The Role of In Situ Diversification in Island Endemism: Adaptive Introgression in Scutellaria in Taiwan

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.