Evolutionary biologists have remarked that when physical barriers and eco-physiological explanations fail to explain a specie’s range limit, that range limit represents a conundrum of evolvability: why do species not continue to evolve and adapt to a broader range of habitats and expand their ranges? Here we present an evolutionary analysis of the range limit of the Trinidadian guppy ( Poecilia reticulata ) and ask: why has it been unable to adapt to the brackish waters directly beyond its current range in Trinidad despite its ability to survive in brackish water in the lab and other parts of its range? We specifically test the hypothesis that a tradeoff between salinity tolerance and competitive ability is what prevents range expansion. To test this, we conducted several experiments which allowed us to connect behavior, fitness, and gene expression to the distribution of the guppy. First, we conducted a common garden study and found a genetic basis for this tradeoff. Next, competition experiments revealed that the tradeoff is mediated by a change in social dominance. Network analysis of gene expression data of fish under different competitive and salinity conditions allowed us to uncover evidence for this tradeoff at the transcriptomic level as well. The consistent negative correlation between salinity tolerance and competitive ability across different levels of biological organization (fitness, performance, transcriptomic) provides strong evidence that this tradeoff constrains adaptive evolution in P. reticulata and prevents range expansion.