Comparing neutral and adaptive genetic variation in shaping phenotypic diversity of the polymorphic bush cricket Isophya rizeensis along an altitudinal gradient

The accelerating impacts of global environmental and climatic change present significant threats to biodiversity, emphasizing the need to understand how natural populations adapt to environmental gradients. Isophya rizeensis, a univoltine bush cricket endemic to Turkey's Fırtına Valley, offers a compelling model for studying local adaptation, owing to its striking altitudinal color polymorphism in males. Lower-altitude populations predominantly exhibit black morphs, while higher altitudes are characterized by pale-green individuals, a pattern suggesting adaptation to microclimatic conditions rather than classical thermal melanism. Using RAD-seq data from 71 individuals across 10 populations, we identified 92,048 high-confidence polymorphic loci, including 1,113 putatively adaptive loci. Genetic analyses revealed significant differentiation driven by altitude, with Mantel tests confirming isolation by distance and selection. Mid-altitude populations exhibited higher genetic diversity, while extreme-altitude populations showed reduced diversity and fixation of adaptive loci, highlighting the dual influence of gene flow and selection. Genome-wide association studies identified 42 loci significantly associated with altitude, supporting the role of disruptive selection in shaping genetic variation and maintaining polymorphism along the gradient. Discriminant analysis of principal components (DAPC) revealed strong genetic differentiation between color morphs, with three major loci contributing to this separation. These loci exhibited bimodal genotype distributions, indicating the existence of distinct genetic groups linked to color morphs and their altitudinal distribution. This study underscores how environmental pressures shape adaptive and neutral genetic variation, maintaining diversity despite high connectivity. Our findings highlight the interplay between gene flow, selection, and local adaptation, providing valuable insights into the mechanisms driving biodiversity in montane ecosystems and informing conservation strategies in the face of climate change.