Evolutionary history of a vanishing radiation: isolation-dependent persistence and diversification in Pacific Island partulid tree snails

Taehwan Lee, Jingchun Li, Celia KC Churchill, Diarmaid Ó Foighil

BackgroundPartulid tree snails are endemic to Pacific high islands and have experienced extraordinary rates of extinction in recent decades. Although they collectively range across a 10,000 km swath of Oceania, half of the family¿s total species diversity is endemic to a single Eastern Pacific hot spot archipelago (the Society Islands) and all three partulid genera display highly distinctive distributions. Our goal was to investigate broad scale (range wide) and fine scale (within¿Society Islands) molecular phylogenetic relationships of the two widespread genera, Partula and Samoana. What can such data tell us regarding the genesis of such divergent generic distribution patterns, and nominal species diversity levels across Oceania?ResultsMuseum, captive (zoo) and contemporary field specimens enabled us to genotype 54 of the ~120 recognized species, including many extinct or extirpated taxa, from 14 archipelagoes. The genera Partula and Samoana are products of very distinct diversification processes. Originating at the western edge of the familial range, the derived genus Samoana is a relatively recent arrival in the far eastern archipelagoes (Society, Austral, Marquesas) where it exhibits a stepping¿stone phylogenetic pattern and has proven adept at both intra¿and inter¿ archipelago colonization. The pronounced east¿west geographic disjunction exhibited by the genus Partula stems from a much older long-distance dispersal event and its high taxonomic diversity in the Society Islands is a product of a long history of within¿archipelago diversification.ConclusionsThe central importance of isolation for partulid lineage persistence and diversification is evident in time-calibrated phylogenetic trees that show that remote archipelagoes least impacted by continental biotas bear the oldest clades and/or the most speciose radiations. In contemporary Oceania, that isolation is being progressively undermined and these tree snails are now directly exposed to introduced continental predators throughout the family¿s range. Persistence of partulids in the wild will require proactive exclusion of alien predators in at least some designated refuge islands.