Within the Balkan Peninsula, topographic and climatic agents have promoted biodiversity and shaped the speciation history of many ectotherms. Here, we targeted an iconic European reptile, the nose-horned viper species-complex (Vipera ammodytes), and explored its spatial and temporal evolution. We (1) utilized genome-wide SNPs to infer genetic structure and build a time-calibrated species tree, and (2) applied species distribution modeling (SDM) with niche-divergence tests among major phylogenomic clades. Geographically structured genetic diversity was found. Cycles of recurrent isolation and expansion during glacial–interglacial periods led to allopatric speciation and to secondary contacts and formation of multiple hybrid zones throughout the Balkan Peninsula. Deep divergence is still detected among populations separated by old and imminent biogeographical barriers (e.g. Pindos Mountain Range, the Cyclades islands, etc.), but in most cases, speciation is incomplete. At the other end of the speciation continuum, we recognize two well-differentiated lineages, currently lacking any evidence of gene flow; one is distributed in the Northwestern Balkans and the other in the Southeastern Balkans, further expanding into Asia. Despite their split 5 Mya, there is no evidence of ecological divergence, as speciation probably occurred in niche-pockets of analogous environments. These two lineages probably represent different species, while V. transcaucasiana does not merit species status. By comparing the genomic phylogenies to an updated mitochondrial one, we propose an evolutionary scenario that resolves all mito-nuclear conflicts, according to which the history of the V. ammodytes species-complex was shaped by complex processes, including a major event of introgressive hybridization with asymmetric mitochondrial capture.

Across the distribution of the Caspian whipsnake (Dolichophis caspius), populations have become increasingly disconnected due to habitat alterations. To understand population dynamics and the adaptive potential of this widespread but locally endangered snake, we investigated population structure, admixture and effective migration patterns. We took a landscape-genomic approach to identify selected genotypes associated with environmental variables relevant to D. caspius. With double-digest restriction-site associated DNA (ddRAD) sequencing of 53 samples resulting in 17,518 single nucleotide polymorphisms (SNPs), we identified eight clusters within D. caspius reflecting complex evolutionary patterns of the species. Estimated Effective Migration Surfaces (EEMS) revealed higher-than-average gene flow in most of the Balkan Peninsula and lower-than-average gene flow along the middle section of the Danube River. Landscape genomic analysis identified 751 selected genotypes correlated with seven climatic variables. Isothermality correlated with the highest number of selected genotypes (478) located in 41 genes, followed by annual range (127) and annual mean temperature (87). We conclude that environmental variables, especially the day-to-night temperature oscillation in comparison to the summer-to-winter oscillation, may have an important role in the distribution and adaptation of D. caspius.