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Genetic divergence among populations of marine broadcast spawners in the absence of past geological barriers presents an intriguing challenge to understanding speciation in the sea. To determine how differences in life history affect genetic divergence and demographic histories across incomplete dispersal barriers, we conducted a comparative phylogeographic study of three intertidal limpets (Siphonaria spp.) represented on either side of a biogeographic disjunction separating tropical and subtropical marine provinces in southeastern Africa. Using a combination of mitochondrial and nuclear sequence data, we identified two distinct evolutionary lineages each in both Siphonaria concinna (a planktonic disperser) and S. nigerrima (a direct developer), and panmixia in a second planktonic disperser, S. capensis. Although phylogeographic breaks were present in two species, how these became established differed depending on their life histories. In the direct developer, lack of gene flow following divergence, and demographic expansion from a small initial size in the species' subtropical population, point to a single colonisation event. In contrast, the evolutionary lineages of the planktonic disperser split into two genetic lineages with much larger initial population sizes and southward gene flow continued at least periodically, indicating that divergence in this species may have been driven by a combination of reduced larval dispersal and divergent selection. These findings help explain why the presence or absence of phylogeographic breaks often appears to be independent of species' dispersal potential., (© 2011 Blackwell Publishing Ltd.)

Aim Based on mitochondrial DNA (mt DNA) data, many organisms with ranges spanning multiple biogeographical regions exhibit genetic structure across the transition zones between these regions, while others appear to be genetically homogenous. No clear link has been found between the presence or absence of such spatial genetic discontinuities and species' dispersal potential, confounding the formulation of general predictions concerning genetic structure. The fact that discrepancies between mt DNA and nuclear markers are common across semi-permeable barriers suggests that a lack of structure could be attributable to mt DNA-specific properties of inheritance. We re-examined genetic structure in the coastal crab Hymenosoma orbiculare, a species that is represented by a single mt DNA lineage across the Atlantic/Indian Ocean biogeographical transition zone, by comparing mt DNA data with nuclear DNA data. Location South Africa's cool-temperate and warm-temperate marine biogeographical provinces. Methods DNA sequence data from the mitochondrial COI gene and the intron of the nuclear ANT gene were generated for 150 individuals of H. orbiculare. For each locus, we determined whether the sharing of alleles between provinces was the result of either the retention of ancestral polymorphism or of secondary contact. Results We recovered two nuclear intron lineages whose spatial genetic structure reflects contemporary biogeographical and oceanographical conditions, indicating that the existence of a single mt DNA lineage is not a function of unexpectedly high levels of dispersal. Main conclusions Mt DNA-based genetic homogeneity is increasingly being reported in coastal organisms with ranges spanning biogeographical transition zones that define distinct evolutionary lineages in other species. Our results stress the importance of revisiting single-locus data sets by means of multilocus genetic approaches before any conclusions can be drawn about the role of biogeographical transition zones in driving genetic structure. [ABSTRACT FROM AUTHOR]