The relative importance of evolutionary and environmental controls on broad-scale patterns of species richness in North America

Abstract

Species richness varies greatly through space. It has been hypothesized that, over broad spatial scales, richness is determined by contemporary environmental characteristics, especially climatic energy. This hypothesis predicts that spatial variation of richness should be correlated with climatic variables. In contrast, it has also been hypothesized that differing net rates of speciation and subsequent dispersal may have given rise to contemporary patterns of species richness. The predictions of this hypothesis are less clear, but many proponents of the hypothesis derive the prediction that the average degree of evolutionary advancement of taxa should be related to patterns of richness. We have tested these predictions by examining spatial variation across North America of richness and of indices of evolutionary rate in the families Cicindelidae (tiger beetles), Cyprinidae (minnows), Catastomidae (suckers) and Percidae (perches and darters). We observed that richness in all three groups was related to climate in ways consistent with the richness-energy hypothesis. In contrast, richness in these four families was only weakly and inconsistently related to measures of evolutionary advancement. We conclude that, in the taxa for which we could find both phylogenies and distribution data, there is little reason to postulate evolutionary explanations for contemporary continental patterns of species richness in North America.