Cichlid fish diversity threatened by eutrophication that curbs sexual selection

Cichlid fish species of Lake Victoria can interbreed without loss of fertility but are sexually isolated by mate choice. Mate choice is determined on the basis of coloration, and strong assortative mating can quickly lead to sexual isolation of color morphs. Dull fish col-oration, few color morphs, and low species diversity are found in areas that have become turbid as a result of recent eutrophication. By constraining color vision, turbidity interferes with mate choice, relaxes sexual selection, and blocks the mechanism of reproductive isolation. In this way, human activities that increase turbidity destroy both the mechanism of diversification and that which maintains diversity. In the Great Lakes of Africa, large and di-verse species flocks of cichlid fish have evolved rapidly (1, 2). Lake Victoria, the largest of these lakes, had until recently at least 500 species of haplochromine cichlids (3). They were ecologically so diverse that they utilized almost all resources available t