A model is presented for age-specific selection on the genotypes in a population. Each genotype is assigned a life table that specifies the viability and fecundity of its age classes. Breeding and reproduction occur at regular intervals, and generations overlap. Examples were generated on a digital computer. Gene frequencies and the distribution of individuals among the various age classes may oscillate until equilibrium is reached. Moreover, age structure and gene frequencies are intimately related; a change in either factor alone may bring about a change in the other. In an extension of the basic model, the fecundities of the genotypes were regulated by population density. Under the joint action of logistic control and age-specific selection, the growth curve of the population can, for some schedules of selection, show plateaus in an otherwise sigmoid increase. The relevance of the growth patterns obtained in different types of environments to current ideas of "r" and "K" selection is discussed.