Flawed inference: why size-based tests of heterochronic processes do not work

Abstract

Allometric (or size-based) heterochrony is widely used in drawing inferences about heterochronic processes. Whereas problems associated with substituting size for age in heterochronic analysis are acknowledged, it is also generally believed that comparisons of ancestral and descendant growth allometries often reveal real processual shifts in growth processes. At the very least, they are presumed to discriminate systemic perturbations in “size” and “shape” (“ontogenetic scaling”) from local dissociations of “size” and “shape” (“neoteny,” “acceleration,” and so on). When ancestor and descendant follow the same growth trajectory in size (relative to age), it is further presumed that size-based heterochronies will match “true” (age-dependent) heterochronies. In this paper we argue that growth allometries are a poor vehicle for inferring heterochronic process; the actual processual shifts that produce them can be counter-intuitive. Systemic shifts can result in dissociated allometries, and overlapping growth allometries can conceal perturbations in “shape” relative to “size”. Changes in growth allometries are influenced as much by the initial ancestral relationship between “size” and “shape” as they are by any perturbations that may occur during the evolution of the descendant from the ancestor. Finally, it is emphatically notthe case that size-based heterochrony will reveal true heterochrony as long as the size trajectories of ancestor and descendant remain constant relative to age. We urge careful consideration of the actual shapes of biological growth trajectories, recognition of the fact that they are rarely simple power functions of age, and calculation of possible relationships between complex and/or lazy- Sgrowth trajectories.