Species divergence and phylogeography of Corylus heterophylla Fisch complex (Betulaceae): Inferred from molecular, climatic and morphological data.

Historical geo-climatic changes have shaped the geographical distributions and genetic diversity of numerous plant taxa in East Asia, which promote species divergence and ultimately speciation. Here, we integrated multiple approaches, including molecular phylogeography, ecological niche modeling, and morphological traits to examine the nucleotide diversity and interspecific divergence within Corylus heterophylla complex (C. heterophylla, C. kweichowensis, and C. yunnanensis). These three sibling taxa harbored similar high levels of nucleotide diversity at the species level. The molecular data (SCNG and cpDNA) unanimously supported the division of C. heterophylla complex into two major clades, with C. yunnanensis diverged earlier from the complex, whereas C. heterophylla and C. kweichowensis could hardly be separated. The split between the two clades (c. 12.89 Ma) coincided with the formation of Sichuan Basin in the middle Miocene, while the divergence among and within the five subclades (YUN1-YUN3, HK1-HK2) occurred from the late Miocene to the Pleistocene. C. heterophylla of northern China experienced glacial contraction and interglacial expansion during the Quaternary, whereas C. kweichowensis and C. yunnanensis of southern China presented population expansion even during the last glacial maximum. Despite of high levels of genetic admixture between C. heterophylla and C. kweichowensis, significant ecological and morphological discrepancy as well as incomplete geographic isolation indicated that adaptive evolution triggered by divergent selection may have played important roles in incipient ecological speciation.