Abstract
Explaining global variation in geographic and taxonomic diversity gradients represents a central focus of macroecology and macroevolution. Ultimately, these diversity gradients have been generated over deep timescales as a consequence of historical variation in rates of dispersal, diversification, and the capacity of different geographic areas to preserve taxa through time. Here, I assess the relationships among these processes, to elucidate the causes of geographic and taxonomic variation in species richness among the most speciose order of birds: the Passeriformes (c. 6,500 species). To achieve this, I comparatively analyze phylogenetic, distributional and eco-morphological trait data collated at broad taxonomic and spatial scales. The results of my analyses strongly support the prevalence of historical dispersal events across large geographic scales, in addition to spatiotemporal variation in diversification rates. These findings reflect that some lineages and some geographic regions (notably tropical mountain regions and island archipelagoes) generate and maintain a much larger amount of species diversity than others. In addition, the evolution of particular life-history and eco-morphological traits, specifically pair breeding systems, and higher wing aspect-ratios, increased rates of range expansion and diversification. In summary, geographic variation in historical diversification combined with differences in the capacity of lineages to colonize new areas, determines spatial and taxonomic differences in passerine species richness.
Highlights
Despite over two centuries of investigation, biologists are yet to comprehensively explain why the distribution and diversity of species varies so much across the world
Kennedy rich vertebrate orders, having diversified into approximately 6,500 species, (2) these species are very unevenly distributed among the major lineages, (3) species are found across all terrestrial habitats, and the majority of continental and insular landmasses, (4) geographic distributions vary extensively among clades, with some having radiated over large geographic areas, while others are restricted to small habitat patches that extend over only a few km2, (5) advances in the phylogenetics and systematics of this radiation are rapidly being made, which in turn is increasing knowledge about their biogeographic history, and (6) extensive study and observation mean that ecological and distributional information are comparatively well known
(1) Is there a consistent relationship between the extent of historical dispersal and diversification? (2) Is there geographic variability in the areas that support lineages which vary in their current rate of diversification? (3) Are there life-history or ecomorphological traits that concurrently account for differences in the propensity for lineages to establish in new areas and radiate there? I address all three questions through the comparative analysis of well-resolved phylogenetic, eco-morphological and distributional data collated across broad geographic and taxonomic scales
Summary
Despite over two centuries of investigation, biologists are yet to comprehensively explain why the distribution and diversity of species varies so much across the world. Determining the relative importance of these processes necessitates the study of organismal groups that are variable in their geographic distributions, species diversity, and are relatively well known in respect of their evolutionary relationships. Considering these criteria, passerine birds represent an ideal study group for a number of reasons: (1) they are one of the most species. I propose and evaluate a hypothesis that potentially explains taxonomic and geographic disparity in passerine species diversity: that geographic variation in opportunities for diversification, combined with the extent to which lineages disperse to and colonize new areas, simultaneously determine the number of species that accumulate among clades and across space.
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