Macroevolutionary patterns and processes in Neotropical freshwater fishes: cichlids as a model

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South and Central America, known as the Neotropical region, have the highest freshwater fish diversity anywhere on the planet, exceeding 7000 species. Yet we know very little about the evolutionary processes that originated such incredible diversity. My students and I study the evolution of a common group of Neotropical freshwater fishes, the family Cichlidae, and use it as a model to understand the forces that have shaped the evolution of Neotropical freshwater fishes. Cichlids have adapted to live in nearly every environment of South and Central America by diversifying into a large variety of forms and ecological habits. We hypothesize that the diversity and ecological versatility of cichlids resulted from early expansions into new, available ecological resources such as food and habitats (ecological opportunity), originating distinct, ecologically specialized genera. Contrastingly, within most of those genera, species tend to be similar to each other, sharing both their morphology (like body size and shape) and their ecological preferences (including habitat and diet). This lack of diversity among species within genera may stem from the opposite of ecological opportunity, adaptive constraint. Once a lineage becomes an ecological specialist, it may not be able to survive under conditions too different from those to which it is specialized. This forces the evolutionary path of the specialist to remain close to a so-called “adaptive peak”, around which all ecological, morphological and physiological conditions favor its survival. If the evolutionary path moves too far away from the peak, the species cannot survive. In this project, we want to understand how ecological opportunity and adaptive constraint produce different evolutionary outcomes. We will do this by studying the major evolutionary lines (genera) of American cichlids, and by comparing two genera within the group: “pike cichlids” in the genus Crenicichla, and “dwarf cichlids” in the genus Apistogramma. Pike cichlids have diversified into remarkably different forms and ecological niches, while the dwarf cichlids share similar ecologies and morphologies. Although both genera have comparable numbers of species, pike cichlids may have diversified through ecological opportunity, whereas dwarf cichlids may be evolving under severe adaptive constraint. This provides a natural system to study how these two forces can generate diversity through different mechanisms. We will use hundreds of DNA sequences and the fossil record to reconstruct the evolutionary relationships (the family tree) and the when American cichlids, pike cichlids, and dwarf cichlids originated. We will then use the dated family trees as roadmaps to understand how and when different features of these fishes appeared in their evolutionary history. We will trace the speed and form of evolutionary change in morphology (feeding mechanisms, swimming attributes) and ecology of the different groups of cichlids. By comparing different groups of cichlids, we will learn how ecological opportunity and adaptive constraint contributed to the diversification of one the most diverse families of fishes in the Americas. Understanding how evolutionary forces shape biological diversity is also relevant for biologists working on conservation or trying to understand the long term effects of climate change or other environmental transformations. How species respond to environmental changes depends on how adaptable they are to varying ecological conditions. It follows that species belonging in groups with severe adaptive constraints may not be able to tolerate changes to their environment, whereas species in less constrained groups may be more tolerant of ecological transformations.