Mitochondrial DNA (mtDNA) restriction fragment length polymorphisms were assayed among
40 species of Lake Malawi haplochromines (Cichlidae) including representatives of ecologically
divergent genera. Six distinctive mtDNA lineages were distinguished, two of which were major
clades, represented by a large number of species. The other four lineages were each represented
by a single species with a divergent mtDNA haplotype. One of the two major clades was composed
of the shallow-water, rock-dwelling mbuna species, whereas the other included a diverse array
of sand-dwelling and pelagic species. A number of taxa, found to be firmly embedded within the
mbuna clade, are quite distinct in morphology and generally inhabit deeper, sediment-rich
areas rather than the rocky habitats typical of other mbuna. The mbuna group is generally
thought to be a monophyletic assemblage, but these results suggest that it is actually
paraphyletic. In contrast to the high morphological diversity among Malawi haplochromine
species, mtDNA sequence divergence was found to be remarkably low. This finding underscores
the unprecedented rapidity of speciation and evolutionary plasticity in this fish species flock.