A study has found that the cichlid fish radiation of Lake Tanganyika has been diversifying at a rate six times slower than those in the species flocks of Lakes Victoria and Malawi.
Julia Day, James Cotton and Timothy Barraclough studied the molecular phylogeny of 152 Lake Tanganyika cichlid species using mitochondrial markers (ND2 and control region), and use statistical models (Bayesian relaxed molecular clock approach) to reconstruct the timing of diversification of the Lake Tanganyika species flock.
The authors found that Lake Tanganyika cichlids have diversified six times more slowly than endemic radiations in the other lakes, even during peak episodes of diversification.
The Lake Tanganyika species flock is derived from a prolonged accumulation of species, rather than rapid, recent radiation (in contrast to Lakes Victoria and Malawi).
The reasons for the differences in the diversification rates of Lake Tanganyika against the other lakes are not clear, but the authors offer two hypotheses:
One possibility is that the physical environment of Lake Tanganyika is less conducive for cichlid speciation, perhaps by inhibiting whatever special mechanism causes fast speciation in Lake Malawi and Lake Victoria.
Turbid waters have been shown to impede sexual selection in Lake Victoria, raising the possibly that a similar phenomenon may have occurred during the history of Lake Tanganyika, although currently these waters are very transparent.
Alternatively, the remarkable diversifying abilities of haplochromines could have been inhibited because the lake was already occupied by older radiations, such as the lamprologines and ectodines, which occupy similar habitats.
For more information, see the paper: Day, JJ, JA Cotton and TG Barraclough (2008) Tempo and mode of diversification of Lake Tanganyika cichlid fishes. PLoS ONE 3(3), e1730 doi:10.1371/journal.pone.0001730.