This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 hind wings 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, Japan's largest island.
The findings are published in the Biological Journal of the Linnean Society. Snail-feeding carabid beetles, including Carabus blaptoides, have undergone adaptive radiation on continents and continental islands across the Northern Hemisphere. Konuma, the lead author of the study, explained, "Adaptive radiation has often been discussed in the context of populations isolated on oceanic islands.
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