Data from: Parallel evolution of local adaptation and reproductive isolation in the face of gene flow

Parallel evolution of similar phenotypes provides strong evidence for the
operation of natural selection. Where these phenotypes contribute to
reproductive isolation, they further support a role for divergent,
habitat-associated selection in speciation. However, the observation of
pairs of divergent ecotypes currently occupying contrasting habitats in
distinct geographical regions is not sufficient to infer parallel origins.
Here we show striking parallel phenotypic divergence between populations
of the rocky-shore gastropod, Littorina saxatilis, occupying contrasting
habitats exposed to either wave action or crab predation. This divergence
is associated with barriers to gene exchange but, nevertheless, genetic
variation is more strongly structured by geography than by ecotype. Using
approximate Bayesian analysis of sequence data and AFLP markers, we show
that the ecotypes are likely to have arisen in the face of continuous gene
flow and that the demographic separation of ecotypes has occurred in
parallel at both regional and local scales. Parameter estimates suggest a
long delay between colonisation of a locality and ecotype formation,
perhaps because the postglacial spread of crab populations was slower than
the spread of snails. Adaptive differentiation may not be fully
genetically independent despite being demographically parallel. These
results provide new insight into a major model of ecologically-driven
speciation.