This project aimed to identify within-generation allele frequency changes in subpopulations of Fundulus heteroclitus inhabiting distinct environmental niches. Repeated, within-generation selection may allow populations inhabiting heterogeneous landscapes to locally adapt to environmental niches, even at small spatial scales when gene flow is high. This project identified significant, within-generation allele frequency changes that led to fine spatial structure among niche populations indicative of local adaptation. Identifying repeated, within-generation adaptation at small temporal and spatial scales has far-reaching consequences for our understanding of the mechanisms and rate of evolution.