Hierarchical population genetic analysis reveals metapopulation structure in a phytophagous Galapagos beetle

The Galapagos Archipelago has long been considered a living laboratory for the study of evolution. Due to geographic isolation and speciation many endemic animal and plant groups have radiated on the islands. Although the vertebrate fauna of these islands (e.g. giant tortoises, Darwin's finches) has been studied in great detail, little is known about invertebrates and especially insects. Results are given of a population genetic study on the phytophagous beetle Nesaecrepida darwini. This small alticine beetle is present on all major islands but shows a discontinuous population distribution. To obtain population genetic information we used cellulose acetate gel electrophoresis to study allozyme variation in 6 populations from 3 islands. Twelve presumptive loci, including 9 polymorphic ones, were analysed The results show low heterozygosity values, with the lowest genetic diversity on the youngest island. F-statistics (mean F-ST = 0.431) indicate a very large amount of genetic differentiation between populations. Hierarchical analysis indicates little inter-island gene flow but also considerable genetic variation between populations occurring on the same island. These results strongly suggest a metapopulation structure with recurrent extinctions and recolonisations of populations within each island. Recent field observations support these findings.