POPULATION-STRUCTURE AND GENE FLOW IN STOMION - A SPECIES SWARM OF FLIGHTLESS BEETLES OF THE GALAPAGOS-ISLANDS

Stomion is a swarm of 13 flightless tenebrionid beetle species endemic to the Galapagos Islands. Their distribution is patchy and largely re stricted to the littoral and arid zones of the archipelago. Each taxon is found on one or a few geographically close islands. Thirty-five po pulations representing nine of the taxa were examined at eight polymor phic enzyme loci using cellulose acetate electrophoresis to measure pa tterns of gene flow and investigate models of dispersal in a relativel y young species group exhibiting spatially isolated populations. Genet ic subdivision is high, particularly among populations of taxa which i nhabit more than one island, revealing restricted gene flow and confir ming the high potential for reproductive isolation among subpopulation s. The mean F-ST across taxa was 0.30. The genetic differentiation occ urring between spatially isolated populations may explain the exuberan t speciation of the genus in the Galapagos. Tests of gene flow models give support to the stepping-stone model of dispersal. Because of thei r lack of flight wings, interisland dispersal of Stomion probably occu rred by oceanic drift as pleuston or on floating debris, with individu als colonizing nearby islands more frequently than ones at a greater d istance.