The Link Between Range Size, Niche Specialisation and Diversification in One of the Most Successful Avian Radiations, the New World Flycatchers (Tyrannoidea)

Abstract

Aim: Range size constitutes a fundamental characteristic of every species and is commonly believed to influence diversification rates. This relationship is potentially dependent on the mode of speciation and other intrinsic factors, including geographic setting and species attributes. Yet, not only the size of the geographic range, but also the heterogeneity of environments it comprises, including its vertical extent (elevational range size) can affect speciation and extinction processes. For instance, mountain species and those whose range encompasses only one ecoregion (specialists) are expected to show higher diversification rates than lowland and generalist species. This study aims to investigate the link between range size, niche specialisation and diversification rates in a speciose bird superfamily that occurs throughout the Americas. Location: America. Taxon: The Tyrannoidea superradiation. Methods: We employed a suite of phylogenetic methods including phylogenetic generalised least squares regressions, ancestral reconstructions and multiple-states speciation-and-extinction models. Results: We observed a triangular pattern in the relationship between geographic and elevational range size, indicating that while species with a broad elevational range generally also have a large geographic range, the opposite is not necessarily true. The strong support for the BAYAREALIKE + j model, along with the negative association between range size and diversification rates, suggests that the predominant mode of speciation in this system is likely parapatric speciation, where divergence arises due to steep environmental gradients or local adaptation. Species inhabiting only one ecoregion and mountain species exhibited higher diversification rates than generalists and lowland species. Main Conclusions: Reconstruction of ancestral geographic ranges and habitat supports the idea that the colonisation of open environments that proliferated in the Miocene fueled the diversification of this superfamily. The absence of a positive relationship between range size and diversification may explain the low conservatism observed for the analysed spatial traits in this study, which contrasts with that observed in other avian groups where the process of speciation has been largely allopatric.