Hypothetical phylogeographic scenario of the Brown Shrike
The southern clade represented the basal lineage of the Brown Shrike (Figure S1.2), suggesting that the ancestral breeding area of this species may have been located in the eastern part of the Asian continent, including CRECS. Given the LDD hypothesis, the archipelagic population may have diverged from this population around the late Early Pleistocene. The northern clade was found to be most distant from the southern clade and genetically close to the archipelagic clade (Figure 2), possibly suggesting divergence of the northern clade from the archipelagic population. Therefore, our interpretation of the results is that divergence between the archipelago and the continent occurred at least twice. Such an interpretation can be explained by the following hypothetical scenario that involves different divergence processes across different geographic barriers.
The ECS has remained open during a glacial period throughout the species’ history. Meanwhile, the ECS shrank and became less effective as a biogeographic barrier during glacial periods because the lower sea level led to the emergence of the continental shelf in the late Early to Middle Pleistocene (Matsuzaki et al., 2019; Ota, 1998). Paleopalynological records are in line with our SDM analysis; the emergent continental shelf of the ECS was covered by grassland with sparse forest (Xu, Lu, Wu & Liu, 2010), which provides suitable breeding habitat for the Brown Shrike (Lefranc & Worfolk, 1997). Present vagrancy records of many migratory species in the Japanese archipelago during spring are probably due to overshooting (birds traveling too far due to favourable winds and anticyclonic conditions; Newton, 2008) from the CRECS (Senzaki et al., 2019). Increased proximity of the two landmasses, therefore, may have resulted in sufficient propagule numbers (accidental migrants), which arrived in the Japanese archipelago from the CRECS (Figure 5a; Gillespie et al., 2012; MacArthur & Wilson, 1967). During a subsequent interglacial period, the breadth of the ECS expanded to almost 1,000 km (Figure 5b). This could have facilitated isolation of the archipelagic shrike population from that in the CRECS since vagrancy to islands becomes increasingly rare as the distance from the continental source area to the island increases (Lees & Gilroy, 2014). Thus, changes in the size of a geographic barrier may have been responsible for LDD-driven divergence between these regions.
In contrast, the following description supports a hypothesis for the northern clade diverging from the archipelagic clade by vicariance. During the LGM, the present Korean peninsula and current northern China, with the Japanese archipelago connected to the continent at the time, were predicted to be highly suitable for L. c. cristatus (Figure S1.3). The Japanese archipelago was predicted to be more suitable than either the east or southern continental coast (current China). This implies that part of the archipelagic population colonized the Korean Peninsula and current northern China. Moreover, a major COI haplotype of the northern clade (H1) was shared among samples from a wide range of localities in Russia, and a star-like pattern of this clade was found (Figure 2a). The structure of this haplotype network suggests a small population during the glacial period that expanded its range, probably northwards, during the subsequent interglacial period (Avise et al., 1987). Therefore, we suggest that a population may have colonised the Korean Peninsula from the Japanese archipelago over a land bridge during a glacial period, and subsequently expanded its range during an interglacial period (Figure 5c), as the reverse colonisation hypothesis predicts (Hikida 2003; Nishiumi & Kim, 2015). Disappearance of the land bridge and northward expansion of the archipelagic and northern clades could have resulted in the present vicariant distribution (Figure 5d; Aoki et al., 2018; Saitoh et al., 2010). The genetically intermediate individuals between the most distant clade pair, shown in our multi-locus network, possibly reflect their secondary contact with presumed introgression on the continent (Lefranc & Worfolk, 1997). Although their past population dynamics and gene flow needs to be directly estimated to refine our hypothetical scenario, our integrative approach provides, at least, a new scenario for this region (Motokawa, 2017) and a better phylogeographic interpretation of the Brown Shrike.