<i>Caenorhabditis</i> diversity on Pohnpei, Micronesia, provides evidence that the Elegans Supergroup has its roots in the Americas and diversified in the Pacific <i>en route</i> to Asia

Model organisms are powerful tools for discovery in cell and molecular biology, and studies of their natural history have the potential to provide bridges between these fields and ecology and evolutionary biology. The nematode Caenorhabditis elegans is a preeminent model, and recent findings place its center of diversity in the cool, high-elevation forests of Hawaii. To test models of biogeography and species coexistence, we investigated Caenorhabditis on Pohnpei, an island in Micronesia, home to the largest patch of high-elevation forest between Hawaii and Asia. We found nine species of Caenorhabditis, five of them new. Using the distribution of nematodes among habitat patches, we parameterized models of Caenorhabditis population biology that help explain species coexistence patterns. We inferred a phylogeny for 70 species of Caenorhabditis and performed the first quantitative biogeographic analysis for the group. Our analysis suggests that the deep ancestors of the Elegans Supergroup of species lived in the Americas. The Supergroup's subsequent diversification occurred in Oceania, giving rise to a diverse Oceanian fauna and ultimately to multiple lineages that moved into Asia, Africa, Australasia, and back into the Americas. We infer a slow trans-Pacific migration, with the islands of Oceania serving as sources rather than sinks for biodiversity.