How many species of cichlid fishes are there in African lakes?

The endemic cichlid fishes of Lakes Malawi, Tanganyika and Victoria are tex tbook examples of explosive speciation and adaptive radiation, and their st udy promises to yield important insights into these processes. Accurate est imates of species richness of lineages in these lakes, and elsewhere, will be a necessary prerequisite for a thorough comparative analysis of the intr insic and extrinsic factors influencing rates of diversification. This revi ew presents recent findings on the discoveries of new species and species f locks and critically appraises the relevant evidence on species richness fr om recent studies of polymorphism and assortative mating, generally using b ehavioural and molecular methods. Within the haplochromines, the most speci es-rich lineage, there are few reported cases of postzygotic isolation, and these are generally among allopatric taxa that are likely to have diverged a relatively long time in the past. However, many taxa, including many whi ch occur sympatrically and do not interbreed in nature, produce viable, fer tile hybrids. Prezygotic barriers are more important, and persist in labora tory conditions in which environmental factors have been controlled, indica ting the primary importance of direct mate preferences. Studies to date ind icate that estimates of alpha (within-site) diversify appear to be robust. Although within-species colour polymorphisms are common, these have been ta ken into account in previous estimates of species richness. However, overal l estimates of species richness in Lakes Malawi and Victoria are heavily de pendent on the assignation of species status to allopatric populations diff ering in male colour. Appropriate methods for testing the specific status o f allopatric cichlid taxa are reviewed and preliminary results presented.