The limitations of both population demography and genetics highlight the need to combine these approaches when inferring the influence of demographic processes and modes of migration on genetic structure. The aim of this study was to use spatiotemporal genetic and demographic surveys to reveal the microevolutionary forces acting on the metapopulation dynamics of the freshwater snail Biomphalaria pfeifferi. We also analyzed the consequences of population turnover on temporal genetic differentiation, an aspect that has been little explored. Genetic drift was revealed by both the demographic survey, which indicated severe bottlenecks or extinction during the rainy (resp. dry) season in open (resp. closed) habitats, and the genetic approach, which indicated high selfing rates and strong temporal differentiation. Genetic reassignments and temporal differentiation both confirmed the results of the demographic survey, which suggests that migration occurs in closed (resp. open) habitats during the hot and dry (resp. rainy) season, and indicated that source-sink functioning may be envisaged. A propagule pool mode of colonization was inferred in the open habitats during the rainy season and a migrant pool in the closed habitats during the dry season. Our study also suggests that selection might be inferred from patterns of neutral genetic markers when recombination is limited.