The species flocks of East African cichlid fishes: recent advances in molecular phylogenetics and population genetics

Abstract

With more than 3,000 species, the fish family Cichlidae is one of the most species-rich families of vertebrates. Cichlids occur in southern and central America, Africa, Madagascar, and India. The hotspot of their biodiversity is East Africa, where they form adaptive radiations composed of hundreds of endemic species in several lakes of various sizes and ages. The unparalleled species richness of East African cichlids has been something of a conundrum for evolutionary biologists and ecologists, since it has been in doubt whether these hundreds of species arose by allopatric speciation or whether it is necessary to invoke somewhat less traditional models of speciation, such as micro-allopatric, peripatric, or even sympatric speciation or evolution through sexual selection mediated by female choice. Ernst Mayr's analyses of these evolutionary uniquely diverse species assemblages have contributed to a more direct approach to this problem and have led to a deeper understanding of the patterns and processes that caused the formation of these huge groups of species. We review here recent molecular data on population differentiation and phylogenetics, which have helped to unravel, to some extent, the patterns and processes that led to the formation and ecological maintenance of cichlid species flocks. It is becoming apparent that sexually selected traits do play an important role in speciation in micro-allopatric or even sympatric settings. Species richness seems to be roughly correlated with the surface area, but not the age, of the lakes. We observe that the oldest lineages of a species flock of cichlids are often less species-rich and live in the open water or deepwater habitats. While the species flocks of the Lake Malawai and the Lake Victoria areas were shown to be monophyletic, the cichlid assemblage of Lake Tanganyika seems to consist of several independent species flocks. Cichlids emerge as an evolutionary model system in which many fundamental questions in evolution and ecology can be tested successfully, yet for other fish species flocks the relative importance of alternative mechanisms of speciation is likely to differ from that in cichlid fish.

On the origin of Lake Malawi cichlid species: A population genetic analysis of divergence

“…The oldest parts of Lake Tanganyika have been estimated to be 9-12 million years old (22), and we used a date of 7 million years for the common ancestor of the outgroup and Tropheops (23). A representative of Eretmodus cyanostictus was obtained from a Lake Tanganyika fish importer, and DNA sequences were obtained from those regions corresponding to the sequence portions of the HapSTR loci.…”

High-Dimensional Fitness Landscapes and Speciation

“…Classical examples include the diversification of Darwin's finches on the Galápagos islands, Anolis lizards on Caribbean islands, Hawaiian silverswords, and cichlids of the East African Great Lakes, among many others (Gillespie 2004; Givnish and Sytsma 1997;Losos 1998;Salzburger and Meyer 2004;Schluter 2000;Seehausen 2007;Simpson 1953). Adaptive radiation typically follows the colonization of a new environment or the establishment of a "key innovation" (e.g., nectar spurs in columbines, Hodges 1997) which opens new ecological niches and/or new paths for evolution.…”

“…Which of these scenarios is more general is controversial. The large time scale involved and the lack of precise data on its initial and intermediate stages make identifying general patterns of adaptive radiation very difficult (Gillespie 2004;Losos 1998;Salzburger and Meyer 2004;Schluter 2000;Seehausen 2007;Simpson 1953). Further, it is generally unknown if the patterns identified in specific case studies apply to other systems.…”

“…Adaptive radiation is both a spectacular and a remarkably complex process, which is affected by many different factors (genetical, ecological, developmental, environmental, etc.) Gillespie 2004;Losos 1998;Salzburger and Meyer 2004;Schluter 2000;Seehausen 2007;Simpson 1953). Further, it is generally unknown if the patterns identified in specific case studies apply to other systems.…”

To be or not to be a hamlet pair in sympatry

“…The lack of well-described marine fish radiations is surprising given the multitude of examples in freshwater fishes and the fact that, as with the species flocks of cichlids in East Africa, some of the most spectacular adaptive radiations have occurred in fishes (Kocher 2004;Salzburger & Meyer 2004;Seehausen 2006). One reason for this discrepancy might be that adaptive radiations are simply more apparent in geographically well-defined areas.…”