Introduction

The explosive radiation of cichlid fishes in the African Great Lakes has intrigued biologists for many decades. These lakes are outstanding, both in species richness and in the composition of their fish fauna. Several of them contain as many or even more fish species than all the rivers and lakes of Europe together (Lowe-McConnell 1987; Kottelat 1997). About 90% of the fish species in each lake belong to a single family, the cichlids (Cichlidae; Teleostei) and are endemic to that lake. Estimates of the phylogenies of these species flocks suggest that the species of Lakes Victoria, Malawi, and Tanganyika have evolved in situ (Meyer et al. 1990; Lippitsch 1993; Nishida 1997). Even more remarkable, for Lakes Malawi and Victoria the species flocks are derived from one or only a few closely related ancestral species and are all haplochromines. In comparison to the diversity of these lakes, riverine cichlid fish faunas in Africa and South America are considerably less diverse.

The unusually fast ecological radiation of haplochromine cichlids and the exceptionally dense species packing of these fishes demands an explanation. Most lacustrine species flocks of other fish taxa, even other cichlid taxa, are less diverse in ecology and species numbers. The versatility of the pharyngeal jaw apparatus, physiological properties, and their mouth-brooding behavior may all be necessary attributes, but these alone are not sufficient to explain the exceptional diversification of haplochromines. In this chapter, we argue that it is the combination of a number of factors.