Comparing geographical genetic differentiation between candidate and noncandidate loci for adaptation strengthens support for parallel ecological divergence in the marine snail Littorina saxatilis.

The Galician sympatric ecotypes of Littorina saxatilis have been proposed as a model system for studying parallel ecological speciation. Such a model system makes a clear prediction: candidate loci (for divergent adaptation) should present a higher level of geographical differentiation than noncandidate (neutral) loci. We used 2356 amplified fragment length polymorphisms (AFLPs) and four microsatellite loci to identify candidate loci for ecological adaptation using the F(ST) outlier method. Three per cent of the studied AFLP loci were identified as candidate loci associated with adaptation, after multitest adjustments, thus contributing to ecotype differentiation (candidate loci were not detected within ecotypes). Candidate and noncandidate loci were analysed separately at four different F(ST) partitions: differences between ecotypes (overall and local), differences between localities and micro-geographical differences within ecotypes. The magnitude of F(ST) differed between candidate and noncandidate loci for all partitions except in the case of micro-geographical differentiation within ecotypes, and the microsatellites (putatively neutral) showed an identical pattern to noncandidate loci. Thus, variation in candidate loci is determined partially independent by divergent natural selection (in addition to stochastic forces) at each locality, while noncandidate loci are exclusively driven by stochastic forces. These results support the evolutionary history described for these particular populations, considered to be a clear example of incomplete sympatric ecological speciation.