Adaptive radiation on an isolated oceanic island: the Cryptorhynchinae (Curculionidae) of Rapa revisited



Rapa Island in SE Polynesia hosts a remarkable adaptive radiation of small, flightless weevils in the genus Miocalles. Sixty-seven species are known at present, of which 26 are described as new. One new name, two new combinations, and two new synonyms are established. The paradoxical occurrence of a large adaptive radiation on a small (40 km2), isolated, oceanic island is analysed in its evolutionary and ecological aspects: how did so many species of weevils evolve, and how is such a diversity of weevils maintained? Most of the speciation has taken place on Rapa itself. Two principal methods of intra-island isolation of weevil populations have led to speciation: between high mountain ranges, and between Rapa and its satellite islets. Glacial sea level fluctuations aided in speciation by connecting the satellite islets to Rapa at times, and by the downward extension, and connection of high-altitude cloud forests. Some speciational events may have taken place in 15000–150 000 years. Close relatives of Rapan weevils are known from nearby Marotiri, an almost sunken island, and from the neighbouring Austral archipelago, with which some inter-island speciation has taken place. The weevil species are almost all host-plant specific. There are often several species occupying the same host plant, in which case they may inhabit different parts of it. Some plants with a longer history on Rapa host more weevil species than newer arrivals.