Equations are derived for the rate of increase under selection of a gene present at low frequency in a population with overlapping generations. These equations are used to support the idea that the evolution of life histories can be understood in terms of the relative effects on the intrinsic rate of increase of changes at different ages in the age-specific mortality and fecundity rates. It is shown that an analogous idea can be applied to density-regulated populations. The results can also be used to determine sufficient conditions for the selective maintenance of a polymorphism in an age-structured population.