Patterns of genetic variation in Pacific island land snails: the distribution of cytochrome b lineages among Society Island Partula

The radiation of Partula land snails has produced a large array of distinct morphological, ecological and behavioural types occupying many tropical volcanic islands in the Pacific Ocean. Within the Society Islands of French Polynesia, the mode of evolution is thought to have involved a single colonization event on each island, with later speciation occurring largely in situ. The present study examines genetic variation in the mitochondrial cytochrome b gene among taxa within the Society Island archipelago. Levels of intraspecific variation are found to be high, but variation among species is sometimes small. Mitochondrial variants do not always cluster according to island and some species are found to be polyphyletic in the cytochrome b tree, despite other morphological and molecular evidence that strongly supports their monophyly. A possible explanation for the polyphyly of species is that different variants are derived from ancestral mitochondrial polymorphisms that have been retained despite speciation events. Although it is possible that there has been some gene flow among islands, the distribution of mitochondrial lineages across islands strongly indicates that their origins predate the colonization of the islands in the study, and that they are very unlikely to have evolved entirely in situ.