Phylogeny of a rapidly evolving clade: The cichlid fishes of Lake Malawi, East Africa

Lake Malawi contains a flock of >500 species of cichlid fish that have evol ved from a common ancestor within the last million years. The rapid diversi fication of this group has been attributed to morphological adaptation and to sexual selection, but the relative timing and importance of these mechan isms is not known. A phylogeny of the group would help identify the role ea ch mechanism has played in the evolution of the flock Previous attempts to reconstruct the relationships among these taxa using molecular methods have been frustrated by the persistence of ancestral polymorphisms within speci es. Here we describe results from a DNA fingerprinting technique that overc omes this problem by examining thousands of polymorphisms distributed acros s the genome. The resulting dendrogram averages the evolutionary history of thousands of genes and should accurately reflect the evolutionary history of these species. Our tree resolves relationships among closely related Lak e Malawi cichlids and provides insights into the pattern of speciation in t his group. We demonstrate that adaptive divergence in trophic morphology pl ayed an important role during the early history of the lake. Subsequent spe cies diversity has arisen with little change in trophic morphology, which s uggests that other forces are responsible for the continued speciation of t hese fishes.