Islands are widely recognized as natural laboratories of evolution. The ocean restricts movement among islands, allowing isolated populations to adapt to different environments and, at times, diverge into new species. However, on large islands, geographic distance may restrict movement, while environmental variation may impose divergent selection, thereby promoting adaptive divergence. Does adaptive divergence occur not only among islands but also within islands?
The beetle Carabus blaptoides, found throughout the Japanese archipelago, is an ideal system for investigating this question. This beetle feeds primarily on terrestrial snails and has diversified into eight subspecies. Because its hindwings are reduced, it cannot fly and disperses by walking. This limitation on mobility makes it particularly well-suited for investigating how geographic factors restrict gene flow and promote population differentiation.
Variation in the beetle's body shape is caused by a functional trade-off between two feeding strategies. Slender beetles have long, narrow heads and thoraxes, allowing them to insert their heads deep into the shells of large snails. In contrast, stout beetles have short, wide heads and powerful mandibles, making them well-suited for crushing the shells of relatively small snails. Previous studies suggest that regional variations in the size of snails available in different areas have contributed to the diversification of this beetle's body shape.
A research team led by Associate Professor Junji Konuma of Toho University, in collaboration with Nobuaki Nagata of the National Museum of Nature and Science and Professor Teiji Sota of Kyoto University, reconstructed the evolutionary and geographic history of this beetle species by combining genetic data from 170 individuals sampled at 70 sites with morphological data from 50 populations. These analyses suggested that body shape underwent adaptive divergence not only among islands—including Hokkaido and Sado Island—but also between regions within Honshu, the largest island of Japan.
Snail-feeding carabid beetles, including Carabus blaptoides, have undergone adaptive radiation on continents and continental islands across the Northern Hemisphere. Junji Konuma, the lead author of this study, explained: "Adaptive radiation has often been discussed in the context of populations isolated on oceanic islands. Our results suggest that on large continental islands, adaptive divergence may be promoted by geographic distance and environmental variation within the same landmass. This process may be particularly important for flightless animals because their limited dispersal reduces gene flow among populations."
These findings were published in the Biological Journal of the Linnean Society on August 17, 2026.
Journal：Biological Journal of the Linnean Society
Article Title：Morphological differentiation driven by adaptive divergence among and within islands in the snail-feeding carabid beetle Carabus blaptoides
Authors：Junji Konuma, Nobuaki Nagata, Teiji Sota
DOI：10.1093/biolinnean/blag065
