Speciation and Radiation in African Haplochromine Cichlids

The explosive radiation of cichlid fishes in the African Great Lakes has intrigued biologists for many decades. The faunas of these lakes are outstanding, both in species richness and in the composition of their fish fauna. Several of these lakes 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 understanding of what is so special about them. 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 mouthbrooding 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. Of the three African great lakes, Lake Tanganyika is the oldest and its age has been estimated to be between 9 and 12 My (Cohen et al. 1993). Lake Malawi’s age has been estimated to be 1–2 My (Fryer and Iles 1972). Lake Victoria is the youngest (0.25–0.75 My: Fryer 1997). Most likely it dried up 200 000 years ago (Martens 1977) and seems to have dried up again in the late Pleistocene, after which it refilled between 15 500 (Beuning et al. 1997) and 14 600 years ago (Johnson et al. 1996a). Paradoxically, the young Lakes Victoria and Malawi contain more endemic species (between 500 and 1000 each) than the old Lake Tanganyika, with about 250 species. In particular, the Lake Victoria species flock must have been the result of truly explosive speciation, even if the lake did not completely dry out during the late Pleistocene.