Island vs. Mainland: Genetic Divergence of Calotes versicolor (Daudin, 1802) (Squamata: Agamidae) in Thailand

Geographic isolation can shape genetic variation and structure, leading to divergence between island and mainland populations. The Oriental Garden lizard (<i>Calotes versicolor</i> Daudin, 1802) is a widespread agamid reptile in Asia, occurring across diverse habitats from continental Southeast Asia to offshore islands. We examined mitochondrial cytochrome c oxidase subunit I (<i>CO1</i>) sequence variation in 143 individuals from 23 localities across the Andaman Sea and Gulf of Thailand to assess genetic diversity and structure between insular and mainland populations. Forty-six haplotypes (Cve1-Cve46) were identified, with haplotype diversity (Hd) ranging from 0.500 to 1.000 and nucleotide diversity (π) from 0.0057 to 0.0265. AMOVA revealed low to moderate differentiation between island and mainland groups in the Andaman Sea (<i>F</i>_CT = 0.075, <i>p</i> > 0.05) and negligible differentiation in the Gulf of Thailand (<i>F</i>_CT = 0.009, <i>p</i> > 0.05). Haplotype networks and PCoA showed clustering of most island and mainland populations within regions, with some localized divergence. Divergence-time analysis indicated that lineages split within the last 0.5 million years ago (Ma), coinciding with late Pleistocene climatic oscillations and sea-level changes. Species delimitation analyses supported three major lineages, including a geographically restricted clade confined to Trat Province and Phuket Island. These results suggest that <i>C. versicolor</i> populations are structured more by regional geography than strict island-mainland separation, reflecting historical connectivity and contemporary gene flow. The findings contribute to understanding reptile biogeography in Southeast Asia and highlight populations of conservation value.