Spatiotemporal reorganization of growth rates in the evolution of ontogeny

Heterochrony, evolutionary changes in rate or timing of development produci ng parallelism between ontogeny and phylogeny, is viewed as the most common type of evolutionary change in development. Alternative hypotheses such as heterotopy, evolutionary change in the spatial patterning of development, are rarely entertained. We examine the evidence for heterochrony and hetero topy in the evolution of body shape in two clades of piranhas. One of these is the sole case of heterochrony previously reported in the group; the oth ers were previously interpreted as cases of heterotopy. To compare ontogeni es of shape, we computed ontogenetic trajectories of shape by multivariate regression of geometric shape variables (i.e., partial warp scores and shap e coordinates) on centroid size. Rates of development relative to developme ntal age and angles between the trajectories were compared statistically. W e found a significant difference in developmental rate between species of S errasalmus, suggesting that heterochrony is a partial explanation for the e volution of body shape, but we also found a significant difference between their ontogenetic transformations; the direction of the difference between them suggests that heterotopy also plays a role in this group. In Pygocentr us we found no difference in developmental rate among species, but we did f ind a difference in the ontogenies, suggesting that heterotopy, but not het erochrony, is the developmental basis for shape diversification in this gro up. The prevalence of heterotopy as a source of evolutionary novelty remain s largely unexplored and will not become clear until the search for develop mental explanations looks beyond heterochrony.