Roles of divergent selection and environmental heterogeneity in speciation of Lake Victoria cichlid fish

Cichlid fish are one of the most diverse families of vertebrates and are among the most spectacular examples of rapid speciation and adaptive radiation. Their explosive radiations in the East African Great Lakes, known to harbor an enormous diversity of endemic species that are morphologically, ecologically and behaviorally extremely diverse, are some of the most astonishing examples of adaptive radiation among vertebrates and have rightfully become models in the study of evolutionary biology. Unfortunately they have also become famous examples for the collapse of species due to increased pollution and eutrophication.

During my PhD thesis I studied the roles of divergent selection and environmental heterogeneity in speciation of Lake Victoria cichlid fish. Fish from this lake have become a model system for the study of mechanisms of evolution. The main conclusion from my research is that divergent selection, sexual and/or ecological, acting on several types of traits can maintain high levels of phenotypic diversity and differentiation despite high gene flow among Lake Victoria cichlids. Variation in the steepness of environmental gradients, which are mediated by light, can explain much of the variation in the extent of phenotypic differentiation: better differentiation is associated with shallower gradients. These findings have important implications for conservation of species and ecological diversity as the eutrophication of Lake Victoria in recent decades has lead to a massive increase of turbidity and associated loss of light-mediated habitat heterogeneity. In several locations of the lake this increase in turbidity has lead to the collapse of species.