Testing for latitudinal bias in diversification rates: An example using new world birds

Study of the latitudinal diversity gradient to date has focused largely on pattern description, with relatively little work on the possible mechanisms underlying the pattern. One proximate mechanism is a latitudinal bias in the discrepancy between speciation and extinction rates, leading to higher rates of species diversification toward lower latitudes. Despite being central to many explanations for high tropical diversity, this mechanism is tested very rarely. We discuss some of the problems involved in testing for latitudinal bias in diversification rates and present an example phylogenetic analysis for endemic bird genera of the New World. The results provide evidence for higher diversification rates in clades inhabiting lower latitudes, both when genera are considered independent and when phylogeny is controlled for using independent contrasts. High rates of diversification are also associated with larger geographic area and higher net primary productivity, although these do not fully account for the latitudinal effect. The latitudinal pattern is stronger in younger clades, supporting the prediction of a simple model in which the signal of latitudinal bias in diversification rates diminishes as clades age and become saturated with species. Our study demonstrates that a clade-based approach can help answer important questions that a geographic approach cannot; but large phylogenies and geographic databases are needed to cope with the large amount of noise inherent in this type of analysis.