White clover is polymorphic for cyanogenesis (HCN production after tissue damage), and this herbivore defense polymorphism has served as a classic model for studying adaptive variation. The cyanogenic phenotype requires two interacting biochemical components; the presence/absence of each component is controlled by a simple Mendelian gene (Ac/ac and Li/li). Climate-associated cyanogenesis clines occur in both native (Eurasian) and introduced populations worldwide, with cyanogenic plants predominating in warmer locations. Moreover, previous studies have suggested that epistatic selection may act within populations to maintain cyanogenic (AcLi) plants and acyanogenic plants that lack both components (acli plants) at the expense of plants possessing a single component (Acli and acLi plants). Here we examine the roles of selection, gene flow, and demography in the evolution of a latitudinal cyanogenesis cline in introduced North American populations. Using 1,200 plants sampled across a 1,650 km transect, we determine the distribution of cyanogenesis variation across the central U.S. and investigate whether clinal variation is adaptive or an artifact of population introduction history. We also test for evidence of epistatic selection. We detect a clear latitudinal cline, with cyanogenesis frequencies increasing from 11% to 86% across the transect. Population structure analysis using nine microsatellite loci indicates that the cline is adaptive and not a byproduct of demographic history. However, we find no evidence for epistatic selection within populations. Our results provide strong evidence for rapid adaptive evolution in these introduced populations, and they further suggest that the mechanisms maintaining adaptive variation may vary among populations of a species.