Résumé
Despite no obvious barriers to gene flow in the marine realm,
environmental variation and ecological specializations can lead to genetic
differentiation in highly mobile predators. Here, we investigated the
genetic structure of the harbor porpoise over the entire species
distribution range in western Palearctic waters. Combined analyses of ten
microsatellite loci and a 5,085 bases-pairs portion of the mitochondrial
genome revealed the existence of three ecotypes, equally divergent at the
mitochondrial genome, distributed in the Black Sea, the European
continental shelf waters, and a previously overlooked ecotype in the
upwelling zones of Iberia and Mauritania. Historical demographic
inferences using Approximate Bayesian Computation (ABC) suggest that these
ecotypes diverged during the Last Glacial Maximum (~23–19 kilo-years ago,
kyrBP). ABC supports the hypothesis that the Black Sea and upwelling
ecotypes share a more recent common ancestor (~14 kyrBP) than either does
with the European continental shelf ecotype (~28 kyrBP), suggesting they
likely descended from the extinct populations that once inhabited the
Mediterranean during the glacial and post-glacial period. We showed that
the two Atlantic ecotypes established a narrow admixture zone in the Bay
of Biscay during the last millennium, with highly asymmetric gene flow.
This study highlights the impacts that climate change may have on the
distribution and speciation process in pelagic predators and shows that
allopatric divergence can occur in these highly mobile species and be a
source of genetic diversity.