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).