4.2 Geographical isolation and population divergence associated
with Pleistocene climatic oscillations and mountain ranges
All populations are monophyletic and closely aligned with geographical
regions (Figure 1, 3), suggesting that they evolved mostly via local
diversification. This is thought to occur especially when geographic
isolation plays a dominant role (Harrington, Hollingsworth, Higham, &
Reeder, 2018; Hughes, 2017; Hughes & Atchison, 2015; Kadereit, 2017;
Nevado, Contreras-Ortiz, Hughes, & Filatov, 2018; Xing & Ree, 2017).
Analysis of the molecular variance with significantly high population
divergence (Fst = 0.99, P = 0.00) also indicates low
inter-population gene flow (Table 2). Mountain ranges sometimes are
considered as poorly conducive for facilitating long-distance dispersal,
thus contributing to limited gene flow and geographic isolation (Oyama
et al., 2018). In our study, isolation between WYS1 and WYS2 (V1) was
attributed to the Wuyi Mts. acting as geographical barrier to separate
the populations from each other (Figures 1, 3). The rising of Wuyi Mts.
during the early Pleistocene has been reported to cause geographical
isolation and genetic divergence for many species in southern China
(Liu, 1984; Yan et al., 2013). Notably, the central Luoxiao Mts., with a
north-south orientation, are assumed to serve as a geographic barrier
particularly for east-west colonization. This appears to apply to LXS1
and LXS2 in the Luoxiao Mts., which are currently isolated from each of
their eastern or western populations (V2 and V3) (Figures 1, 3). We
infer that the geographical isolation between the populations of the
Nanling Mts. and those to the east (V4) has arisen through the lack of
geographical corridors with a west-to-east orientation. Vicariance
events also exist between the western and eastern (V5) as well as the
middle and northwest (V6) populations within the Nanling Mts. The
Nanling Mts. present a general north-south orientation, which we infer
as disadvantageous for east-west colonization, thus contributing to
vicariance involving V5. In contrast with the populations
NLW2~5, NLW1 is located alone on one ridge of the
Nanling Mts. and geographically distant from the remaining populations,
thus resulting in the vicariance involving V6. Therefore, the
geographical barriers formed by the associated mountain ranges including
the Wuyi, Luoxiao, and Nanling Mts. have directly limited long-distance
colonization and are considered a major factor contributing to the
historical isolation of C. chuniana populations (Jiang, Xu, &
Deng, 2019; Li et al. , 2019; Yang et al. , 2019). Similar
patterns have been found in many other plant species with a wide
distribution range in subtropical China, such as Machilus pauhoi(Zhu et al. , 2017), Loropetalum chinense (Gong et
al. , 2016) and Liriodendron chinense (Shen, Cheng, Li, &
Li, 2019), all of which were analyzed with traditional molecular markers
and data analysis.
Our study suggests that population divergence of C. chunianaoccurred in the Pleistocene and has been affected by the glacial cycles.
These cycles periodically changed suitable habitat and are thought to
have promoted range contraction and expansion coupled with geographic
isolation (Knowles, 2010; Qu et al. , 2011). Based on Bayesian
estimation, the time of divergence (0.68 Ma) between the populations in
the Nanling Mts. and those of the east coincides with the last third
glacial period in China in the Middle Pleistocene (Figure 4). The time
may fall in the Naynayxungla Glacial period (0.5–0.7 Ma; Zheng, Xu, &
Shen, 2002; Zhou & Li, 1998), Dagu Glacial period (0.5–0.6 Ma; Duan,
Pu, & Wu, 1980), Kunlun Glacial period (0.62–0.78 Ma; Zhao, Shi, &
Wang, 2011), or Poyang-Dagu Interglacial (Duan, Pu, & Wu, 1980).
Although the time for the glaciations is uncertain, primarily the third
(last) glaciation drove the genetic divergence between populations in
the Nanling Mts. and those to the east, and shaped the geographical
patterns of genetic variation. The estimated divergence time of the best
fit model in FSC2 is older, i.e. 1.60 Ma (Figure 5), which overlaps with
the earliest known Quaternary glacial of the Xixiabangma Glacial period
ca. 1.6 Ma (Wan et al., 2016), or the Sizishan Periglacial period (Duan,
Pu, & Wu, 1980), when the temperature was 10℃ lower than at present.
The discrepancy between the results of Bayesian time estimation and FSC2
may be partially attributed to the wider time range under the
log-uniform setting in FSC2. The secondary calibration used in BEAST is
thought to generate smaller time estimates (Foster et al., 2017; Kong,
2017; Kong, Zhang, Hong, & Barker, 2017). The climate during glacial
periods tended to be dry and cool, which would favor the populations
shifting to lower elevations with contracted distribution ranges. The
glacial period in the Middle Pleistocene have been shown to drive spruce
fir forests to lowlands in northern China (Liu, 1988). In our study, the
geographical distribution of C. chuniana in southern China is
also associated with the Pleistocene glacial cycles (Figure 4). The Dagu
Glacial period primarily affected the population divergence between the
east and west, whereas the Dagu-Lushan Interglacial period and Lushan
Glacial period primarily affected population diversification. The
dominant role for Pleistocene glacial cycles on the geographic
distribution of populations is also apparent in ENM, where several
isolated glacial refugia were identified during the LGM, although the
climatic conditions may not be analogous to that of other glacial
cycles.