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 coloration,
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.