Digging Into the General Dynamic Model: How Island Ontogeny Shifts the Evolutionary Drivers of Lineage Divergence

ABSTRACT Aim The general dynamic model (GDM) of oceanic island biogeography proposes that evolutionary processes shift predictably with island ontogeny: opportunities for ecological speciation on young islands drive a rapid increase in diversity, which peaks on middle‐aged islands, then declines on older islands as extinction increases. Most studies reporting support for the GDM have not assessed the evolutionary processes associated with these phases: ecological speciation on young islands, genetic drift within species on middle‐aged islands, and finally, extinction on old islands. Here, we refine and extend the GDM by developing a set of testable predictions to show that the dominant evolutionary drivers of lineage divergence shift as islands progress through successive ontogenetic stages. We then assess whether patterns of genomic divergence are consistent with our predictions in an iconic flowering plant radiation. Location The Mascarene archipelago (Rodrigues, Mauritius, Réunion) in the Indian Ocean. Taxon Hibiscus section Lilibiscus (Malvaceae). Methods Using genome‐wide data from 2bRAD sequencing, we conducted phylogenomic, population genomic and demographic analyses (approximate Bayesian computation in DIY‐ABC) of six single‐island endemic species sampled across the three islands. Results On the youngest island, we detected signatures of ecological differentiation and high intraspecific gene flow, suggesting ecological speciation as the dominant driver of lineage divergence. On the middle‐aged island, we found that recent population structure was shaped predominantly by genetic drift, with some evidence of ancient interspecific gene flow. On the oldest island, phylogenetic and population genetic patterns were consistent with lineage loss and increased phylogenetic distance, suggesting extinction as the dominant evolutionary force. Main Conclusions Our findings provide rare empirical support for the GDM's process‐based predictions and offer a general framework for linking geomorphological change on oceanic islands to evolutionary dynamics in island systems. Our framework lays the groundwork for future population genetic studies to directly test the evolutionary processes shaping insular biodiversity.