Selection theory with overlapping generations

Abstract

The implications for selection of genetic differences between animals of different ages and between progeny of the same age but from parents of different ages are discussed. Correct definitions of generation length and of selection differential when replacements are selected disproportionately from parental-age subgroups can be made in two ways. Seeking most rapid genetic gain may involve using many young dams despite their poorer maternal ability. Making proper allowances for genetic age effects is similar in principle to making allowance for genetic trend and for parental breeding value in BLUP methods. Matrix methods of predicting response to selection are illustrated by an example with disproportionate selection from parental-age subgroups. Proper specification of an appropriate control population is essential for evaluation of genetic gains, but if a control population is established from a line previously under selection it may show appreciable fluctuations in mean quite separate from genetic drift, and the final control population mean may be significantly different from the initial mean even in the absence of environmental change. The relative contributions of the control line and the selection line to fluctuations in genetic gain depend on the initial conditions.