Living in a tilted world: climate change and geography limit speciation in Old World anchovies (Engraulis; Engraulidae)

Historical changes in the distributions of temperate species in response to Milankovitch climate cycles have been well documented in palaeontological studies and recently evaluated with phylogeographical methods. How these cycles influence biological diversity remains a matter of debate. Molecular surveys of terrestrial and freshwater fauna demonstrate glacial refugia in low latitudes and range expansions into high latitudes, but few genetic studies have assessed the corresponding impact on marine fauna. In the present study, mtDNA sequences (N = 84) are surveyed to understand the impact of long-term climate oscillations on 'Old World' anchovies (genus Engraulis), a monophyletic group occurring in north and south temperate zones of the eastern Atlantic and the western Pacific. The analysis of a 521-bp sequence of mtDNA cytochrome b indicates a late Miocene or Pliocene dispersal from the north-eastern Pacific (California-Mexico) to the north-western Pacific (Japan), followed by Pleistocene dispersal from the north-western Pacific to Europe. Geography mandates that populations in southern Africa and Australia were stepping-stones for this dispersal. However, neither population occupies an intermediate position in the mtDNA genealogy; both populations are more recently derived from their northern neighbours. Haplotype diversity is high (h = 0.93-0.97) in European, Australian, and Japanese anchovies, but low (h = 0.22) in the southern African population, where all haplotypes are more closely related to European specimens than to each other. These southern populations occupy a precarious position, lacking north-south coastlines that allow range shifts during climatic extremes. Recurring extinctions and episodic recolonizations from northern hemisphere populations are the likely results. In this case, ocean-climatic changes retard rather than enhance opportunities for evolutionary radiations.