Fine scale population structure linked to neutral divergence in the common sole (Solea solea), a marine fish with high dispersal capacity

The Baltic Sea provides a classical example of how an environmental gradient is associated with the distribution of marine species. Here, numerous genetic studies have revealed clear patterns of population structuring linked to the physical features of the gradient itself. Nevertheless, it remains difficult to distinguish clearly between the different micro-evolutionary processes that shape these structured populations in situ. The common sole (Solea solea) is a benthic flatfish that rarely occurs within the Baltic Sea, but that exhibits a clear genetic break between populations from the North Sea - Baltic Sea transition zone and the remainder of the Atlantic Ocean. Here, we aim to evaluate the extent to which natural selection is involved in the observed patterns of divergence of sole populations occurring in the transition zone by comparing them with population structures of other flatfish species that have successfully colonized the Baltic Sea. By using several thousand of ddRAD-derived SNPs, we identified a fine-scale pattern of isolation-by-distance (IBD) of sole populations in the region. However, despite strong biological similarities among the flatfishes compared here, the sole IBD was, by far, the lowest detected across the transition zone. While selection was inferred to strongly influence all other flatfishes evolutionary histories, the analytical inference on the sole demographic history suggests that this fine-scale IBD is mainly maintained by neutral processes due to low effective population size of sole in the transition zone and asymmetrical gene flow. Our work contributes to a growing body of evidence suggesting that the strength of the different micro-evolutionary processes is species-specific, even when species occur in the same environment.