Geographic and environmental isolations of islands and the mainland offer excellent opportunity to investigate colonization and survival dynamics of island populations. We inferred and compared evolutionary processes and the demographic history of Rhododendron tsusiophyllum, in the Izu Islands and the much larger island Honshu, treated here as the mainland, using thousands of nuclear SNPs obtained by ddRAD-seq from eight populations of R. tsusiophyllum and three populations of R. tschonoskii as an outgroup. Phylogenetic relationships and their habitats suggest that R. tsusiophyllum had evolved and migrated from cold north to warm south regions. We detected clear genetic divergence among populations in three regions of Honshu and the Izu Islands, suggesting restricted migration between them due to isolated habitats on mountains even in the mainland. The three regions have different changes in effective population size, especially, genetic diversity and population size of the Izu Islands are small compared to the others. Further, habitats of populations in the Izu Islands are warmer than those in Honshu, suggesting that they have undergone adaptive evolution. Our study provides evidences of montane rather than insular isolation on genetic divergence, survival of populations and significance of adaptive evolution for island populations with small population size and low genetic diversity, despite close proximity to mainland populations.

岛屿和大陆的地理和环境隔离为调查岛屿种群的殖民化和生存动态提供了极好的机会。我们使用 ddRAD-seq 从 8 个R. tsusiophyllum 种群和三个R. tschonoskii的种群作为外群。系统发育关系及其栖息地表明R. tsusiophyllum进化并从寒冷的北方迁移到温暖的南方地区。我们在本州和伊豆群岛的三个地区发现了种群之间明显的遗传差异，这表明即使在大陆，由于山区的孤立栖息地，它们之间的迁移也受到限制。三个地区在有效种群规模上有不同的变化，尤其是伊豆群岛的遗传多样性和种群规模与其他地区相比较小。此外，伊豆群岛的种群栖息地比本州的更温暖，表明它们经历了适应性进化。我们的研究提供了山地而不是岛屿隔离对遗传差异、种群生存以及适应性进化对种群规模小和遗传多样性低的岛屿种群的重要性的证据，