is observed at times t = 0, 1, 2 , . . . . It is assumed that for each t there is the same number of individuals of a particular sex and age group. Another assumption we make is that an offspring of a specified sex has at all times the same probability of having a parent of a particular age and sex. It is shown that the rate of substitution of neutral mutants is v/L, where v and L are respectively equal to the mean fraction of mutants among gametes

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... ucceeding in forming newborn individuals and the mean age of reproduction. This result also applies to monoecious populations. The substitution rate is also derived for advantageous mutants in a monoecious population. Once again, the mutation rate in the usual expression is replaced by v/L. Implications of these results are discussed. THE DERIVATION FOR NEUTRAL ALLELES We consider a population that is observed at times t = 0, 1, 2 , .