Past climatic change as a driving force of marine diversification is still largely unclear, particularly for Southern Hemisphere species. Here, we present a case study using the brown fur seal, Arctocephalus pusillus, and assess the geographical structure and demographic history using mitochondrial and nuclear data. Results show the two previously defined subspecies (one from Australia and the other from Southern Africa) are phylogeographically distinct. Migration analyses based on nuclear data suggest the absence of migrants among the two genetically close assemblages. The demographic history of A. pusillus is characterized by a glacial population expansion (~18 Kya) in the Southern African taxon, which coincides with time estimates of population expansion of prey species of seals. Approximate Bayesian Calculations support an eastward dispersal event during the Last Glacial Maximum (LGM) when sea levels were lower, followed by a postglacial divergence event, ~ 13 Kya. The demographic history of the brown fur seal in the Southern Oceans provides support that recent paleoclimatic changes could have facilitated expansions in some marine species and that postglacial sea-level rise may have acted as a dispersal barrier for species mostly confined to continental shelves.  

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