The existence of sympatric speciation — that populations diverge into species in the
absence of physical or ecological barriers — is controversial. The East African Great
Lakes harbour hundreds of cichlid species representing only a few monophyletic lineages,
although palaeolimnological evidence and local restrictions on species distribution
suggest that speciation in these lakes could have been allopatric. The case for sympatry
in restricted areas of Lakes Malawi and Tanganyika is stronger but not unassailable.
A better case might be made for cichlid species flocks in small, ecologically monotonous
crater lakes. Here we present a mitochondrial DNA analysis of cichlid species flocks
endemic to two such lakes in Cameroon. The results suggest that the flocks in each lake
are monophyletic: the implication being that each lake was colonized once only, the size
and shape of each lake being such that subsequent diversification would have been sympatric.