Abstract
BACKGROUND: Insects comprise one of Earth's most diverse animal groups, but the adaptive capacity of most species, especially those with weak genetic structure, remains understudied. Psolodesmus mandarinus is an endemic damselfly in Taiwan, where its populations show latitudinal variation in wing traits despite limited genetic differentiation in mitochondrial and ribosomal sequences. We hypothesised that weak genome-wide structure may obscure the signals of local adaptation driven by environmental variation. To test this, we integrated genome-wide SNPs, phenotypic measurements, environmental associations, and species distribution models. RESULTS: Although genome-wide population structure was generally weak, pairwise F(ST) values exceeded 0.35 between southeastern and northeastern populations, and genetic-environment association analyses identified outlier loci and individuals associated with environmental variables. Wing traits, particularly wing colours, exhibited a latitudinal divergence and exceeded expectations from neutral structure (P(ST) >F(ST)), indicating selection. Species distribution models showed ecological differentiation and predicted range expansion for clear-winged individuals but range contraction for dark-winged individuals under future climate scenarios. CONCLUSION: Our findings demonstrate that phenotypic divergence can arise and persist under weak genetic structure, highlighting the evolutionary potential for local adaptation in structured environments even in species with high dispersal potential. An integrative framework provides essential insights for predicting biodiversity responses to environmental change and guiding climate-resilient conservation strategies.