Island habitats are natural laboratories that offer unique opportunities
to study speciation mechanisms. Recent work indicates that both
anagenetic and cladogenetic speciation processes, driven by allopatric
and sympatric modes, contribute to island species diversity. However,
clear evidence of local adaptation of endemic plant species on islands
requires in-depth studies, which are scarce. This study underscores the
importance of local adaptation in maintaining species entity by
examining how adaptive introgression, hybridization, and local
adaptation contribute to genetic variation in island species.
Specifically, multilocus genome scanning of 51 nuclear genes was used to
investigate the evolutionary relationships of the Scutellaria species
complex on Taiwan Island and assess the role of in situ
diversification in generating high endemism and genetic diversity.
Recurrent interspecies introgressions were detected by phylogenetic
networks and ABBA-BABA-based analysis, suggesting ongoing or recent
speciation processes. Approximate Bayesian computation identified hybrid
speciation in S. taiwanensis and S. hsiehii, with evidence of
hybridization between more than two parental species.
Genotype-environment association studies revealed that the influence of
climate, particularly precipitation- and temperature-related factors,
contributed to adaptive genetic divergence between species.
Additionally, adaptive introgressions related to environmental pressures
that may have facilitated the colonization of new island habitats were
identified. This research illustrates how hybridization, introgression,
and adaptation shaped the evolutionary histories and divergence of this
island-endemic plant species complex and sheds light on the multifaceted
mechanisms of speciation on semi-isolated islands.