Determinants of dispersal and phylogeographic history of a highly mobile cetacean species : The North Atlantic harbour porpoise.

Understanding the determinants of individual dispersal, population structure, and evolutionary history can provide insights about how species will evolve with climate change. However, such a task is complex for highly mobile marine species such as cetaceans for which it is intuitively difficult to infer what could limit their dispersal. The harbor porpoise (Phocoena phococena) is widely distributed in the North Atlantic. Its biogeographic history has been shaped by environmental variation during the last Glaciations with the divergence of three ecotypes (or sub-species) in the Eastern North Atlantic (ENA). However, we still do not know the degree to which populations in the ENA are connected to those in the Western North Atlantic (WNA), what environmental determinants drive the dispersal of porpoises, and whether distinct ecotypes exist in the WNA. Here we analyze the genetic diversity of 10 microsatellite loci and one-quarter of the mitogenome for an unprecedented sampling of 1,533 individuals. Using spatially explicit population genetics approaches, we investigated fine-scale population structure over the entire North Atlantic. Our study suggests that porpoises from the WNA and ENA are part of the same “continental shelf ecotype” that stretches from the northern Bay of Biscay to the WNA. We identified a clear signal of restricted dispersal in the mitogenome data supporting previous evidence of female philopatry. We also discovered a cryptic divergent mitochondrial lineage in one individual from Western Greenland suggesting a fourth distinct ecotype may exist. Finally, we reconstructed the phylogeographic history of these porpoises using coalescent simulations of population evolution, shedding light on the likely scenarios that shaped the current pattern of genetic diversity. These results provide key insights into the factors and processes shaping population structure in this species and will help model its evolution in the forecasted climate changes.