Genetic differentiation and reproductive isolation in a Cyprinodon fish species flock from Laguna Chichancanab, Mexico.

The Cyprinodon species flock from Laguna Chichancanab, aged 8000 years, provides another potential case of sympatric speciation. The flock consists of seven morphologically distinct species, each within partially different trophic niches, and a group of specimens which cannot unequivocally be assigned to one of these species. Genetic analyses, based on mtDNA and five microsatellite loci, revealed significant genetic differentiation of one species, C. maya, from other members of the species flock, providing strong evidence for reproductive isolation. For the remaining members of the flock significant genetic structuring was detected, with some evidence of gene flow with the most abundant species C. beltrani. These analyses suggest that speciation proceeds with ongoing hybridisation, and further suggest that the morphologically unidentifiable specimens found in the lake are probably hybrids. I propose that in the Cyprinodon species flock besides disruptive selection sexual selection plays an important part in achieving and maintaining reproductive isolation.