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Parallel evolution of phenotypic divergence offers compelling evidence supporting the influence of natural selection on the divergence process. However, the existence of ecotypes adapted to different habitats in separate locations is not a definitive proof of parallel evolution. Here, we leverage a large pool-seq dataset of the rocky-shore gastropod, Littorina saxatilis to compare and contrast two explicit scenarios of ecotype formation. Populations of this species inhabit contrasting habitats, where the main selective pressure is either wave action or crab predation. Using approximate Bayesian computation to jointly infer demographic parameters and account for pool-seq specific sources of uncertainty, our analysis reveals that ecotype formation at a large geographical scale has occurred in parallel. Parameter estimates provide strong support for a demographic history marked by the spatial separation of the ancestral populations that later gave rise to parallel evolution of ecotypes in Spain and Sweden. Additionally, ecotype formation occurred in the face of continuous gene flow between the diverging ecotypes. These results shed new light on a important model of speciation driven by ecological factors and emphasize the effectiveness of combining pool-seq with ABC for studying parallel evolution across diverse geographical regions.