The ecological, behavioral, and genetic factors influencing the diversification of Lake Malawi's rock -dwelling cichlids
Rapid evolutionary radiations provide insight into the fundamental processes involved in species formation. Here I examine the diversification of one such group, the rock-dwelling cichlid fishes of Lake Malawi (mbuna), which have radiated from a single ancestor into more than 200 species over the past 700,000 years. Through the examination of several molecular phylogenies, a phylogenetic history of the mbuna is proposed. The diversification of this group has proceeded in three major bursts of cladogenesis each of which has been dominated by a single selective force (selection on habitat preferences, trophic morphology, and male nuptial coloration, respectively). A divergence with gene flow model is advocated with reference to this phylogenetic pattern. A population genetic survey of allele frequency distributions at four microsatellite loci is used to quantify levels of gene flow within the mbuna genus Metriaclima. Results from this study indicate that migration rates between adjacent populations are low such that slight variation in the selective environment may drive the divergence of populations. Speciation models involving genetic drift, population bottlenecks, founder events, microallopatric divergence, and divergence with gene flow are discussed with respect to these findings. Some of the factors contributing to the existence of highly diverse, densely packed mbuna communities are identified by examining male territorial behavior and habitat utilization within Metriaclima. By observing the recolonization of artificially vacated territories, I show that (1) territories are species-specific and (2) this species-specificity is associated with the width of the breeding caves. These results are discussed with respect to character displacement and sexual selection. The speciation of the mbuna has been closely tied to the diversification of male color patterns, yet little is known concerning the genetic basis of such a fundamental character. By (1) examining female mating preferences, (2) describing the cellular basis of color pattern variation, and (3) identifying the genetic basis of this variation, I show that differential male melanistic patterning, which plays a role in the reproductive isolation of two Metriaclima species, is controlled by a limited number of genes. The rapid diversification of the mbuna is discussed with reference to this finding.
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