Without the locals' aid: no evidence for a role of admixture in the colonisation success of Italian wall lizards.

The successful establishment of small founding populations introduced into novel environments often represents a paradox, given the genetic challenges they face. Genetic admixture, whether intra- or interspecific, may offer a solution by enhancing the genetic diversity and adaptability of the population. Here, we explore the role of genetic admixture in the rapid establishment and adaptation of the Italian wall lizard (Podarcis siculus) introduced on Pod Mrčaru, a small islet in the Adriatic Sea. Introduced in 1971 in a small group of ten individuals from a nearby island, this population rapidly adapted to its new environment, outcompeting the native Dalmatian wall lizard (Podarcis melisellensis) and exhibiting striking ecological and phenotypic shifts. Using whole-genome sequencing data from P. siculus populations on Pod Mrčaru and neighbouring islands and from P. melisellensis, we investigated population structure, admixture, and gene flow to test whether inter- or intraspecific genetic exchange contributed to the successful establishment and divergence of the introduced population. Despite the sympatric presence of P. melisellensis during the introduction of P. siculus on Pod Mrčaru, and thus the opportunity for genetic exchange, we found no evidence of hybridisation between the two species. Even amongst neighboring island populations of P. siculus, we observed only limited gene flow, suggesting relatively independent evolution of the populations since their establishment on the islands. These findings highlight the potentially significant roles of distinct selective pressures and/or ecological and phenotypic plasticity, rather than genetic exchange, in driving the population's rapid adaptation to a novel environment.