Phylogeography: the Nanling Range is not a simple barrier to gene flow
Although a modest mountain chain with respect to elevation (Körner et al. 2017), the Nanling Range is an effective barrier to dispersal of relatively sedentary taxa such as amphibians and many plant species (Chen et al. 2017, Wang et al. 2017). But, for vagile taxa like birds, the efficacy of the range as a barrier has until now yet been emphasized. Our phylogeographic examination indicates that the mountains have little effect on gene flow of populations of the 5 sylvioid bird species whose phylogeography we compared: Alcippe hueti ,Leiothrix lutea , Pterorhinus pectoralis , Staphida torqueola , and Ixos mcclellandii . Indeed, the range may act more as a conduit than a barrier for these birds. This effect appears to have been the case even during the dramatic climatic (and consequent environmental) shifts from the LGM forward.
Although the 5 species move readily past the mountains, their population structures vary, suggesting differences in their dispersal pathways and histories of demographic changes. For example, only one species,A. hueti , display a significant correlation between genetic and geographic distances and, hence, IBD. For this species, genetic distance correlates best with penalized cost-distance rather than spheric or non-penalized distances (Supporting information). This bird more likely took routes flying under rather than over preferred elevation. Elevational niche gradient is known to have driven variable seasonal migration in birds (Williamson and Witt 2021, Céspedes Arias et al. 2022), and it could also shape birds’ behavior of activity range and direction of colonization. The closely related species to A. hueti , A. fratercula (Zou et al. 2007, Song et al. 2009) is distributed in the mountains in Yunnan, southwesten China. The specific preference for certain elevation range could be the echo of the its origination from the higher mountains. Another species, P. pectoralis showed correlation of genetic distance to geographic distance in part of the scenarios (Supporting information). Moreover, it was also the only species with clustered populations in the genetic clustering analysis (Supporting information), indicating its divergence was gradual over time as well as distance. Combined to the negative Tajima’ D (Table 2) which suggests a selective sweep or population expansion after a bottleneck or founder event, and threshold timing, this species likely went through population expansion recently, and most likely after the LGM period.
The 3 other species differ from A. hueti and P. pectoralisand from one another in their genetic patterns. Leiothrix luteadisplays little population clustering and no IBD. This unusual pattern likely results from a history of anthropogenic transport. At one time, this species was a popular cage bird in China, and only recently has it been legally protected. As such, it was transported in unpredictable ways by pet traders and owners across the country. Staphida torqueola displays hardly any variation across its range. This species has a Tajima’s D close to zero, indicating a stable demographic history and no significant selection (Tajima 1989, Rozas et al. 2017), a pattern that may have resulted from redistribution of ancestral groups by population contraction from a continuous genetic pool, masking its geneflow patterns.
A general pattern in all 5 species is that no obvious shift in genotype is evident north-to-south across the mountains (Figure 2). This observation agrees with the BARRIER analysis (Figure 5), which identified relatively few barriers in the mountains and even fewer configured west-to-east, thus inhibiting north-south dispersal. Another general pattern is the apparently small effect imposed by differences in bioclimatic (or related) variables and elevation on bird divergence. In the three of the species with little or no IBD (I. mcclellandii ,L. lutea , S. torqueola ), dispersal may have been more strongly influenced by habitat (IBE) than distance, given that in the absence of IBD some other factors must be at play to differentiate populations (Wright 1943, Nosil et al. 2005). However, we found no correlation between genetic distances and environmental variable differences in these species (Supporting information). Although no significant habitat barrier was detected, the rugged terrain of the mountains (an environmental factor that is difficult to quantify) likely influenced the route taken by birds to some degree.