What's growth got to do with it? Process and product in the evolution of ontogeny

Abstract

Growth allometries are regularly used to diagnose evolutionary perturbations in the rate and timing of development, and to study their effects on morphological change. Metric variables are measured for ancestor and descendant, plotted on logarithmic scales, and the two estimated linear relationships are compared. The relationship between these two lines has been a tool of choice for inferring heterochronic history, particularly when age data are not available. Size, plotted on the abscissa, is often used as a proxy for age; the variable plotted along the ordinate typically measures some aspect of ontogenetic change. Under the rubric of allometric heterochrony, changes in the slopes and intercepts of ancestral and descendant growth allometries are taken to reveal heterochronic processes (changes in the rates of growth, offset timing, and onset timing of relative to and their two products—paedomorphosis (the retention in mature descendants of ancestral juvenile characteristics) and peramorphosis (the acquisition in mature descendants of features that transcend ancestral adult characteristics). This framework is said to be grounded in the pioneering work of Gould (1977) and Alberch et al. (1979). In this paper, we examine the meanings of shape in heterochronic analysis. We argue that the current diagnostic toolkit departs significantly from those constructed by Gould (1977) and Alberch et al. (1979), and bears little relevance to the problems they addressed. We show why growth allometries (as typically read) do not capture evolutionary relationships between growth (Gould's size) and development (Gould's shape).