Identifying how the interplay between neutral and adaptive evolutionary processes shapes the spatial structure of natural
populations is a central task in ecology and conservation biology. In marine populations, generally characterised by large
population sizes and high gene flow, the simultaneous use of multiple descriptors (i.e. genetic, morphological, life-history,
etc.) can be particularly helpful in unravelling the often subtle and complex spatial patterns observed. The gastropod Buccinum
undatum (common whelk) lacks a planktonic larval stage, which could promote isolation of local populations and lead to phenotypic
divergence through genetic drift or local adaptation. Despite the commercial significance of this species, the relative importance
of these 2 forces remains unknown. Here, we used microsatellites, geometric morphometrics and shell thickness measurements
to investigate the evolutionary dynamics generating spatial variation in 10 whelk populations in coastal waters around Ireland.
Genetic diversity was generally high and genetic structure moderate but significant (overall FST = 0.019), in accordance with
an isolation-by-distance pattern. Phenotypic divergence, as measured by PST, was uncorrelated with, and much more pronounced
than, neutral divergence (FST), indicating that environmental variation rather than population isolation drives phenotypic
differentiation. For some traits, at least, diversifying selection is likely to be involved unless additive genetic components
of phenotypic variation among populations are very low. Our results document a lack of correspondence between neutral and
adaptive divergence, and highlight the need to couple connectivity estimates with the assessment of ecologically relevant
traits in fisheries management and conservation.