Molecular systematics, speciation, and radiation in the haplochromine cichlid fish species flock of Lake Malawi, East Africa (mbuna species)
Mitochondrial DNA (mtDNA) restriction fragment length variation was used to study
the evolution of the haplochromine cichlids of Lake Malawi in East Africa. This
investigation was directed at three levels of taxonomic hierarchy. First, phylogenetic
relationships were examined in a broad sampling of the most ecologically and
morphologically diverse of Lake Malawi's haplochromines. Second, a more detailed
study was initiated to explore the phylogenetic relationships and taxonomic
distribution of mtDNA haplotypes among the members of a single trophic guild,
the rock-dwelling mbuna. Finally, a population study of selected mbuna species was
conducted to explore the demographics of speciation in these very recently derived
taxa. The first study revealed six primary mtDNA lineages, two of which included
large numbers of diverse taxa, whereas each of the other four were found in single
species. All the mbuna examined comprised one lineage; however, a number of other
species, not previously recognized as mbuna, were also included in this group,
indicating a paraphyletic relationship. Sequence divergence was found to be remarkably
low, indicating unprecedented rapidity of speciation and radiation. Further examination
of the mbuna revealed two mtDNA subgroupings. Interestingly, these mtDNA lineages were
each found in multiple species and are interpreted as a retained ancestral polymorphism.
This result has significant implications for the further use of mtDNA in phylogenetic
reconstruction of this group because the mtDNA gene tree is incongruent with the
mbuna species tree. The final study sought to examine the potential role of founding
events in the speciation of local endemics restricted to a single group of islands.
Haplotype diversity and frequency distribution indicated that one of these species
did indeed experience a bottleneck during or subsequent to its isolation. For the other
species, significant differences in haplotype frequency in the presence of high
haplotype diversity, presents a somewhat equivocal result; several potential
explanations are examined. In two species, replicate samples from different sites
at single locations produced significant differences in haplotype frequency.
This result suggests a fine scale of spatial structuring, resulting from extreme
philopatry. Taken as a whole these results provide substantial insight into both
pattern and process of speciation and radiation of this evolutionarily interesting
group.