4.1 Genetic divergence between eastern and western populations
The population divergence in the eastern portion of the geographic range
of C. chuniana is estimated to be older (0.51‒0.34 Ma) and with
smaller population sizes than within the Nanling Mts., where more recent
and rapid population diversification occurred (0.37‒020 Ma) with larger
population sizes (Figure 4). The phylogenetic analysis also revealed
that populations of the Nanling Mts. formed a monophyly group and were
separated distinctly from the eastern populations. This pattern agrees
with the general pattern of genetic divergence observed between eastern
and western China in other plant species with wide distributions
(Chen,
Deng, Zhou, & Sun, 2018; Gong et al. , 2008; Ha, Kim, Choi, &
Kim, 2018; Hohmann et al., 2018; Lu et al. , 2018; Qiu, Guan, Fu,
& Comes, 2009). One main factor contributing to the differences of
population divergence time and level of diversification between the east
and west is likely to be the different orientations of mountain ranges
(Chen et al. , 2018). The southwest-northeast orientation of the
Wuyi Mts. and East China Mts. are thought to present geographical
barriers blocking southward migrations in times of cooler climate or
northward postglacial population expansion, which is disadvantageous for
increasing population size and diversification, and may have contributed
to an older divergence as is seen in the eastern populations (WYS1,
WYS2, LXS1 and LXS2). Conversely, the north-south orientation of the
Nanling Mts., allowing various elevational shifts of plant species, can
facilitate gradual retreat from north to south and short-distance
migrations during glacial and interglacial periods, thus promoting more
population diversification, larger population size and younger
divergence as is seen in the populations of the Nanling Mts. (NLE1 and
NLE2, and NLW1 through NLW5). Therefore, the orientation and
physiography of the mountain ranges appear to have contributes to the
geographic pattern of genetic variation between the eastern and western
populations of C. chuniana .