Late Miocene Speciation
Limited gene flow promotes evolutionary diversification by facilitating
genetic and phenotypic divergence that may culminate in speciation
(Schluter, 2001; Slatkin, 1987). The migrations between C. bifidaand C. micholitzii in both directions were very low, indicating
restricted gene flow among them (Figure 5b). Since C. bifidaoccupies a lower altitude habitat in southwest China and the northern
Vietnam compared to C. micholitzii in the Annam Highlands region
of central Vietnam, eastern Laos and the northern Cambodia (Figure 1),
it is impossible for the two species to have a second contact. Due to
the separation of habitats by the RRFZ, the two taxa have obvious niche
differentiation, which led to the gradual formation of two independent
species. This can be proved by the two obvious clusters corresponding to
the two species in genetic structure (Figure 3a) and haplotype networks
(Figure 2). Based on the analyses of the three combined cpDNA and five
low copy nuclear gene datasets, divergence between C. bifida andC. micholizii was estimated to have occurred approximately at c.
5.605 Mya (90% HPD interval: 2.580 - 25.407 Mya) (Figure 5c). These
estimates imply that they diverged from their common ancestor in the
late Miocene at a long time after India-Asia collision which formed the
RRFZ. As a natural barrier, the RRFZ hindered the gene flow between the
two taxa, making them more and more differentiated and finally
speciation. In the haplotype networks (Figure 2), not all nuclear gene
haplotypes can be separated according to the two species (e.g.AAT , SAMS ), and even some nuclear gene haplotypes can be
shared by the two species (e.g. PHYP , AAT ), implying that
the two species once came from the same ancestor or they were the same
species, but the long-term geographical isolation resulted in high
genetic differentiation between them.