A GENETIC METAPOPULATION MODEL FOR REEF FISHES IN OCEANIC ISLANDS - THE CASE OF THE SURGEONFISH, ACANTHURUS-TRIOSTEGUS

Allozyme data on a surgeonfish, Acanthurus triostegus, were analysed f rom 10 islands in French Polynesia. We compared estimates of gene flow according to the hypothesis of an equilibrium between genetic drift a nd migration, and estimated genetic divergence times assuming complete genetic isolation without gene how since foundation. The significant correlation between genetic divergence and geographic distance, at the within-archipelago level (r = 0.709, P = 0.024) indicates exchange of individuals mainly between neighbouring populations. The correlation was, however, not significant at the among-archipelagoes level (r = 0. 325, P = 0.330), suggesting that long distance migrations are more spo radic. This addresses the problem of scale in population biology. Acco rding to the spatial scale of analysis, results can change from an isl and model, with no relation between genetic differentiation and geogra phical distances between archipelagos, to an isolation-by-distance mod el within an archipelago. These factors lead us to propose a ''patchy population'' model, in which all patches are occupied and reproductive ly active, though with few successful migrations between neighbouring populations. This model describes a subdivided population that is stab le through time, with an amount of gene flow small enough to allow sig nificant local differentiation in neutral gene frequency, but high eno ugh to prevent differential fixation in the long term, and therefore p reserving the genetic cohesion of the species.