Atypical panmixia in a European dolphin species (Delphinus delphis): implications for the evolution of diversity across oceanic boundaries.

Despite the scarcity of geographical barriers in the ocean environment, delphinid cetaceans often exhibit marked patterns of population structure on a regional scale. The European coastline is a prime example, with species exhibiting population structure across well-defined environmental boundaries. Here we undertake a comprehensive population genetic study on the European common dolphin (Delphinus delphis, based on 492 samples and 15 loci) and establish that this species shows exceptional panmixia across most of the study range. We found differentiation only between the eastern and western Mediterranean, consistent with earlier studies, and here use approximate Bayesian computations to explore different scenarios to explain the observed pattern. Our results suggest that a recent population bottleneck likely contributed significantly to the differentiation of the Eastern Mediterranean population (in Greek waters). This interpretation is consistent with independent census data that suggest a sharp population decline in the recent past. The implication is that an unperturbed population may currently show panmixia across the full study range. This exception to the more typical pattern of population structure seen for other regional dolphin species (and for common dolphin populations elsewhere in the world) suggests particular ecological or life-history traits distinct to this species in European waters.