Mechanisms of speciation in cichlid fish were investigated by analyzing population genetic
models of sexual selection on sex-determining genes associated with color polymorphisms.
The models are based on a combination of laboratory experiments and field observations on
the ecology, male and female mating behavior, and inheritance of sex-determination and
color polymorphisms. The models explain why sex-reversal genes that change males into
females tend to be X-linked and associated with novel colors, using the hypothesis of
restricted recombination on the sex chromosomes, as suggested by previous theory on the
evolution of recombination. The models reveal multiple pathways for rapid sympatric
speciation through the origin of novel color morphs with strong assortative mating that
incorporate both sex-reversal and suppressor genes. Despite the lack of geographic
isolation or ecological differentiation, the new species coexists with the ancestral
species either temporarily or indefinitely. These results may help to explain different
patterns and rates of speciation among groups of cichlids, in particular the explosive
diversification of rock-dwelling haplochromine cichlids.