Data from: Phenotypic and genetic divergence among harbour porpoise populations associated with habitat regions in the North Sea and adjacent seas.

Determining the mechanisms that generate population structure is essential to the understanding of speciation and the evolution of biodiversity. Here, we investigate a geographic range that transects two habitat gradients, the North Sea to North Atlantic transition, and the temperate to sub-polar regions. We studied the harbour porpoise (Phocoena phocoena), a small odontocete inhabiting both sub-polar and temperate waters. To assess differentiation among putative populations we measured morphological variation at cranial traits (N=462 individuals) and variation at eight microsatellite loci for 338 of the same individuals from Norwegian, British and Danish waters. Significant morphological differentiation reflected the size of the buccal cavity. Porpoises forage in relatively shallow waters preying mainly on benthic species in British and Danish waters, and on mesopelagic and pelagic fish off the coast of Norway. We suggest that the observed differentiation may be explained by resource specialization and either adaptation or developmental responses to different local habitats.