Lake level fluctuations synchronize genetic divergences of cichlid fishes in African lakes

Water level fluctuations are important modulators of speciation processes i n tropical lakes, in that they temporarily form or break down barriers to g ene flow among adjacent populations and/or incipient species. Time estimate s of the most recent major lowstands of the three African Great Lakes are t hus crucial to infer the relative timescales of explosive speciation events in cichlid species flocks. Our approach combines geological evidence with genetic divergence data of cichlid fishes from the three Great East African Lakes derived from the fastest-evolving mtDNA segment. Thereby, we show fo r each of the three lakes that individuals sampled from several populations which are currently isolated by long geographic distances and/or deep wate r form clusters of equally closely related haplotypes. The distribution of identical or equally closely related haplotypes in a lake basin allows deli neation of the extent of lake level fluctuations. Our data suggest that the same climatic phenomenon synchronized the onset of genetic divergence of l ineages in all three species flocks, such that their most recent evolutiona ry history seems to be linked to the same external modulators of adaptive r adiation. A calibration of the molecular clock of the control region was el aborated by gauging the age of the Lake Malawi species flock through the di vergence among the utaka-cichlid and the mbuna-cichlid lineages to minimall y 570,000 years and maximally 1 Myr. This suggests that the low-lake-level period which established the observed patterns of genetic relatedness dates back less than 57,000 years, probably even to 17,000-12,400 years ago, whe n Lake Victoria dried up and Lakes Malawi and Tanganyika were also low. A r apid rise of all three lakes about 11,000 years ago established the large-s cale population subdivisions observed today. Over that period of time, a mu ltitude of species originated in Lakes Malawi and Victoria with an impressi ve degree of morphological and ecological differentiation, whereas the Tang anyikan taxa that were exposed to the same habitat changes hardly diverged ecologically and morphologically. Our findings also show that patterns of g enetic divergences of stenotopic organisms provide valuable feedback on geo logical and sedimentological time estimates for lake level changes.