The role of evolutionary time, diversification rates and dispersal in determining the global diversity of a large radiation of passerine birds

Aim: Variation in species diversity among different geographical areas may result from differences in speciation and extinction rates, immigration and time for diversification. An area with high species diversity may be the result of a high net diversification rate, multiple immigration events from adjacent regions, and a long time available for the accumulation of species (known as the ‘time-for-speciation effect’). Here, we examine the relative importance of the three aforementioned processes in shaping the geographical diversity patterns of a large radiation of passerine birds. Location: Global. Taxon: Babblers (Aves: Passeriformes). Methods: Using a comprehensive phylogeny of extant species (~90% sampled) and distributions of the world's babblers, we reconstructed their biogeographical history and analysed the diversification dynamics. We examined how species richness correlates with the timing of regional colonization, the number of immigration events and the rate of speciation within all 13 geographical distribution regions. Results: We found that babblers likely originated in the Sino-Himalayan Mountains (SHM) in the early Miocene, suggesting a long time for diversification and species accumulation within the SHM. Regression analyses showed the regional diversity of babblers can be well explained by the timing of the first colonization within of these areas, while differences in rates of speciation or immigration have far weaker effects. Nonetheless, the rapid speciation of Zosterops during the Pleistocene has accounted for the increased diversification and accumulation of species in the oceanic islands. Main Conclusions: Our results suggest that the global diversity patterns of babblers have predominantly been shaped by the time-for-speciation effect. Our findings also support an origin centred in tropical and subtropical parts of the SHM, with a cradle of recent diversification in the oceanic islands of the Indo-Pacific and Indian Ocean regions, which provides new insights into the generation of global biodiversity hotspots.