The exotic round goby (Neogobius melanostomus), (hereafter round goby), originally from the Ponto- Caspian region, was first discovered in North America in 1990;; it now inhabits the Great Lakes system. The goal of the project is to quantify the evolutionary mechanisms underlying the habitat margin of the exotic round goby across the environmental gradient in water ion chemistry. I predict two possible outcomes of the spread of the round goby from the high-ion waters of the Great Lakes to the low-ion muddy waters of the Upper St. Lawrence river. First, the spread of the invasion associated with local adaptations to the habitat margin will be limited by gene flow of maladaptive alleles from populations from the Great Lakes. Second, the invasion will be promoted by evolution of phenotypic plasticity in the low-ion habitat margin. These two outcomes are not exclusive and could both happen at the same time. In order to quantify this adaptation, I will be measuring selection on heritable and non-heritable sources of variation of the round goby. More specifically, using laboratory experiment, I will be looking at quantitative traits such as survival, feeding rates, mass gain, body size and fecundity in both high-ion and low-ion waters. The round goby is a destructive species that has already invaded the Great Lakes system. It is critical to understand the eco-evolutionary dynamics of this exotic species as it is a key component in restricting it’s spread beyond the Great Lakes system as well as for future conservation efforts.