Rapid Evolutionary Responses in Cichlids: Genetics of Adaptation, Morphology and Taxonomic Implications

The haplochromine cichlids of Lake Victoria exhibit the fastest adaptive radiation of vertebrates known. Species identification of these lacustrine cichlids has been subject of debate for many years. Over the past 35 years, environmental perturbations have resulted in phenotypic change and possibly in hybridization, making species delimitation even more problematic. In this chapter, we document the effects of a changed environment on the ecomorphology of the zooplanktivore Haplochromis (Yssichromis) pyrrhocephalus over a 20-year period. We found that this species has extended its habitat to shallower waters and adjusted its diet to larger and more robust prey. Adaptive morphological responses to predation, to larger and tougher prey and to a hypoxic environment were found. Whether these morphological changes are the result of hybridization, phenotypic plasticity or evolutionary adaptation remains unclear. The morphological responses are sometimes so large that they transcend morphological species boundaries and even one of the limits of the genus Yssichromis. The extent of the morphological responses makes clear that nuptial colouration is indispensable for species distinction and the assessment of species richness. We review advanced molecular techniques that might make it possible to distinguish species genetically. These techniques may also reveal how adaptive responses might have played a role in the remarkable resurgence of these rapidly adapting cichlids.