SIDLAUSKAS, B.L.; University of Chicago and The Field Museum: Unequal rates of morphological diversification in sister clades of characiform fishes

Why have certain groups of organisms diversified greatly in morphology, while closely related groups have not? This study took advantage of a rare natural experiment to determine whether apparent differences in morphological diversity between the Anostomoidea and Curimatoidea, two sister clades of South American fishes with approximately equal species richness, should be taken as evidence that the tempo or mode of evolution has differed between them. Despite identical ages of origin and similar net speciation rates and geographic distributions, the morphological and ecological diversity of the Anostomoidea dwarfs that of the Curimatoidea. I investigated the source of unequal diversification in these fishes by combining simulations of cladogenesis with models of morphological drift in an empirically determined morphospace. Likelihood ratio tests showed with 90% confidence (using variance among species as the measure of morphological diversity) or 99% confidence (using volume of occupied morphospace) that the rate of morphological change per unit time in the Anostomoidea was likely to have been higher than that of the Curimatoidea. Variation in rates of speciation or extinction was not found to be a likely explanation of the observed difference in morphological diversity. Uniquely derived morphological and ecological features of these clades may have accelerated or decelerated the rate of morphological change, such as a marked lengthening of the quadrate that may have relaxed structural constraints on the evolution of the anostomoid jaw.