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Aim: Adaptation to local environments has been considered a driving force of ecological speciation. Previous phylogenetic studies at higher taxonomic levels have strongly suggested that herbivore diversification occurs through local adaptation to host plants. However, whether similar mechanisms contribute to within‐species diversification remains poorly understood. We reconstructed species expansion processes using the phylogenetic relationships between two Luehdorfia butterflies and their host plants and tested the local adaptation hypothesis during species diversification. Location: Japanese archipelago. Taxa: Luehdorfia japonica and L. puziloi. Methods: We analysed mitochondrial DNA and single‐nucleotide polymorphisms to trace the expansion processes of two species in the Japanese archipelago. We analysed data collected during trans‐host plant feeding experiments to test for local host plant adaptation in L. japonica. Results: Luehdorfia japonica differentiated 17.0 million years ago (MYA) from a group of four Luehdorfia species in East Asia, and L. puziloi differentiated 13.3 MYA from the remaining three species. We observed several genetic groups in both species, reflecting geographical differences among populations. Some host plants had detrimental effects on the pupation rates of L. japonica, demonstrating the presence of genetic barriers between populations of this species. Main Conclusions: Although these two sibling species were influenced by the geological formation of the Japanese archipelago, their present population structures differ greatly: L. japonica expanded its distribution and adapted to local host plants along at least four expansion routes independent of host plant phylogeny, and L. puziloi exhibited a more isolated population structure and was less strongly influenced by gene flow among populations. These results suggest that both isolation by distance and isolation by adaptation have been important drivers resulting in the present‐day population structure of L. japonica. Unidirectional gene flow through local adaptation to the host plant may have played a role in initial speciation. [ABSTRACT FROM AUTHOR]