Diversification in African cichlid fishes: From speciation to macroevolutionary patterns

African cichlid fishes are a celebrated example of evolutionary diversification, and a thorough understanding of this diversification requires studies of both the origins of their diversity, and of the factors influencing broad scale evolutionary patterns. This body of work addresses evolutionary questions at these both of these levels. I first test the influence of habitat and distance on fine-scale population genetic structure in three sympatric cichlid species in Lake Tanganyika. These species show striking differences in their patterns of genetic subdivision within the same geographical region, implying substantially different patterns of gene flow. This suggests that both ecological and behavioral traits have a strong influence on the scale and degree of population subdivision, a finding which has implications for understanding differential propensities for diversification among lineages. Next, I study the population genetics of recent speciation in sympatric color morphs from Lake Tanganyika. I report genetic evidence that these color morphs diverged only recently, yet that barriers to gene flow exist which prevent extensive admixture despite their sympatric distribution. This is an unusual example of active diversification in Lake Tanganyika’s ancient cichlid fauna. Moving to a macroevolutionary perspective, I next examine the factors that influence cichlid adaptive radiation. Cichlids have radiated within more than thirty African lakes, and in another seventy instances, colonizing lineages are present in lakes without diversifying. Using this “natural experiment”, and a dataset including both environmental variables and information about the traits of colonizing cichlid lineages, I find that lineage-specific traits related to sexual selection, and environmental factors related to ecological opportunity, both strongly influence whether cichlids radiate. Finally, I examine the environmental influences on the species richness of cichlids within African lakes. I show that total species richness per lake is correlated with measures of lake size and energy, and that the species richness of radiations is limited by these same environmental variables. I conclude that ecological carrying capacities exist which render the total diversity of cichlids predictable within these lakes, but that these diversities are achieved by lineage-specific diversification outcomes, thereby producing marked differences in the faunal composition of different lakes.