Fossil-calibrated phylogeny and historical biogeography of Southeast Asian water monitors (Varanus salvator Complex)

• Divergence of V. salvator Complex ∼14 mya, with subsequent diversification during the Pliocene and Pleistocene.
• Ancestral area for the V. salvator Complex recovered as Philippines + Sundaland.
• Island (Philippine) diversification of water monitors with dispersal back to Indochina and Indonesia.
• Primary dispersal corridors: the Sulu Archipelago, and the Sangihe Ridge.

We utilize robust geographical genetic sampling, a multilocus dataset, a new synthesis of numerous fossil calibration points, a time-calibrated phylogeny, and the Dispersal–Extinction–Cladogenesis model to test the prediction that widespread Southeast Asian water monitor species initially diversified on the Asian mainland and subsequently invaded the island archipelagos of the Philippines, Sundaland, and Wallacea. Our results strongly contradict these expectations and instead infer an initial water monitor radiation of range-restricted but highly divergent evolutionary lineages (now recognized as endemic species) in one archipelago around 3.6 mya, followed by an out-of-the-Philippines reinvasion of the mainland (2.2 mya), resulting in a few, widespread species that now inhabit most the islands of the Sunda Shelf and the Southeast Asian mainland as far north as Myanmar, as well as an out-of-the-Philippines invasion of Sulawesi (2.1 mya). Our analyses both confirm the importance of island archipelagos as drivers of diversification for mainland biodiversity and emphasize the global evolutionary significance and conservation priority of the Philippines for understanding processes of diversification in island archipelagos.

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