1 | INTRODUCTION
Climatic oscillations, especially during the Quaternary, have played a critical role in shaping the genetic diversity of animals and plants by altering species distributions, periods of population isolation, and influencing opportunities for secondary gene flow as previously isolated species come into spatial contact (Hewitt, 2000; Hoffmann & Sgro, 2011). Researchers commonly refer to climatic oscillations such as glacial interglacial cycles as drivers of biodiversity and evolutionary change (Oswald et al., 2016). During climate oscillations, speciation events may increase as glacial barriers cause large and previously continuous populations to split into several smaller isolated populations (Wallis et al., 2016). Speciation may also occur during interglacial periods in response to the expansion of suitable habitat, leading to rapid population expansion followed by subpopulation isolation due to factors such as changes in the size and course of river systems, habitat fragmentation, or geological events (Chen et al., 2015), as well as in response to changes in the direction, intensity, and opportunities for gene flow within and between species (Savolainen et al., 2013).
Several recent studies using genomic data and Pairwise Sequentially Markovian Coalescent (PSMC) and Multiple Sequentially Markovian Coalescent modeling (MSMC) (Schiffels & Durbin, 2014) have found that climate-driven changes in species’ distributions are often associated with major increases or decreases in effective population size (Liang et al., 2018). For example, throughout the Quaternary, changes in the effective population size of the giant panda (Ailuropoda melanoleuca ) closely tracked climate change. Genetic data indicated that the population size of giant pandas decreased significantly during both the longest Quaternary glacial event (800,000-200,000 years ago) and the last glacial event (20,000-10,000 years ago), but increased rapidly during interglacial periods (Zheng et al., 2013). The effective population size of golden snub-nosed monkey (Rhinopithecus roxellana ), with a sympatric distribution with giant panda, showed a similar demographic pattern (Zhou et al., 2014).
Over time, persistent barriers to gene flow in response to subpopulation isolation, especially during periods of population contraction are expected to result in the population extinction or adaptive evolution of sister species (Abbott et al., 2013; Butlin et al., 2008; Crow et al., 2010; Feder et al., 2012). Subsequent environmental changes or geologic events (e.g. volcanism) can result in secondary contact between sister species, leading to periods of interspecific gene flow and introgression (Hewitt, 2000; Hampe & Jump, 2011). Gene flow between closely related species can act to retard speciation or result in species’ extinction by merging gene pools or in response to competition between hybrids and one or both parental species (Grant & Grant, 2014; Moyle & Nakazato, 2010). Although methods based on genomic data have been developed to detect gene flow during the differentiation of related species (Han et al., 2017; Liu et al., 2014), lacking considering the species’ adaptation to their environment through genetic changes might lead to be difficult to understand the mechanism of species differentiation, particularly for closely related species, only based on changes of distribution, effective population size and gene flow.
François’ langur (Trachypithecus francoisi ) (Endangered) and White-headed langur (T. leucocephalus ) (Critically Endangered) (IUCN, 2020, https://www.iucn.org) represent two threatened species of nonhuman primates that are endemic to the limestone forests and karst hills of Southeast Asia. François’ langur distributes only in tropical and subtropical regions of southwestern China (Guizhou, Chongqing, and Guangxi Province) and northern Vietnam with a remaining wild population of approximately 2,000 individuals, and White-headed langur is confined to a narrow triangular area of karst hills totaling some 200 km2 in southern Guangxi province (107–108°E, 22°06′–22°42′N), China (Zhou & Huang, 2021). The population size of White-headed langur is estimated at less than 1,000 individuals, distributing in several small isolated subpopulations with being surrounded by populations of François’ langur (Li et al., 2018).
Since White-headed langur were first described in 1955, there have existed conflicting hypotheses concerning whether it represents an independent species or a subspecies of François’ langur (Huang et al., 2002). A recent study suggests that François’ langur and White-headed langur diverged at the beginning of the Quaternary’s glacial maximum, approximately 350,000 years ago (Liu et al., 2020). Following this initial divergence, both taxa continuously experienced a second glacial event (at approximately 200,000 to 100,000 years ago) and a third glacial period (LGM 20,000-10,000 years ago) with a drop in temperatures, then followed by a Holocene period of rising temperatures (Liu et al., 2020). Glacial conditions during the period of 20,000-10,000 years ago appear to have resulted in population bottlenecks among a diverse set of animal taxa, such as the Nujiang catfish (Creteuchiloglanis macropterus ) (Kang et al., 2017), Chinese alligators (Alligator sinensis ) (Wan et al., 2013), Great tits (Parus major ) (Qu et al., 2015), Gray wolves (Canis lupus ) (Fan et al., 2016), and snub-nosed monkeys (Rhinopithecus spp.) (Zhou et al., 2016), suggesting that the Guxiang glaciation (300,000 to 130,000 years ago) event should be a primary driver of species diversification in the Tibetan Plateau and the Hengduan Mountains of southeast China (Qu et al., 2015).
However, it remains unclear how changes in climatic suitability during glacial and interglacial periods affected population contraction/expansion and opportunities for gene flow between the these two closely related langur species. In addition, a clearer understanding how the closely related and endangered species historically respond to changes in habitat availability and climate is beneficial for elucidating the process of speciation and developing species conservation strategies, especially in the face of climate change in the future. To examine these relationships, we used climatic suitability, effective population change, and gene flow to explain the diverse of the two langur species and propose conservation strategies based on climate change (Figure 1). We assumed that for the two species, the separation of climatic suitability caused the changes of effective population size and interspecific gene flow, which leaded to niche separation between François’ langur and White-headed langur.