Island bird populations often provide exemplary cases of evolution based on historical colonization and diversification events. However, capturing contemporary cases of evolution is rare. Introductions of nonnative birds into the Hawaiian Islands, USA, provide numerous potential opportunities to assess evolutionary changes over a relatively short time frame. One introduced species, the Japanese Bush-Warbler (Cettia diphone), has a well-established history in the Hawaiian Islands, with a documented introduction in 1929 to the island of Oahu and natural colonization of the other main islands by 1997. We sampled 143 Japanese Bush-Warblers from 5 of the main Hawaiian Islands and amplified 12 microsatellite loci (9 were variable) and sequenced portions of the cytochrome b and cytochrome c oxidase subunit I (COI) genes to assess the genetic structure and potential original source of these populations. As predicted, genetic diversity, measured by allelic richness and private alleles, was greatest on Oahu (the original introduction site) and was significantly lower in birds on the islands farthest from Oahu. Accordingly, there was a clear isolation-by-distance effect, with highest FST values between island pairs farthest apart. The population on the westernmost island of Kauai appears to be diverging from the easternmost populations on the islands of Maui, Molokai, and Hawaii. The results provide a unique opportunity to document the microevolutionary process of genetic drift in action, and we speculate on the potential role of behavior in diversification.
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