Quaternary climatic oscillation promoted speciation of two closely related endangered langurs (François’ langur and White-headed langur)

Climate oscillations may drive the divergence of ancestral species through habitat isolation, species distribution patterns, and the cessation of gene flow. However, traditional ecological niche models lack the consideration for the genetic adaptation of species, thus it’s difficult to understand how climate oscillations affect species divergence by changing their climatic suitability and historical demography. We analyzed the impact of climate change on the divergence of two langur species, François’ langur and White-headed langur, using Maxent, Genotype–Environment Association (GEA) models and genomic data. Our results indicate that 1) Climatic suitability for François’ langur and White-headed langur has separated from the Last Interglacial (LIG, 120,000-140,000 years ago) to the Last Glacial Maximum (LGM); 2) Although the effective population size for both species decreased between the LIG and the LGM, the effective population size of François’ langur increased more quickly than that of White-headed langur from the LGM to the present; 3) Historical patterns of climate change have resulted in periods of expanded and contracted climatic suitability, with each species experiencing periods of interspecific genetic isolation and periods of interspecific genetic exchange; and 4) Human activities and future climate change have contracted the range of François’ langur (in China) and White-headed langur. Climate oscillation, population isolation, and in situ evolution in refugia from the LIG to the LGM appear to have played a critical role in langur speciation and adaptive evolution. Overall, our results demonstrate how climatic oscillations and historical demography drive speciation, providing a new perspective on species divergence and conservation.