The Channel Islands of California are part of the California Floristic Province, one of the world’s biodiversity hotpots. These islands occur very close to the mainland (< 100 Km), and to one another (5–45Km). Due to their geography and their many shared species both among the islands and with the neighboring mainland, they are frequently seen as a unit. However, many of the hypothesized widespread island species are known to be poor dispersers or even salt-water intolerant, so it remains unclear wether these islands are truly connected through gene flow or if they are isolated. Various hypotheses of colonization exist for the endemic island taxa, and many of these hypotheses invoke human-mediated dispersals in both pre-European and recent times. 

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Biogeography and species delimitation. We evaluated genomic patterns of variation of two island natives, the endemic island night lizard Xantusia riversiana, hypothesized to be an ancient island dweller with classic island gigantism, and the widespread pacific treefrog Pseudacris regilla, thought to be a recent immigrant potentially by human mediation. Using next-generation sequencing technologies, we found that the divergences among the islands are much higher than initially thought. The widespread treefrog P. regilla has two highly distinct groups on the islands, which suggests an absence of contemporary and recent gene flow. The night lizard X. riversiana is highly genetically distinct on the three islands it occurs. Our results support the existence of unique cryptic species on the islands, and suggest that biodiversity and endemicity may be vastly underestimated in the California Channel Islands. This work is soon to be submitted to Molecular Ecology! 

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