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The nearly neutral theory of molecular evolution predicts that small population size is essential for non-adaptive evolution. Evolution of whole-body left-right reversal in snails is generally a compelling example of non-adaptive speciation, because vari-ants with reversed chirality would suffer from reduced mating opportunities within a population. Despite this reproductive dis-advantage, sinistral snail species have repeatedly originated from dextral ancestors in terrestrial pulmonates. Here I show that snail speciation by reversal has been accelerated on oceanic islands. Analysing the global biogeography of 995 genera across 84 sty-lommatophoran families, I found that the proportion of sinistral snail genera was enhanced in genera endemic to oceanic islands. Oceanic islands are relatively small land masses offering highly fragmented habitats for snails. Thus, the upper limit of popula