Ontogeny, Timing of Development, and Genetic Variance-Covariances Structure

evolutionary perspective is to regulate a number of ontogenetic processes to arrive eventually at a final adult size. Optimal final size is important since there are a number of processes which, if not dependent on body size, are at least highly correlated with it including metabolic rate, onset of reproductive maturity, and competitive ability for mates. Growth patterns during development are widely thought to be under genetic control and arise mainly from the activation and repression of structural and regulatory genes which vary the metabolic activity and chemical composition of differentiated cells. These activation-repression cycles are responsible for changes in cell number and cell size, fluctuations in enzyme and hormone levels, varying rates of protein and fat accretion, skeletal growth, etc. The timing of ontogenetic cycles of gene activity determines the total length of the developmental process as well as the timing of critical steps during various ontogenetic processes. The evolution of gene regulation patterns during ontogeny and the resultant timing of events for complex, polygenically controlled traits is central to general discussions in evolution and development. Hall (1982, p. 46) stated that "we do not know when during development, nor by what developmental process, growth of the mandible, or for that matter of any other region of the embryo, is controlled." Bonner (1982, p. 8) states that "everyone agrees that the most effective way to elicit big phenotypic changes with the least genetic fuss is by heterochrony. Nevertheless, we seem unable to attack the problem." The inability to attack experimentally the problem of gene regulation in polygenic traits is reflected in the absence of genetic models for ontogenetic changes in polygenic traits. Further, critical genetic data are lacking about general morphogenetic models which emphasize heterochronic phenomena (e.g., Gould 1977; Alberch et al. 1979). This paper provides a discussion of ontogenetic variation in quantitative genetic parameters in mammals, examines the genetic aspects of several general growth phenomena, and describes their relative importance in mammalian growth.