Phylogeography of Bornean land snails suggests long-distance dispersal as a cause of endemism

Islands are often hotspots of endemism due to their isolation, making colonization a rare event, and as such facilitating allopatric speciation. Dispersal usually originates from nearby locations according to a stepping-stone model. We aimed to elucidate colonization and speciation processes in an endemic-rich system of tropical land-based islands that does not seem to follow the obvious stepping-stone model of dispersal. We studied the phylogeographic relations within several island radiations of three unrelated species complexes of land snail, each common on limestone outcrops in the study region in Sabah, Malaysian Borneo. We found that most archipelagos have been colonized multiple times over the past three million years, from within the region. Gene flow in these Bornean land snails is rare and has been affected both by small-scale dispersal, where it leads to isolation-by-distance, and long-distance colonization. Our results demonstrate that long-distance dispersal, and resultant colonization, is actually relatively frequent and seems the origin of endemism in our study system. The formation of highly localized, isolated “endemic populations” appears to form the onset for a complex radiation of endemic taxa.