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Interactions between evolutionary forces such as natural selection, genetic drift and gene flow are complex. Natural selection can lead to parallel formation of phenotypes under similar environmental conditions. Phylogenetic relationships can be inferred from the accumulation of genetic variation caused by genetic drift, regardless of phenotypes. Gene flow between populations can sometimes facilitate the formation of species by natural selection. In this thesis, two groups of marine snails in the diverse subfamily Littorininae were studied. The three rocky-shore species Littorina saxatilis, Littorina arcana and Littorina compressa form one such group, whereas the two sister-species Littoraria cingulata and Littoraria filosa form the other group. Previous studies of the L. saxatilis complex have shown high levels of phenotypic and genetic diversity both at local and broader scales. Previous studies of L. cingulata and L. filosa have found some of the typical signatures of reinforcement. Chapters II and III focused on analysing morphological and genetic variation, respectively, within and among species in the Littorina saxatilis complex from the British Isles. Geometric morphometrics analyses revealed a diversity of shell shape among species, but especially so within L. saxatilis. Shell shape was better explained by environment rather than by geography. Molecular data obtained by high-throughput targeted capture showed the opposite pattern, i.e. genetic variation showed a strong phylogeographic pattern. Chapter IV focused on testing whether reinforcement had contributed towards speciation between Littoraria cingulata and Littoraria filosa. Approximate Bayesian computation analysis supported absence of gene flow between the species in sympatry, suggesting that reinforcement did not contribute towards speciation. However, the results need to be validated and more complex models tested. This thesis highlights the relevance of marine snails in the subfamily Littorininae as model species for addressing a wide range of evolutionary questions. It also provides a wealth of data for many potential follow-up studies.