Demographic history has shaped the strongly differentiated corkwing wrasse populations in Northern Europe

Understanding the biological processes involved in genetic differentiation
and divergence between populations within species is a pivotal aim in
evolutionary biology. One particular phenomenon that requires
clarification is the maintenance of genetic barriers despite the high
potential for gene flow in the marine environment. Such patterns have been
attributed to limited dispersal or local adaptation, and to a lesser
extent to the demographic history of the species. The corkwing wrasse
(Symphodus melops) is an example of a marine fish species where regions of
particular strong divergence are observed. One such genetic break occurred
at a surprisingly small spatial scale (FST ~0.1), over a short
coastline (<60 km) in the North Sea-Skagerrak transition
area in southwestern Norway. Here, we investigate the observed divergence
and purported reproductive isolation using genome resequencing. Our
results suggest that historical events during the post-glacial
recolonization route can explain the present population structure of the
corkwing wrasse in the northeast Atlantic. While the divergence across the
break is strong, we detected ongoing gene flow between populations over
the break suggesting recent contact or negative selection against hybrids.
Moreover, we found few outlier loci and no clear genomic regions
potentially being under selection. We concluded that neutral processes and
random genetic drift e.g., due to founder events during colonization have
shaped the population structure in this species in Northern Europe. Our
findings underline the need to take into account the demographic process
in studies of divergence processes.