Predator-prey interactions are major processes promoting phenotypic evolution. However, it remains unclear how predation causes morphological and behavioural diversity in prey species and how it might lead to speciation. Here, we show that substantial divergence in the phenotypic traits of prey species has occurred among closely related land snails as a result of adaptation to predator attacks. This caused the divergence of defensive strategies into two alternatives: passive defence and active defence. Phenotypic traits of the subarctic Karaftohelix land snail have undergone radiation in northeast Asia, and distinctive morphotypes generally coexist in the same regions. In these land snails, we documented two alternative defence behaviours against predation by malacophagous beetles. Furthermore, the behaviours are potentially associated with differences in shell morphology. In addition, molecular phylogenetic analyses indicated that these alternative strategies against predation arose independently on the islands and on the continent suggesting that anti-predator adaptation is a major cause of phenotypic diversity in these snails. Finally, we suggest the potential speciation of Karaftohelix snails as a result of the divergence of defensive strategies into passive and active behaviours and the possibility of species radiation due to anti-predatory adaptations.

Hybridization between different taxa is likely to take place when adaptive morphological differences evolve more rapidly than reproductive isolation. When studying the phylogenetic relationship between two land snails of different nominal genera, Ainohelix editha and Ezohelix gainesi, from Hokkaido, Japan, using nuclear internal transcribed spacer and mitochondrial 16S ribosomal DNA, we found a marked incongruence in the topology between nuclear and mitochondrial phylogenies. Furthermore, no clear association was found between shell morphology (which defines the taxonomy) and nuclear or mitochondrial trees and morphology of reproductive system. These patterns are most likely explained by historical introgressive hybridization between A. editha and E. gainesi. Because the shell morphologies of the two species are quite distinct, even when they coexist, the implication is that natural selection is able to maintain (or has recreated) distinct morphologies in the face of gene flow. Future studies may be able to reveal the regions of the genome that maintain the morphological differences between these species. , 115, 77-95 CD. 2012. Adaptive introgression across species boundaries in heliconius butterflies. PLoS Genetics 8: e1002752. Parham JF, Papenfuss TJ, van Dijk PP, Wilson BS, Marte C, Schettino LR, Simison WB. 2013. Genetic introgression and hybridization in Antillean freshwater turtles (Trachemys) revealed by coalescent analyses of mitochondrial and cloned nuclear markers. Molecular Phylogenetetics and Evolution 67: 176-187. Rambaut A, Drummond AJ. 2007. TRACER, Version 1.5. Available at: http://beast.bio.ed.ac.uk/Tracer Rieseberg LH. 2011. Adaptive introgression: the seeds of resistance. Current Biology 21: R581-R583. Rieseberg LH, Archer MA, Wayne RK. 1999. Transgressive segregation, adaptation and speciation. Heredity 83: 363-372. Rieseberg LH, Raymond O, Rosenthal DM, Lai Z, Livingstone K, Nakazato T, Durphy JL, Schwarzbach AE, Donovan LA, Lexer C. 2003. Major ecological transitions in wild sunflowers facilitated by hybridization. Science 301: 1211-1216. Roca AL, Georgiadis N, O'Brien SJ. 2005. Cytonuclear genomic dissociation in African elephant species. Nature Genetics 37: 96-100. Rundle HD, Nagel L, Boughman JW, Schluter D. 2000. Natural selection and parallel speciation in sympatric sticklebacks. Science 287: 306-308. Sang T, Zhong Y. 2000. Testing hybridization hypotheses based on incongruent gene trees. Systematic Biology 49: 422-434. Schileyko AA. 2004. Treatise on recent terrestrial pulmonate molluscs, part 12: Bradybaenidae, Monadeniidae, Xanthonychidae, Epiphragmophoridae, Helminthoglypridae, Elonidae, Humboldtianidae, Sphincterochilidae, Cochlicellidae. Moscow: Ruthenica. Schluter D. 2000. The ecology of adaptive radiation. New York, NY: Oxford University Press. Schluter D. 2001. Ecology and the origin of species. Trends in Ecology and Evolution 16: 372-380. Schluter D, Nagel LM. 1995. Parallel speciation by natural selection. American Naturalist 146: 292-301. Seehausen O. 2004. Hybridization and...

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BackgroundIslands have traditionally been the centre of evolutionary biological research, but the dynamics of immigration and differentiation at continental islands have not been well studied. Therefore, we focused on the Japanese archipelago, the continental islands located at the eastern end of the Eurasian continent. While the Japanese archipelago is characterised by high biodiversity and rich freshwater habitats, the origin and formation mechanisms of its freshwater organisms are not clear. In order to clarify the history of the planorbid gastropod fauna, we conducted phylogenetic analysis, divergence time estimation, ancestral state reconstruction, and lineage diversity estimations.ResultsOur analyses revealed the formation process of the planorbid fauna in the Japanese archipelago. Most lineages in the Japanese archipelago have closely related lineages on the continent, and the divergence within the Japanese lineages presumably occurred after the late Pliocene. In addition, each lineage is characterised by different phylogeographical patterns, suggesting that immigration routes from the continent to the Japanese archipelago differ among lineages. Furthermore, a regional lineage diversity plot showed that the present diversity in the Japanese archipelago potentially reflects the differentiation of lineages within the islands after the development of the Japanese archipelago.ConclusionsAlthough additional taxon sampling and genetic analysis focused on each lineage are needed, our results suggest that immigration from multiple routes just prior to the development of the Japanese archipelago and subsequent diversification within the islands are major causes of the present-day diversity of the Japanese planorbid fauna.Electronic supplementary materialThe online version of this article (10.1186/s12862-018-1273-3) contains supplementary material, which is available to authorized users.