Latitudinal Clines in Gene Flow and Demographic Stability Reveal Drivers of Microendemism in a Radiation of Alpine Grasshoppers

Understanding the geological, ecological, and microevolutionary processes that shape range size in alpine organisms is key to explaining the high rates of local endemism that characterise mountain ecosystems. Here, we investigate the geography of speciation in Oropodisma, a radiation of alpine grasshoppers from the Balkan Peninsula that includes several narrow-endemic species distributed along a latitudinal gradient of > 600 km. Phylogenomic-based species delimitation clarified taxonomic uncertainties and revealed striking variation in range size, with the northernmost species occupying an area nearly as large as all other congeneric taxa combined. Phylogenetic network analyses and divergence dating indicate a Pleistocene origin for the group, with recent speciation events (< 0.2 Ma) and limited post-divergence gene flow, suggesting rapid evolution of reproductive isolation and short speciation times. Distributional shifts inferred from environmental niche modelling show that all taxa underwent severe range contractions during interglacial periods, leading to population fragmentation and demographic declines. Despite the ubiquity of these processes across taxa, both demographic stability and genetic connectivity among conspecific populations decreased at lower latitudes relative to the genus' distribution. This suggests that greater genetic connectivity at higher relative latitudes, likely driven by regional range shifts and a more continuous availability of suitable habitats through time, has limited opportunities for microgeographic speciation and maintained genetic cohesiveness among populations across broader distributional ranges. Collectively, our findings support a model of interglacial speciation and illustrate how latitudinal variation in the balance between lineage formation and fusion-shaped by Pleistocene climatic oscillations-determines the rates at which alpine microendemic species emerge and accumulate in temperate mountain regions.