Association between Genetic Admixture and Morphological Patterns in a Hybrid Zone between the Two Iberian Vipers, Vipera aspis and V. latastei

Understanding how hybridization influences the morphology and fitness of hybrids is essential for studying adaptive evolution and ecological speciation. Secondary contact zones, where separately evolving populations meet and hybridize, offer valuable insights into the evolutionary processes driving speciation and provide an excellent system to address these questions. In this study, we investigate patterns of morphological and genetic variation of two congeneric viper species, Vipera aspis and V. latastei, across a contact zone in northern Spain (Oja-Tirón), where vipers with mixed morphology are often detected, but genetic studies addressing hybridization and relating patterns of genetic and morphological admixture are lacking. Using nine morphological traits (scalation and colouration) and 18 microsatellite markers, we (1) estimated the extent of hybridization, (2) morphologically characterized parental species and hybrids, and (3) evaluated the correlation between patterns of genetic and morphological admixture. Analyses revealed a bimodal hybrid zone with high rate of hybridization (22%) and prevalence of late-generation hybrids (F2 and backcrosses). Morphological analyses differentiated the two parental species, and a positive correlation (r = 0.95) was found between morphological and genetic patterns. The hybrid group displayed on average an intermediate morphology between the parentals, yet morphologically intermediate hybrids were rare in our dataset. Instead, most hybrids resembled the parental species with whom they share most of the genetic background. Notably, the hybrid group exhibited greater morphological variation than the parental groups. Traits with adaptative value, such as ventral scales and dorsal marks, showed significant differences between hybrids and the two parental species. Introgression of these traits may confer ecological advantages to hybrids, enhancing local adaptation. Overall, this study reveals a positive correlation between patterns of morphological and genetic variation across a hybrid zone and provides insights into the phenotypic consequences of hybridization on these viper species.