Passerine sister clade comparisons reveal variable macroevolutionary outcomes of interhemispheric dispersal

Dispersal events offer a unique window into macroevolutionary processes. While several hypotheses predict alternative outcomes from such events, many empirical studies have been limited in geographic and phylogenetic scale. Here, we tested some of these hypotheses by comparing an assemblage of 16 oscine passerine clades, representing independent dispersal events into the Western Hemisphere, to their sister clades in the Eastern Hemisphere. We also compared the diversity of this assemblage of clades to an older, incumbent passerine clade in the Western Hemisphere, the suboscines. We tested for ecological opportunity and incumbency-mediated constraints versus neutrality by integrating estimates of clade-specific morphological disparities with models of multivariate regimes of trait evolution. While there was no consistent outcome of oscine dispersal and macroevolution in the Western Hemisphere relative to their Eastern Hemisphere sister groups, most clades supported a role for ecological opportunity or incumbency effects, and such effects were better explained by differences in species accumulation than by differences in rates of trait evolution or colonization timing. However, we found that this general pattern was not evident when comparing the entire oscine assemblage of the Western Hemisphere to the incumbent suboscine clade. Oscines and suboscines occupy comparable regions of functional trait diversity and, despite oscines possessing higher rates of trait evolution, these observations were consistent with simulated null expectations. This result suggests that oscine and suboscine assemblages most likely evolved in relative isolation until 5-10 million years ago, with oscines primarily in North America and suboscines in South America, which limited incumbency effects of the autochthonous suboscine radiation on subsequent oscine diversification.