Explosive Speciation and Adaptive Radiation of East African Cichlid Fishes

Abstract

Cichlid fishes are the most species-rich group of all teleost fishes. Their diversity is centered in the East African Great Lakes where more than 2,000 species evolved within the past 10 million years, representing the fastest vertebrate radiation known. Ongoing molecular phylogenetic analyses indicate that the cichlid radiation originated within Lake Tanganyika. Within the Tanganyikan radiation, seven lineages diversified during the primary radiation to occupy all available freshwater fish niches. The Tanganyikan radiation is the oldest, containing the greatest phenotypic diversity of all East African cichlid radiations, and is ancestral to the cichlid radiations found within Lakes Victoria and Malawi. The radiations in Victoria and Malawi are reciprocally monophyletic and are rooted within the C-lineage of the primary Tanganyikan radiation. While greater numbers of species are found within both Lakes Victoria and Malawi relative to the Tanganyikan radiation, these species flocks have a lower phenotypic diversity relative to the older Tanganyikan radiation. The construction of phylogenetic hypotheses has allowed researchers to explore the extraordinary morphological and behavioral diversity within an evolutionary framework. As a result, the study of cichlids has begun to answer many fundamental questions about the driving forces, mechanisms, and pathways of diversification. These studies demonstrated that cichlid diversification has been influenced by a complex combination of micro-allopatry, natural and sexual selection, facilitated by genetic mechanisms. Here we discuss these patterns, processes, and influences and also point to specific biological conservation problems of cichlid species flocks due to their extreme species richness and restricted species distribution. The current threats are not (yet) caused by habitat destruction and pollution but by overharvesting. Possible tactics to maintain diversity are proposed.