Résumé
Contact zones between ecotypes are windows for understanding how species
may react to climate changes. Here, we analysed the fine-scale genetic and
morphological variation in harbour porpoises (Phocoena phocoena) around
the UK by genotyping 591 stranded animals at nine microsatellite loci. The
data were integrated with a prior study to map at high resolution the
contact zone between two previously identified ecotypes meeting in the
northern Bay of Biscay. Clustering and spatial analyses revealed that UK
porpoises are derived from two genetic pools with porpoises from the
southwestern UK being genetically differentiated, and having larger body
sizes compared to those of other UK areas. Southwestern UK porpoises
showed admixed ancestry between southern and northern ecotypes with a
contact zone extending from the northern Bay of Biscay to the Celtic Sea
and Channel. Around the UK, ancestry blends from one genetic group to the
other along a southwest--northeast axis, correlating with body size
variation, consistent with previously reported morphological differences
between the two ecotypes. We also detected isolation by distance among
juveniles but not in adults, suggesting that stranded juveniles display
reduced intergenerational dispersal. The fine-scale structure of this
admixture zone raises the question of how it will respond to future
climate change and provides a reference point for further study.