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As anthropogenic activities are increasing the frequency and severity of droughts, understanding whether and how fast populations can adapt to sudden changes in their hydric environment is critically important. Here, we capitalize on the introduction of the Cuban brown anole lizard (Anolis sagrei) in North America to assess the contemporary evolution of a widespread terrestrial vertebrate to an abrupt climatic niche shift. We characterized hydric balance in 30 populations along a large climatic gradient. We found that while evaporative and cutaneous water loss varied widely, there was no climatic cline, as would be expected under adaptation. Furthermore, the skin of lizards from more arid environments was covered with smaller scales, a condition thought to limit water conservation and thus be maladaptive. In contrast to environmental conditions, genome-averaged ancestry was a significant predictor of water loss. This was reinforced by our genome-wide association analyses, which indicated a significant ancestry-specific effect for water loss at one locus. Thus, our study indicates that the water balance of invasive brown anoles is dictated by an environment-independent introduction and hybridization history and highlights genetic interactions or genetic correlations as factors that might forestall adaptation. Alternative water conservation strategies, including behavioral mitigation, may influence the brown anole invasion success and require future examination.

Some of the most important insights into the ecological and evolutionary processes of diversification and speciation have come from studies of island adaptive radiations, yet relatively little research has examined how these radiations initiate. We suggest thatAnolis sagreiis a candidate for understanding the origins of the CaribbeanAnolisadaptive radiation and how a colonizing anole species begins to undergo allopatric diversification, phenotypic divergence and, potentially, speciation. We undertook a genomic and morphological analysis of representative populations across the entire native range ofA. sagrei, finding that the species originated in the early Pliocene, with the deepest divergence occurring between western and eastern Cuba. Lineages from these two regions subsequently colonized the northern Caribbean. We find that at the broadest scale, populations colonizing areas with fewer closely related competitors tend to evolve larger body size and more lamellae on their toepads. This trend follows expectations for post‐colonization divergence from progenitors and convergence in allopatry, whereby populations freed from competition with close relatives evolve towards common morphological and ecological optima. Taken together, our results show a complex history of ancient and recent Cuban diaspora with populations on competitor‐poor islands evolving away from their ancestral Cuban populations regardless of their phylogenetic relationships, thus providing insight into the original diversification of colonist anoles at the beginning of the radiation. Our research also supplies an evolutionary framework for the many studies of this increasingly important species in ecological and evolutionary research.