HISTORICAL PATTERNS OF DEVELOPMENTAL INTEGRATION IN PIRANHAS

TO test the hypothesis that developmental integration coordinates evol utionary change through history, we dissect the spatial and temporal i ntegration of ontogenetic allometries of piranha body form and examine the evolutionary coordination among ontogenetic features by a phyloge netic analysis. Few of our characters provide evidence in support of t he hypothesis. In general, we find that developmental integration is h istorically labile, being modified at virtually every speciation event . Most of the ontogenetic features are dissociated in their phylogenet ic changes and evolve in a mosaic fashion. Indeed, developmental integ ration is so labile that primitively integrated features of ontogeny u sually evolve subsequently as independent characters. Evolutionary cha nges in developmental integration can result in increased or decreased integration on the ontogenetic time scale. When localized features ar e deleted from ontogeny, or when spatially integrated features are gai ned, the derived ontogenies may be more integrated in a spatial sense. The end result of phylogenetic dissociations may be a more highly dev elopmentally integrated ontogeny, Thus, in the piranhas we studied, we find a historically coupled increase in developmental integration cau dally and a decrease in developmental integration cranially.