Color polymorphism and sex ratio distortion in a cichlid fish as an incipient stage in sympatric speciation by sexual selection

We investigated a Lake Victoria cichlid with a complex colour polymorphism that apparently represents one original species and two incipient species, all of which are sympatric. In laboratory breeding experiments we observed sex ratio distortion in certain matings between original and incipient spec ies. Mate choice experiments show that males of the incipient species exhib it mating preferences against the original species, and males and females o f the original species exhibit strong mating preferences against the incipi ent species. Mating preferences might evolve by sex ratio selection to avoi d matings with distorted progeny sex ratios. Phenotype frequencies in natur e suggest that mating preferences translate into mating frequencies, thus r estricting gene flow and exerting disruptive sexual selection between the o riginal and incipient species. The incipient species do not differ in morph ology or ecology from the original species, implying that colour polymorphi sm, associated with sex ratio distortion, can be an incipient stage in symp atric speciation, and that disruption of gene flow can precede ecological d ifferentiation.