Islands are often perceived as endpoints of colonization. However, recent research has highlighted the pivotal role of islands as sources of both speciation and dispersal. In regions of high species turnover such as the Indo-Australian Archipelago, dispersals have typically been treated as a dichotomy between the main continental shelves of Sunda and Sahul. This undermines the role of intervening islands to act not only as sites of insular diversification but also as sources of radiation following upstream colonization events. We aim to explore the substantial role of islands using large-scale ancestral state estimations on passerine birds of the Indo-Australian Archipelago. We tested the effects of historical changes in connectivity, compared within-area diversifications along with inwards and outwards dispersal between areas, and investigated speciation events occurring post-upstream dispersal. We show that islands such as Wallacea, New Guinea, and the East Melanesian arc disproportionately influence the generation of lineages that further diversify into major clades of the passerine super radiation. These findings support recent perspectives that position islands as catalysts for colonization. Our study underscores the important biogeographical role of islands, highlighting their dual capacity as both sources and sinks. By shedding light on the ability of islands to drive radiations across a region, our research offers insights with broad implications for understanding the dynamics of species diversification and biogeography.
Beyond Endpoints: Islands drive radiation of passerine birds in the Indo-Australian Archipelago