Speciation is generally thought to occur as a result of geographic separation that causes a disruption in dispersal and gene flow between populations. It is difficult to imagine effective geographic barriers to dispersal in the ocean, especially for species with planktonic larvae. However many studies of marine animals, such as echinoderms and fishes seem to support such a pattern of allopatric speciation. Published molecular phylogenetic studies of marine molluscs show that patterns of speciation depend on the taxonomic group and on the region of the world in which the phylogeographic patterns are studied. Research from the Indo-Pacific show a pattern of allopatry between sister species and many small areas of regional endemism and cases with more distantly related species occurring in sympatry. Studies along the Pacific coast of the Americas support a situation where many sister species occur in sympatry. Comparisons across datasets show that those species that occur in sympatry are generally separated by smaller genetic distances than those that are allopatric but that divergences also correlate across geographic region. This suggest that the tempo and mode of speciation varies geographically.