Parallel speciation can occur when traits determining reproductive isolation evolve independently in different populations that experience a similar range of environments. However, a common problem in studies of parallel evolution is to distinguish this hypothesis from an alternative one in which different ecotypes arose only once in allopatry and now share a sympatric scenario with substantial gene flow between them. Here we show that the combination of a phylogenetic approach with life-history data is able to disentangle both hypotheses in the case of the intertidal marine snail Littorina saxatilis on the rocky shores of Galicia in northwestern Spain. In this system, numerous phenotypic and genetic differences have evolved between two sympatric ecotypes spanning a sharp ecological gradient, and as aside effect of the former have produced partial reproductive isolation. A mitochondrial phylogeny of these populations strongly suggests that the two sympatric ecotypes have originated independently several times. Building upon earlier work demonstrating size-based assortative mating as the main contributor to reproductive isolation among ecotypes, our analysis provides strong evidence that divergent selection across a sharp ecological gradient promoted the parallel divergence of body size and shape between two sympatric ecotypes. Thus, divergent selection occurring independently in different populations has produced the marine equivalent of host races, which may represent the first step in speciation.
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