The shell color diversification after changing from stabilizing selection to disruptive selection: A mark-recapture study of a land snail Euhadra peliomphala simodae

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Shun Ito, Takahiro Hirano and Satoshi Chiba, Graduate School of Life Science, Tohoku University, Miyagi, Japan, Takahiro Hirano and Satoshi Chiba, Center for Northeast Asian Studies, Tohoku University, Miyagi, Japan, Junji Konuma, Department of Biology, Faculty of Science, Toho University, Chiba, Japan

Background/Question/Methods
Recent studies have shown the mechanisms that cause phenotypic diversification and adaptive speciation and lead to adaptive radiation. Empirical studies demonstrating how natural selection acts is needed to understand these mechanisms more clearly. While theoretical studies have suggested that divergent natural selection, such as disruptive selection, plays an essential role, few empirical studies demonstrate that divergent selection causes phenotypic diversification. Although phylogenetic studies have also suggested that stabilizing selection would act on the ancestral populations before diversification, few ecological studies show that stabilizing selection occurred before adaptive radiation. In land snails, adaptive radiation has occurred on the several oceanic islands, such as Galapagos and Ogasawara. This study focused on the Japanese land snail Euhadra peliomphala simodae distributed in the Japanese mainland and peripheral islands. Phylogenetic studies suggested that shell color diversification would have occurred on the islands after ancestral mainland populations migrated to the islands. To verify this hypothesis, we conducted mark-recapture experiments on the mainland and the island. We estimated the snail survival rates and examined how natural selection acts on both populations. We also quantified the predation pressure by examining evidence of predation on the recaptured shells, conducting trail-camera trap experiments to identify the snails' predators.

Results/Conclusions
We captured and marked approximately 4,000 snail individuals on the mainland and island, and monitored their survivals for 12 and 18 months, respectively. The mainland snails had low diversity, and most snails had a bright shell. The island snails had a high diversity of shell color, classified into dark, bright, and intermediate. The monitoring surveys showed that different regimes of natural selection acted on the mainland and island snails. Stabilizing selection favored bright snails on the mainland, while disruptive selection favored dark and bright snails than intermediate-colored snails on the island. Some of the recovered dead snails were attacked by unknown predators, and the trail-cameras recorded the predators were a large Japanese field mouse Apodemus speciosus. However, predation pressure may not have caused the difference of natural selection between the mainland and island populations, because the predation frequency was not different among the shell colors. Although the shell color's adaptive significance was unclear, our study suggested that the shell color diversification would have occurred after changing from stabilizing selection to disruptive selection. Furthermore, it might explain that the process of adaptive radiation in land snails that occurred on other oceanic islands.