Species delimitation in tree species is notoriously challenging due to shared polymorphisms among species. An integrative survey that considers multiple operational criteria is a possible solution, and we aimed to test it in a species complex of aspens in China. Genetic [four chloroplast DNA (cpDNA) fragments and 14 nuclear microsatellite loci (nSSR)] and morphological variations were collected for 76 populations and 53 populations, respectively, covering the major geographic distribution of the Populus davidiana-rotundifolia complex. Bayesian clustering, analysis of molecular variance (AMOVA), Principle Coordinate Analysis (PCoA), ecological niche modeling (ENM), and gene flow (migrants per generation), were employed to detect and test genetic clustering, morphological and habitat differentiation, and gene flow between/among putative species. The nSSR data and ENM suggested that there are two separately evolving meta-population lineages that correspond to P. davidiana (pd) and P. rotundifolia (pr). Furthermore, several lines of evidence supported a subdivision of P. davidiana into Northeastern (NEC) and Central-North (CNC) groups, yet they are still functioning as one species. CpDNA data revealed that five haplotype clades formed a pattern of [pdNEC, ((pdCNC, pr), (pdCNC, pr))], but most haplotypes are species-specific. Meanwhile, PCA based on morphology suggested a closer relationship between the CNC group (P. davidiana) and P. rontundifolia. Discrepancy of nSSR and ENM vs. cpDNA and morphology could have reflected a complex lineage divergence and convergence history. P. davidiana and P. rotundifolia can be regarded as a recently diverged species pair that experienced parapatric speciation due to ecological differentiation in the face of gene flow. Our findings highlight the importance of integrative surveys at population level, as we have undertaken, is an important approach to detect the boundary of a group of species that have experienced complex evolutionary history.

Chloroplast data and SDMs suggest that P. adenopoda might have survived in multiple glacial refugia in central and southern parts of its range during the LGM. Populations of the Yunnan-Guizhou Plateau in the southern part have high chloroplast DNA diversity, but may have contributed little to the postglacial recolonization of northern and eastern parts. The three major demographic events inferred by DIYABC coincide with the initiation of the LIG, start of the LGM and end of the LGM, respectively. The species may have experienced multiple rounds of range contraction during glacial periods and range expansion during interglacial periods. Our study corroborates the importance of combining multiple lines of evidence when reconstructing Quaternary population evolutionary histories.

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AimThere is increasing interest in the role that biological traits, and historical and biogeographic processes, play in the formation of phylogeographic patterns. An arid belt that once existed in northern China might have affected many plants, but this has yet to be tested in an arid‐tolerant, wind‐dispersed species. Here, we tested how intrinsic and extrinsic factors have affected the phylogeography of Populus davidiana.LocationEast Asia.MethodsGenetic variation was surveyed across 40 populations (555 individuals) covering the Chinese range of P. davidiana, using 16 nuclear microsatellite loci (nSSRs) and four chloroplast fragments (cpDNA). Demographic and migration hypotheses were tested using coalescent‐based approaches, and the present and past potential distributions were predicted using species distribution modelling.ResultsMolecular data divide P. davidiana into two lineages, north‐eastern China (NECR) and central and northern China (CNCR); however, the dividing line is around 118ºE for nSSRs, but 122ºE for cpDNA. The range and habitat of the two lineages barely overlap at present, and their ecological separation may have initiated around the Pliocene‐Quaternary boundary, when major intraspecific cpDNA clades diverged. NECR and CNCR experienced post‐glacial north‐eastward and northward range shifts respectively. Bi‐directional historical gene flow was detected between NECR and CNCR for both bi‐parentally inherited nSSRs and maternally inherited cpDNA. Demographic inferences suggest a severe bottleneck for CNCR and especially NECR, around the latest Pleistocene.Main conclusionsThe phylogeographic break within P. davidiana reflects the impacts of biogeographic history, climate and biological traits. Its plumed, wind‐dispersed seeds might be especially significant because prevailing south‐western spring winds may have moved the NECR‐CNCR boundary further east than similar phylogenetic breaks in other species, and also moved the cpDNA boundary relative to that for nuclear markers. Biological traits, therefore, should also be considered when examining the genetic and ecological differentiation between closely related taxa.

How endemic species originated in eastern Asia has interested botanists for a long time. In this study, we combined experimental and computational modeling approaches to examine the morphological and genetic divergence and reproductive isolation of two tentative species of Sinalliaria (Brassicaceae) endemic to eastern China, S. limprichtiana and S. grandifolia. Most of the examined morphological characters (including hairs of leaf blades and stems, corolla length and width, and flower stalk length) were well-delineated between two species at the same ploidy level, and there was clear evidence of reproductive isolation between them (mainly due to post-pollination barriers) in the common garden environment. There were also strong and consistent divergences in the population genetic data. Coalescent simulations based on sequence variation of the nuclear genes suggest that interspecific divergence began during the Pleistocene when the climate oscillated in eastern Asia. Gene flow between two species appears to have been very limited and asymmetrical. Our results suggested that both species are well-differentiated and that the fast divergence between them might have been together shaped by both stochastic processes and habitat selection pressures.