Theory suggests that speciation is possible without physical isolation of populations (hereafter, nonallopatric speciation), but recent nonallopatric models need the support of irrefutable empirical examples. We collected snails (Littorina saxatilis) from three areas on the NW coast of Spain to investigate the population genetic structure of two ecotypes. Earlier studies suggest that these ecotypes may represent incipient species: a large, thick-shelled 'RB' ecotype living among the barnacles in the upper intertidal zone and a small, thin-shelled 'SU' ecotype living among the mussels in the lower intertidal zone only 10-30 m away. The two ecotypes overlap and hybridize in a midshore zone only 1-3 m wide. Three different types of molecular markers [allozymes, mitochondrial DNA (mtDNA) and microsatellites] consistently indicated partial reproductive isolation between the RB and the SU ecotypes at a particular site. However, each ecotype was related more closely to the other ecotype from the same site than to the same ecotype from another site further along the Galician coast (25-77 km away). These findings supported earlier results based solely on allozyme variation and we could now reject the possibility that selection produced these patterns. The patterns of genetic variation supported a nonallopatric model in which the ecotypes are formed independently at each site by parallel evolution and where the reproductive barriers are a byproduct of divergent selection for body size. We argue that neither our laboratory hybridization experiments nor our molecular data are compatible with a model based on allopatric ecotype formation, secondary overlap and introgression.

Two ecotypes of the rough periwinkle Littorina saxatilis occur at different shore levels, showing assortative mating for size and partial reproductive isolation when they meet at the mid‐shore. This system represents a putative case of incomplete speciation in sympatry. Two processes contribute to the assortative mating: morph‐specific microhabitat aggregation and mate choice. The estimation of mate choice coefficients in nature and a simulation of the aggregation effects on sexual isolation were used to disentangle these processes as well as to test alternative mechanisms of mate choice. Mate choice significantly increased the frequency of within‐morph pairs and significantly decreased the frequency of between‐morph pairs, whereas those pairs including at least one hybrid morph mated randomly. These results allow us to reject a discriminant mate choice and support a model of evolution of sexual isolation as a side‐effect of size‐assortative mating in a context of divergent natural selection for size in the population. This mechanism is more compatible with a model of incomplete by‐product ecological speciation, as suggested by previous evidence.

Populations of the marine gastropod Littorina saxatilis from exposed rocky shores of NW Spain provide one of the few putative cases of sympatric ecological speciation. Two ecotypes with large differences in shell morphology and strong assortative mating are living at different vertical levels of the shore separated by a few meters. It has been hypothesized that shell size is the main determinant for the reproductive isolation observed between the ecotypes, and that several shell shape traits are subject to divergent natural selection and are responsible for the adaptation of each ecotype to its respective habitat. Using embryos extracted from wild females we obtain estimates of genetic variation for shell size and shape and compare them with those from neutral molecular markers. Estimates of heritability are significantly larger for the ecotype found in the upper shore than for that in the lower shore, in concordance with a similar result observed for heterozygosity of neutral markers. The large genetic differentiation between ecotypes for the shell traits, contrasting the smaller close to neutral differentiation between populations of the same ecotype, supports the implication of the traits in adaptation.

On the NW coast of Spain, there is a hybrid zone in natural populations of L. saxatilis, where the largest ecotype is adapted to the desiccation and heat stress of the upper shore, whereas the smaller one is adapted to the wave impacts of the lower shore. The two ecotypes meet and hybridise in a mid-shore area producing intermediate individuals or hybrids. It has been postulated that this hybrid zone is maintained by a selection-gradient model that assumes habitat-dependent selection for the different environments of upper and lower shore areas. In this study, we focus on the hybrid (mid-shore) area, where a transition occurs between upper-shore and lower-shore environments, in order to ascertain whether the habitat-dependent selection is maintained at the micro-habitat scale. We present data on snail density at several levels of the mid-shore (varying accordingly in the relative frequency of mussels and barnacles) obtained for three seasons in three consecutive years and three localities. In the mid-shore, the abundance of the RB ecotype increases with increasing tidal height, whereas the abundance of the SU ecotype increases with decreasing tidal height, suggesting that habitat-dependent selection is maintaining the ecotype micro-distribution.

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