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Environmental gradients constitute an extraordinary opportunity to study adaptation and evolution on a small spatial scale. Along such gradients abiotic and biotic factors fluctuate in space and time and allow studying the distribution of organisms in terms of environmental tolerances and adaptations. Prior to my research there was little knowledge on how differences in the ecology across an environmental gradient influence variation in mating success, population divergence, and physiological parameters determining reproductive success within a single species. I studied these topics in common gobies (Pomatoschistus microps) along a salinity gradient in the Baltic Sea. I first described the actual ecological differences between populations of common gobies. Specifically, I conducted habitat surveys collecting information on nesting resources (empty mussel shells) availability and assessed demographic features as well as phenotypic differences between common goby populations along the salinity gradient in the Baltic Sea (Chapter I). I found that salinity correlated with nest resource quantity and quality, population density and body size of P. microps. I further investigated whether there is any evidence that populations are locally adapted (Chapter II). I demonstrated a clear population genetic structure on a coarse and fine geographic scale, likely driven by divergent selection. Subsequently, I assessed gonad and liver investment of common gobies along the salinity gradient. The investment in both of these organs is linked to the reproductive output. Males originating from low salinity sites had significantly larger gonads (specifically sperm duct glands) and livers than males from intermediate or high salinity sites (Chapter III). With my research I could show that differences in salinity in the Baltic Sea lead to plastic variation (Chapter I, III) in common goby populations, with signs of genetic population divergence (Chapter II). Advisors/Committee Members: Heubel, Katja (Dr.) (advisor).