Climate change and human activities promoted speciation of two endangered langurs (Francois' langur and white-headed langur)

Climate oscillations may drive the divergence of ancestral species through climatic suitability isolation, effective population contraction, and the cessation of gene flow. However, traditional ecological niche models fail to consider gene flow and the historical demography of a species, and thus provide limited insight into how climate oscillations affect species divergence. We analyzed the impact of climate change and human activities on the divergence of two langur species, Francois' langur (Trachypithecus francoisi) and white-headed langur (T. leucocephalus) using Maxent models and genomic data. Our results indicate that 1) although gene flow occurred between Francois' langurs and white-headed langurs during the Last Interglacial (LIG, 140-120 Kya), the two species showed disparate climatic suitability; 2) both taxa were characterized by a decrease in effective population size and a cessation of gene flow during the Last Glacial Maximum (LGM, 26.5-19 Kya), which promoted speciation of the two langurs; and 3) The effective population size and areas of climatic suitability for both species increased from the LGM to the present. More recently, human activities severed the geographical link between the two species, leaving no opportunity for gene flow. Climate oscillation, population isolation, and in situ evolution in refugia appear to have played a critical role in limiting gene flow between these two langur species, however human activities have contributed to species differentiation. Overall, our results demonstrate how climatic oscillations and human activities drive speciation, providing a new perspective on species divergence and conservation.