Diversification in the Tropical Pacific: Comparisons Between Marine and Terrestrial Systems and the Importance of Founder Speciation

Patterns of distribution and processes of differentiation have often been contrasted between terrestrial and marine biotas. The islands of Oceania offer an excellent setting to explore this contrast, because the geographic setting for terrestrial and shallow-water, benthic, marine organisms are the same: the myriad islands strewn across the vast Pacific. The size of species ranges and the geographic distribution of endemism are two biogeographic attributes that are thought to differ markedly between terrestrial and marine biotas in the Pacific. While terrestrial species are frequently confined to single islands or archipelagoes throughout Oceania, marine species tend to have wide to very wide distributions, and are rarely restricted to single island groups except for the most isolated archipelagoes. We explore the conditions under which species can reach an island by dispersal and differentiate. Genetic differentiation can occur either through founder speciation or vicariance; these processes are requisite ends of a continuum. We show that founder speciation is most likely when few propagules enter the dispersal medium and survive well while they travel far. We argue that conditions favorable to founder speciation are common in marine as well as terrestrial systems, and that terrestrial-type, archipelagic-level endemism is likely common in marine taxa. We give examples of marine groups that show archipelagic level endemism on most Pacific island groups as well as of terrestrial species that are widespread. Thus both the patterns and processes of insular diversification are variable, and overlap more between land and sea than previously considered.