UNUSUALLY FINE-SCALE GENETIC STRUCTURING FOUND IN RAPIDLY SPECIATING MALAWI CICHLID FISHES

Mechanisms behind the explosive radiation of over 500 cichlid fish spe cies from a single founding population in Lake Malawi during the last 700 000 years are poorly understood. Recent studies have suggested tha t the degree of population subdivision among the habitat patches withi n the lake may be responsible, but the evidence has been circumstantia l: lack of a dispersal stage in haplochromine cichlids; genetic and co lour variation among populations separated by large-scale geographical barriers; and fluctuating lake levels. One reason for the rapidity of speciation in these fishes may be that population subdivision is on a much finer scale than previously thought. Here we quantify the level of population subdivision and estimate migration at a scale of 700-140 0 m, in order to investigate whether cichlid populations are sufficien tly isolated from each other for allopatric divergence and perhaps spe ciation to take place. Using six microsatellite loci, we demonstrate t he existence of highly significant genetic differentiation between sub populations on adjacent headlands in each of four rock-dwelling haploc hromine cichlid species. Our results suggest that these fish populatio ns are divided into thousands of subunits among which genetic divergen ce is currently occurring, and that this may provide unprecedented opp ortunities for allopatric speciation.