Comparative aspects of adaptive radiation and speciation in Lake Baikal and the great rift lakes of Africa

Abstract

Lakes Baikal, Tanganyika and Malawi have similar origins, are physiographically similar, and of similar size. The hydrological regime of Baikal is, however, very different from that which prevails in its African sisters. Apart from being much cooler, it differs fundamentally in being oxygenated to all depths while the two great African rift lakes possess only a relatively thin oxygenated surface layer and have vast oxygenless, and therefore azoic, abyssal regions. Nevertheless, like Baikal, they have rich endemic faunas.

That these faunas originated largely by intralacustrine speciation and not by multiple invasion is now well established. They provide some of the world's most spectacular examples of species flocks, and some groups display what has been aptly described as explosive speciation. Certain features, and especially the adaptive radiation, of some of the groups involved, are noted. Comparisons between lakes are illuminating. Some species flocks, such as those of amphipods, sponges and turbellarians of Baikal and the atyid prawns and potamid crabs of Tanganyika, have no counterparts in the other lakes. Other groups, such as the prosobranch gastropods, ostracods and harpacticoid copepods of Baikal and Tanganyika, and the fishes of all three, involve representatives of the same major group, though often of different families or even higher taxonomic categories.

That allopatric speciation has been involved is universally acknowledged but the problems posed by species multiplication in deep water in L. Baikal have led to suggestions that sympatric speciation could have played a part. Notwithstanding the difficulties, it is suggested that the process can be explained without invoking the assistance of the sympatric model.

The faunas of these lakes provide immense fields for investigation and enormous intellectual challenges. While each is an entity in itself, comparative studies may be particularly enlightening.