Specialization in narrow ecological niches might not only help species to survive in competitive or unique environments but also contribute to their extermination over evolutionary time. Although the “evolutionary dead end” hypothesis has long been debated, empirical evidence from species with detailed information on niche specialization and evolutionary history is still rare. Here we used a group of four closely related Cnemaspis gecko species that highly depend on granite boulder caves in the Mekong Delta to illuminate the potential impact of ecological specialization on their evolution and population dynamics. Isolated by the unsuitable habitat of a flood plain, these boulder-dwelling geckos are among the most narrowly distributed Squamata species in the world. We applied several coalescence-based approaches combined with the RAD-seq technique to estimate their divergence times, gene flow and demographic fluctuations during the speciation and population differentiation processes. Our results showed long-term population shrinkage in the four geckos and limited gene flow during their divergence. The results suggest that the erosion and fragmentation of the granite boulder hills have had great impacts on these populations’ divergences and population declines. We argue that the specialist gecko’s habitat specialization has facilitated the fine-scaled speciation in these granite rocky hills; in contrast, specialization might also have pushed these species toward the edge of extinction. Our study also emphasizes the conservation urgency of these vulnerable, cave-dependent geckos.