Speciation in rapidly diverging systems: lessons from Lake Malawi

Rapid evolutionary radiations provide insight into the fundamental processe s involved in species formation. Here we examine the diversification of one such group, the cichlid fishes of Lake Malawi, which have radiated from a single ancestor into more than 400 species over the past 700 000 years. The phylogenetic history of this group suggests: (i) that their divergence has proceeded in three major bursts of cladogenesis; and (ii) that different s elective forces have dominated each cladogenic event. The first episode res ulted in the divergence of two major lineages, the sand- and rock-dwellers, each adapted to a major benthic macrohabitat. Among the rock-dwellers, com petition for trophic resources then drove a second burst of cladogenesis, w hich resulted in the differentiation of trophic morphology. The third episo de of cladogenesis is associated with differentiation of male nuptial colou ration, most likely in response to divergent sexual selection. We discuss m odels of speciation in relation to this observed pattern. We advocate a mod el, divergence with gene flow, which reconciles the disparate selective for ces responsible for the diversification of this group and suggest that the nonadaptive nature of the tertiary episode has significantly contributed to the extraordinary species richness of this group.