Contrasting patterns of spatial genetic structure in sympatric rock-dwelling cichlid fishes.

The cichlid fish of Lake Tanganyika in Eastern Africa are a celebrated example of both ecological and species diversification. Because population subdivision is likely to play an important role in the speciation process, understanding how habitat features interact with species' demographic, behavioral and ecological attributes to influence gene flow and population divergence may help explain the causes of high species richness in this and other systems. Here, we test the roles of isolation-by-habitat and isolation-by-distance in generating fine-scale population genetic structure in three sympatric species of habitat-restricted cichlids in Lake Tanganyika. Using multilocus microsatellite genotypes, we contrast patterns of population differentiation in these habitat specialists along a mosaic coastline of both favorable and unfavorable habitat. Despite their close phylogenetic relationship and shared habitat affinity, these species show striking differences in their pattern of genetic subdivision within the same geographical region, suggesting substantially different patterns of gene flow. In particular, two trophically specialized species exhibit much more restricted gene flow over sandy habitat than a trophically opportunistic species. This result suggests that ecological and behavioral traits have a strong influence on the scale and degree of population subdivision, a finding that has potentially important implications for understanding differential propensities for diversification among lineages and phylogenetic patterns of diversity.