Biogeographic history and ecological divergence of cat skinks ( Ristella : Scincidae) in the Western Ghats Mountains of India

Topographically complex tropical landscapes often harbour cryptic biodiversity structured by geography and the environment, where morphologically conserved taxa may conceal deeply divergent evolutionary lineages. We examined whether two traditionally described morphological groups (morphotypes) of the Western Ghats-endemic skink genus Ristella are monophyletic and occupy distinct environmental niches and how climatic and topographic factors influence their distribution and diversification. We quantified niche divergence using spatial data on climate, elevation, and canopy cover and constructed a dated multilocus coalescent phylogeny (∼4 kb; three mitochondrial and three nuclear loci) to assess phylogenetic congruence with niche separation. Ancestral range evolution and ancestral state reconstruction analyses were used to evaluate the role of climatic stability in shaping diversification. Phylogenetic analyses indicate that one morphotype is monophyletic, whereas the other comprises two non-sister clades, suggesting convergence or retention of ancestral traits. Our results also reveal clear environmental niche separation between morphotypes, particularly in elevation, canopy cover, temperature, and precipitation. Diversification probably began in the climatically stable southern Western Ghats, with most phylogenetic splits occurring post Late Miocene, consistent with aridity-driven vicariance. These findings underscore the role of climatic stability in Ristella diversification, revealing early ecological divergence followed by relative niche stability within major lineages.