Global variation in the distribution and diversity of species has been recognized by naturalists for centuries. However, despite extensive scientific study since the initial documentation of these patterns, we still lack a comprehensive explanation as to their underlying causes. Ultimately, this variation must reflect evolutionary history, and how rates of speciation, extinction and dispersal have changed not only among lineages, but also through time and space. Elucidating the relative contribution of these processes can be aided from the study of groups that are not only variable in their geographic distributions and taxonomic diversity, but are also relatively well known in respect of their evolutionary relationships. Considering these criteria, birds represent an ideal study group for several reasons: (1) they are one of the most species rich vertebrate groups, having diversified into c. 10,000 species, (2) these species are very heterogeneously distributed among the major lineages, (3) species are found across the majority of the world’s continental and insular landmasses, (4) advances in the phylogenetics and systematics of this radiation are rapidly being made, which in combination with paleontological information, is leading to an increased understanding about their biogeographic history, and (5) extensive study and observation mean that ecological and distributional information are comparatively well known, even across large geographic and taxonomic scales.
Despite the vast diversity of birds, one order, the Passeriformes, stands out as being extraordinarily species rich, representing approximately 2/3 of all global species (c. 6,500 extant species). Yet, even among passerine lineages, large differences are apparent in both species richness, and in the extent of their phylogenetic isolation. The geographic distribution of passerines are also hugely variable, with some groups having radiated over extensive continental and insular areas, whereas others are restricted to small habitat patches that extend over only a few km2. One hypothesis potentially explaining these observations is that the geographic areas and extent over which clades diversify are correlated with the number of species they accumulate. The major goal of my PhD thesis has been to assess these relationships among passerine clades, while simultaneously attempting to determine their intrinsic and extrinsic causal factors.
To achieve these aims, I have collated phylogenetic, distributional and eco-morphological trait data across regional and global scales. Utilizing comparative methods, I focus my analyses on two study systems: (1) all passerine birds present within the New World (c. 2,300 species), and (2) the global radiation of the “core Corvoidea” (now termed Corvides, c. 760 species). A common finding throughout my chapters is that range expansion, and the colonization of new geographic areas promotes lineage diversification. In addition, the properties of certain geographic areas increase the rate at which lineages diversify. Tropical mountain regions and island archipelagoes are notable in this respect, while the former also maintain lineages over long timescales. These conclusions result from the repeated finding that the accumulation of lineages differing in the extent of their phylogenetic isolation varies spatially. Furthermore, these results imply that opportunities for diversification have differed among areas, and also through time. Why some lineages disperse and radiate extensively, yet others are more restricted in their geographic occurrence, and have accumulated species diversity to a lesser degree, remains poorly understood. My analyses demonstrate that eco-morphological traits are in some instances correlated both with lineage diversification rates, and geographic distributions, accounting for differences in their ability to undergo geographic expansion. Together, these findings support the idea that changes in geographic distributions, diversification rates, and eco-morphological traits are all influenced by common ecological and evolutionary drivers. Continuing to elucidate the interaction between these factors and the causality of their relationships, will improve our understanding about the processes generating, and maintaining, current gradients in biodiversity.