Cichlid fishes as models of ecological diversification: patterns, mechanisms, and consequences

Cichlid fishes are hypothesized to encompass several independent adaptive radiations that display increased diversification rates and impressive ecological heterogeneity. Here, I review major ecological patterns associated with the evolutionary history of cichlids, with particular focus on comparison of Afrotropical and Neotropical lineages. Specifically, I present major patterns of ecological diversification, potential mechanisms that may promote ecological diversification, and possible consequences of ecological diversification. Evolutionary convergence and specialization of ecological (e.g., diet), behavioral (e.g., benthic sifting), and morphological traits (e.g., oral dentition) characterize adaptive patterns that transcend continents. Craniofacial mechanics, the pharyngeal jaw apparatus, phenotypic plasticity, and hybridization may have facilitated diversification of cichlid fishes by generating functional, morphological, and/or genetic diversity. The benthic–pelagic axis has been an important source of divergence during adaptive radiation. Additionally, there are several discrepancies between Afrotropical and Neotropical lineages, such as the relative frequency of herbivorous species, the importance of hybridization in generating diversity, the relative frequency of dentition types, and relationships between dental organization and ecological function. Emphasis on contrasts between Neotropical and Afrotropical lineages improves characterization of patterns at a broader level of organization and indicates that the genetic basis, functional capacity, and ecological opportunity for many traits may be conserved across lineages.