The role of geographic isolation in marine speciation has been contested because of the high dispersal potential of marine species and the apparent lack of dispersal barriers in the sea. Compared to marine invertebrates and fish, most seaweed species are considered poor dispersers. This limited dispersal capacity may reflect on diversity patterns and the spatial scale at which speciation takes place. We assessed species diversity, distribution patterns and phylogenetic relationships within the marine red algal genus Portieria based on multilocus data and analyses combining phylogenetic and population genetic principles. In stark contrast with the current morphology-based assumption that the genus includes about seven species, including one widely distributed species in the Indo-West Pacific (P. hornemannii), DNA-based species delimitation resulted in the recognition of more than 90 species. Species distributions were found to be highly structured with most species having narrow ranges. Our results indicate that speciation in the marine environment may act at much smaller geographical scales than is commonly assumed. The most likely explanation to account for the observed diversity is a peripatric model of speciation, involving founder events resulting from long distance dispersal and subsequent differentiation of the established populations. Temporal analyses of diversification indicate a gradual pattern since the Eocene, and long term persistence of ancient lineages within confined regions of the Indo-Pacific. The Indo Malay archipelago (IMA or Coral Triangle) was found to be particularly diverse. Our phylogenetic data indicate that this diversity resulted from a combination of species accumulation via island integration through tectonic movement, in combination with speciation within the IMA as a result of increased geographical complexity of the region from the Miocene to Pleistocene.
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