Founder Effects and Admixture Shape Genetic and Morphological Variation in Non‐Native Populations of the Lizard Anolis cristatellus Introduced From Puerto Rico to Florida

Biological invasions provide valuable opportunities to examine how stochastic evolutionary processes, such as genetic drift and admixture, interact with deterministic processes like natural selection over short timescales. Disentangling the effects of these processes is often challenging, particularly when invasion sources and timelines cannot be confidently identified. Here, we investigate these dynamics in Anolis cristatellus , a Caribbean lizard introduced to two locations in Florida, for which Puerto Rican source populations have been previously identified using genetic data. We combine genetic analyses with quantitative assessments of morphology for native source populations, 2 independently introduced populations, and 10 additional non‐native populations that arose through post‐establishment spread. We find evidence of reduced genetic variation consistent with founder events during introduction. Multivariate analyses of morphology indicate that the two introductions originated from distinct regions of morphospace and subsequently diverged along different trajectories, consistent with divergence shaped by founder‐induced genetic drift. We further find that post‐establishment admixture among invasive populations contributed to patterns of morphological variation. Our results demonstrate that early‐stage stochastic processes can leave lasting signatures on both genetic and phenotypic variation. These findings underscore the importance of considering historical demographic processes when assessing evolutionary responses of invasive populations to contemporary environmental change.