The cichlid fishes of Lake Victoria in East Africa, the 2nd largest freshwater lake in the world, are famous for their diversity. They are a textbook example of adaptive radiation, that almost every student of evolution will have heard about. What they will also learn about, however, is that the cichlids are en route to extinction and depletion, because of the Nile Perch, a predatory fish introduced to the Lake in the 1960s for food and sport. In combination with eutrophication and pollution, cichlid populations and diversity declined considerably in the following decades.
Now, however, the populations may be recovering, according to scientists studying the lake. Part of the explanation lies in the heavy fishing pressure on the Nile Perch, and on improvements to the quality of water running into the lake, but the adaptive potential of the cichlid to changing conditions may have some role to play. Much of the research has been conducted by Ole Seehausen of Eawag Aquatic Research in Switzerland. His research had previously found that the increased turbidity of the lake impaired the ability of female fishes to discriminate the colors of their prospective mates, reducing the effectiveness of sexual selection and allowing more hybridization between previously reproductively-isolated species. As a result, the cichlid species in the lake today are of a very different composition and genetic makeup than those found before human modification of the lake environment.