On Allometry and Relative Growth in Evolutionary Studies

In his critique of contemporary morphometry, Blackstone (1987) asserts that the conceptual, pattern-oriented framework of allometry ignores developmental processes and is therefore inappropriate for studies in evolutionary morphology. His intent is to emphasize the limitations of allometric studies in the understanding of evolutionary processes. In doing so, however, Blackstone focuses on a descriptive approach to allometry and obscures several fundamental issues. In particular, he claims that information on rates of growth in units of chronological time is necessary to deduce patterns of evolutionary change in developmental processes, and that such information is ignored in allometric analyses. He traces this disregard to a historical departure from the study of underlying processes of growth to the purely descriptive study of size and shape. In contrast, I will argue first that chronological time does not have theoretical or operational priority over other estimates of biological age, such as body size, for the description and comparison of growth patterns; and second that the apparent historical trend he describes corresponds instead to a logical progression from the study of growth rates of individual traits per chronological time, to growth rates of the same traits in terms of more robust estimates of biological time.