Incipient speciation in sympatric Nicaraguan crater lake cichlid fishes: sexual selection versus ecological diversification

The growing body of empirical evidence for sympatric speciation has been complemented hy recent theoretical treatments that have identified evolutionary conditions conducive to speciation in sympatry The Neotropical Midas cichlid (Amphilophus citrinellum ) fits both of the key characteristics of these models, with strong assortative mating on time basis of a colour polymorphism coupled with trophic and ecological differentiation, derived from a polymorphism in the ir pharyngeal jaws. We used microsatellite markers and a 480 bp, fragment of time mitochondrial DNA control region to study four polymorphic populations of the Midas cichlid From three crater lakes and one large lake in Nicaragua in an im investigation of incipient sympatric ic speciation. All populations, were strongly genetically differentiated on the basis of geography We identified strong genetic separation based on colour polymorphism for populations from Lake Nicaragua and one crater lake (Lake Apoyo), but failed to find significant genetic structuring based on trophic differences and ecological niche separation in any of tho four populations studied. These data support the idea that sexual selection through assortative mating contributes more strongly or earlier during speciation in sympatry than ecological separation in these cichlids. The long-term persistence of divergent cichlid ecotypes (as measured by time percentage sequence divergence between populations) in Cenral al American crater lakes, despite a lack of fixed genetic differentiation, differs strikingly from the patterns of extremely rapid speciation in the cichlids in Africa, including its crater lakes. It is unclear whether extrinsic environmental factors or intrinsic biological differences, e.g. in the degree of phenotypic plasticity, promote different mechanisms and thereby rates of speciation of cichlid fishes from the Old and New Worlds.