AbstractHazelnut (Corylusspp.) is known as one of the four famous tree nuts in the world due to its pleasant taste and nutritional benefits. However, hazelnut promotion worldwide is increasingly challenged by global climate change, limiting its production to a few regions. Focusing on the eurytopic SectionPhyllochlamys, we conducted whole-genome resequencing of 125 diverse accessions from five geo-ecological zones in Eurasia to elucidate the genomic basis of adaptation and improvement. Population structure inference outlined five distinct genetic lineages corresponding to climate conditions and breeding background, and highlighted the differentiation between European and Asian lineages. Demographic dynamics and ecological niche modeling revealed that Pleistocene climatic oscillations dominantly shaped the extant genetic patterns, and multiple environmental factors have contributed to the lineage divergence. Whole-genome scans identified 279, 111, and 164 selective sweeps that underlie local adaptation inCorylus heterophylla,Corylus kweichowensis, andCorylus yunnanensis, respectively. Relevant positively selected genes were mainly involved in regulating signaling pathways, growth and development, and stress resistance. The improvement signatures of hybrid hazelnut were concentrated in 312 and 316 selected genes, when compared toC. heterophyllaandCorylus avellana, respectively, including those that regulate protein polymerization, photosynthesis, and response to water deprivation. Among these loci, 22 candidate genes were highly associated with the regulation of biological quality. Our study provides insights into evolutionary processes and the molecular basis of how sibling species adapt to contrasting environments, and offers valuable resources for future climate-resilient breeding.