Variation in ontogeny

The hierarchy that exists from organs to populations is critical to studies of variation. Although growth is a property of an individual, measurements of growth, and the variation in those measurements, exist at several obvious levels of organization: within individual and among individuals, as well as between/among whatever biological groups constitute the hypothesis being tested. The identification of the components of this hierarchical relationship can be simple or complex, but certain elements are consistent. One element, the unit of analysis, is the component for which measurements are collected. This chapter aims to elucidate the significance of variation during growth at different hierarchical levels. It presents an example of an analysis of growth data that includes partitioning within individual and among individual variation to compare differences in variation among externally or experimentally defined groups. In doing so, it shows that a single mechanism affects variation differently at these hierarchical levels.