title: Parallel evolution of local adaptation and reproductive isolation in the face of gene flow.
abstract: 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 amplified fragment length polymorphism 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 colonization 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.
authors.affiliation: Animal and Plant Sciences, The University of Sheffield, Sheffield S10 2TN, United Kingdom; Biological and Environmental Sciences-Tjärnö, University of Gothenburg, SE-452 96, Strömstad, Sweden. r.k.butlin@sheffield.ac.uk.
authors.forename: Roger K
authors.initials: RK
authors.lastname: Butlin
authors.forename: Maria
authors.initials: M
authors.lastname: Saura
authors.forename: Grégory
authors.initials: G
authors.lastname: Charrier
authors.forename: Benjamin
authors.initials: B
authors.lastname: Jackson
authors.forename: Carl
authors.initials: C
authors.lastname: André
authors.forename: Armando
authors.initials: A
authors.lastname: Caballero
authors.forename: Jerry A
authors.initials: JA
authors.lastname: Coyne
authors.forename: Juan
authors.initials: J
authors.lastname: Galindo
authors.forename: John W
authors.initials: JW
authors.lastname: Grahame
authors.forename: Johan
authors.initials: J
authors.lastname: Hollander
authors.forename: Petri
authors.initials: P
authors.lastname: Kemppainen
authors.forename: Mónica
authors.initials: M
authors.lastname: Martínez-Fernández
authors.forename: Marina
authors.initials: M
authors.lastname: Panova
authors.forename: Humberto
authors.initials: H
authors.lastname: Quesada
authors.forename: Kerstin
authors.initials: K
authors.lastname: Johannesson
authors.forename: Emilio
authors.initials: E
authors.lastname: Rolán-Alvarez
mesh_terms: D000220:Adaptation, Biological; D060105:Animal Shells; D000818:Animals; D005075:Biological Evolution; D017753:Ecosystem; D060146:Ecotype; D005060:Europe; D005260:Female; D051456:Gene Flow; D049810:Genetic Speciation; D060047:Reproductive Isolation; D012908:Snails / Q000033:anatomy & histology / Q000235:genetics*; D055871:Tidal Waves
keywords: Gene flow; local adaptation; parallel evolution; speciation