Why do species occupying the same ecological gradient present different evolutionary responses? Why do we find evidence for unstable polygenic adaptations in some species, while others appear to undergo speciation? What are the roles of historical demography, genomic architecture and species ecology in the diversity of these evolutionary trajectories? My PhD project will combine a multi-species approach to comparative genomics with population genomic methods in an attempt to answer these questions. The project will aim to investigate the origins of different evolutionary responses to the marine-lagoon ecological gradient in several species of fish. The project will essentially be based on molecular approaches to evolutionary genomics, but will also include a strong ecological component.