Biogeography and temporal progression during the evolution of striped dolphin population structure in European waters

Aim: We investigated the population genetic structure of a highly mobile marine species, the striped dolphin, Stenella coeruleoalba (Meyen, 1833), along a geographical range with habitat transitions and historical dynamics to identify the causes of genetic divergence, and to assess the effect of past climate change on demography and population connectivity. Location: North-east Atlantic Ocean and Mediterranean Sea. Methods: Twenty microsatellite loci were used in conjunction with coalescent methods to investigate the genetic structure and demographic history of striped dolphins (Stenella coeruleoalba) in the Mediterranean Sea and eastern North Atlantic Ocean. Approximate Bayesian modelling was used to compare the support for alternative scenarios for the pattern of divergence over time in the context of known geographical transitions and environmental change over the course of the Quaternary. Results: We describe a novel pattern of structure among the extant populations along north–south and east–west axes. Modern gene flow shows strong directionality from north-east to south and west in the North Atlantic, and from west to east in the Mediterranean. On a temporal scale we found evidence for a progression starting with a division between the North Atlantic and Mediterranean Sea populations during the middle Pleistocene, followed by a division within the Mediterranean between the east and west basins towards the end of the Pleistocene, and finally an east–west division in the eastern North Atlantic at the start of the Holocene. Main conclusions: In the context of known population structure for other marine species along the same geographical range, our data facilitate inference of the more general processes that shaped patterns of biogeography across this region through the environmental transitions of the Quaternary. In particular, Pleistocene era divisions apparently reflect strong physical habitat boundaries, with later divisions associated with climate warming in the Holocene.