Unusually fine-scale genetic structuring found in rapidly speciating Malawi cichlid fishes

Mechanisms behind the explosive radiation of over 500 cichlid fish species from a single founding population in Lake Malawi during the last 700 000 years are poorly understood. Recent studies have suggested that the degree of population subdivision among the habitat patches within the lake may be responsible, but the evidence has been circumstantial: lack of a dispersal stage in haplochromine cichlids; genetic and colour 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 700-1400 m, in order to investigate whether cichlid populations are sufficiently isolated from each other for allopatric divergence and perhaps speciation to take place. Using six microsatellite loci, we demonstrate the existence of highly significant genetic differentiation between subpopulations on adjacent headlands in each of four rock-dwelling haplochromine cichlid species. Our results suggest that these fish populations are divided into thousands of subunits among which genetic divergence is currently occurring, and that this may provide unprecedented opportunities for allopatric speciation.