Evolution has come to be increasingly discussed in terms of changes in developmental processes rather than simply in terms of changes in gene frequencies. This shift is based in large part on the insight that since all phenotypic traits arise during ontogeny as products of individual development, a primary basis for evolutionary change must be variations in the patterns and processes of development. Comparative psychology has a unique role to play in defining and describing these developmental dynamics, in large part because its specific aims and purposes are not well represented in other sub-disciplines within the life sciences. These include an empirical concern with the developmental and ecological dynamics contributing to behavioral and psychological functioning (especially in terms of degrees of flexibility or malleability) and the significant role of behavior in the evolutionary process (particularly in generating novel phenotypes). Comparative psychologists have provided converging evidence supporting the view that shifts in behavior brought about by changes in the environment and the resulting changes in the activity of the organism can lead to variations within and across generations in morphology and physiology, providing the engine for generating evolutionary change.