INTRODUCTION
Islands are an ideal model to study how phenotypic diversification occurs, including adaptive radiation. Natural selection is a primary influence that favours a certain trait and heightens it according to an adaptive optimum (Schluter, 2000). This influence can be dramatically changed, especially after migration into islands—for example, due to the expansion of habitat resulting from a release from competition (Yoder et al. , 2010; Stroud & Losos, 2016). When this occurs, the adaptive trait in a source population might sometimes be located in a “valley” of the fitness surface in an island population, so natural selection could then push it away from the valley to an adaptive peak (Schluter, 2000; Nosil, 2012). In such a case, it is well known that divergent natural selection, such as disruptive selection, acts on a certain trait as natural selection (Gavrilets & Losos, 2009; Martin & Wainwright, 2013; Gillespie et al. , 2020). Although stabilising selection should act on a trait in a source population if divergent natural selection acts in a population that has migrated, however, it has not often been unclear how selection acts, but determining it could insight the process of adaptive radiation due to release from stabilising selection (Yoder et al. , 2010).
Land snails on islands have also presented cases of trait divergence due to adaptive radiation, such as morphology and shell colour changes (Chiba, 1999a; Chiba & Davison, 2007; Parent & Crespi, 2006, 2009). Studies have demonstrated that character displacement plays a role in preventing interspecific competition (Chiba, 1996, 1999b; Chiba & Davison, 2007; Kimura & Chiba, 2010). Furthermore, land snails have many predators, such as mammals and birds, and it has been shown that some of them could regulate snails’ traits (Barker, 2004; Chiba, 2007). Environmental conditions have also been shown to be a cause of trait divergence (Kraemer et al. , 2019). Overall, these factors cause disruptive selection and promote diversification by their combined effects, and sometimes, the proportions of effects fluctuate according to the degrees of impacts such as the age of the island (Hayashi & Chiba, 2004; Kraemer et al. , 2019; Ito & Konuma, 2020). In many cases, phenotypic diversification has been a focus in adaptive radiation, but there are almost no studies on how the diversified traits are determined in the source population.
The shell colour diversification of snails has also been studied in the Japanese land snail Euhadra peliomphala simodae (Hayashi & Chiba, 2000, 2004; Ito & Konuma, 2020). This snail inhabits the mainland of Japan on the Izu Peninsula, as well as several of the Izu Islands. The mainland populations have monomorphic bright shells, while colour diversification from bright to dark is observed in the peripheral islands’ sympatric populations (Hayashi & Chiba, 2004). Ancestral populations distributed on the mainland have migrated to the islands, where shell-colour diversification has rapidly occurred due to disruptive selection (Hayashi & Chiba, 2004; Ito & Konuma, 2020). However, it is unknown what the cause of the natural selection is in this case. Furthermore, it is also unknown what selection pressure has acted on the source population, what the differences are between the mainland and islands, and what the details of the adaptive radiation are among these land snails.
This study examined what species prey on the snails using trail-camera traps and the differences of predation effects between the mainland and islands. We then conducted a mark-recapture experiment with E. p. simodae on the mainland and estimated the natural selection pressures from predation and other factors. Finally, we compared the estimated selection pressures between the mainland and a peripheral island, Nijima Island (Ito & Konuma, 2020).