Evolutionary history of inshore oceanic island land snails diversified in shell colour

Aim: Oceanic islands provide an excellent opportunity to study
the mode and tempo of phenotypic evolution of terrestrial organisms. Many
studies have focused on oceanic islands far from the mainland. Oceanic
islands near the mainland may provide distinct perspectives on phenotypic
evolution, but a comprehensive understanding is still lacking. To address
this gap, this study aimed to reveal when a land snail species inhabiting
a volcanic archipelago within 40 km of the mainland diverged and how
their shell colours evolved. Location: Southern Izu
Peninsula and five Izu Islands (Oshima, Toshima, Niijima, Shikine, and
Kozu), Japan. Taxon: Euhadra peliomphala simodae
Methods: Double-digest restricted-site associated DNA sequencing
(ddRAD-seq) was conducted using 117 individual snails. Molecular
phylogenetic and population genomic analyses were performed, including
Approximation Bayesian computation (ABC). We then examined whether the
island area, elevation, distance from the surrounding landmasses
and historical event (bottleneck) explain the shell colour diversity on
each island using a phylogenetic generalised
linear mixed model (PGLMM). Results: Snails
could be genetically categorised according to the island they inhabited,
and on Niijima, their genetic structure was further divided within the
island. The divergence times among the extant populations of the snail
dated back to 2.1 million years ago (Ma), which is older than that of
other animals occurring in this region. Additionally, island elevation
positively affected shell colour diversity, and populations with similar
shell colour profiles were phylogenetically different. Main
conclusions: Island land snails diversified early in the
ecosystem of the Izu Islands. This suggests that proximity to the
mainland, immobility and the physiological
tolerance of passive dispersal in land snails were the main
causes of early diversification. Moreover, our study proposes that
environments covarying with elevation would determine shell colour
diversity on each island through natural selection.