Quantitative Genetics of Heterochrony

A model of quantitative-genetic variation in developmental processes is introduced and analyzed. The model is of a bifurcating sequence of events in which traits develop from the same tissue until a transition occurs, after which they develop partially independently. Genetic and environmental variation in both the rates of tissue growth and in the timing of transitions is considered. The model shows how genetic variation in developmental parameters governs variation and covariation in phenotypic traits and how selection on the phenotype alters the distributions of developmental parameters. Particular attention is paid to the conditions under which selection will lead to changes in the average times of developmental events.