TESTING HISTORICAL HYPOTHESES OF MORPHOLOGICAL CHANGE - BIOMECHANICALDECOUPLING IN LORICARIOID CATFISHES

The ''decoupling hypothesis'' has been proposed as a mechanistic basis for the evolution of novel structure and function. Decoupling derives from the release of functional constraints via loss of linkages and/o r repetition of individual elements as redundant design components, fo llowed by specialization of one or more elements. Examples of apomorph ic decoupling have been suggested for several groups of organisms, how ever there have been few empirical tests of explicit statements concer ning functional and morphological consequences of decoupling. Using th e loricarioid catfishes, we tested one particular consequence of decou pling, the prediction that clades possessing decoupled systems having increased biomechanical complexity will exhibit greater morphological variability of associated structures than outgroups having no such dec oupled systems. Morphometric procedures based on interlandmark distanc es were used to quantify morphological variance at three levels of des ign at successive nodes in the loricarioid cladogram. Additional landm ark-based procedures were used to localize major patterns of shape cha nge between clades. We report significantly greater within-group morph ometric variance at all three morphological levels in those lineages a ssociated with decoupling events, confirming our predictions under the decoupling hypothesis. Two of 12 comparisons, however, yielded signif icant variance effects where none were predicted. Localization of the major patterns of shape change suggests that disassociation between mo rphological and functional evolution may contribute to the lack of fit between variance predictions and decoupling in these two comparisons.