It remains unclear how predation causes phenotypic diversity in prey species. Here, we show that substantial divergence in the phenotypic traits of prey species has occurred among subarctic Karaftohelix land snails as a result of adaptation to predator attacks. Two alternative defence behaviours against predation by malacophagous beetles were documented in these snails. Each behaviour was clearly associated with differences in shell morphology among each species. Furthermore, molecular phylogenetic analyses indicated that these alternative strategies against predation arose independently on the islands and continent, suggesting that anti-predator adaptation is a major cause of phenotypic divergence of these snails.