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Why do some lakes have a greater diversity of cichlid fish?

Posted by on August 3, 2012

Austrolebias charrua, cichlid fishGuest blog from Andrew Helmstetter, Savolainen Lab, Imperial College London, who researches ecological adaptation and speciation in the Austrolebias genus of fish (see picture)

One of the major questions that is driving research in speciation biology (the study of how new species evolve) is “why do some groups of species diversify faster than others?”

The role environmental factors (e.g. climate or habitat availability) and traits (e.g. morphology, behaviour and their underlying genetics) play in controlling species diversity has been a topic of research for some time. It is unlikely that any one of the factors acts alone yet there has been little research into how separate factors interact.

The Cichlidae, a group of freshwater fish, is an excellent group for studying speciation biology as their rapid diversification has been documented at numerous sites worldwide. This rapid diversification is known as adaptive radiation. Adaptive radiation is a complex process driven by the evolution of many new adaptive traits in a single species. These traits allow this species to adapt to multiple environments, leading to many new species over a short period of time.

More than 1,600 species of cichlid have been recorded in African lakes alone, making them one of the most species-rich freshwater fish families. Intriguingly, cichlids inhabit many African lakes but adaptive radiation has occurred in only 20% of those lakes that contain cichlids. This begs the question – what environmental conditions and species characteristics are common between those adaptively radiating lineages?

Recently published in Nature, Wagner et al. have attempted to answer this question by compiling data on colonization and diversification in 46 African lakes as well as information on the environmental conditions and characteristics of cichlid species present. The factors that were best at predicting high levels of diversification were lake depth, energy (measured as net solar radiation) and sexual dichromatism (when the sexes of a species differ in colouration).

In this study, sexual dichromatism was taken as a surrogate for sexual selection, suggesting that sexual selection is important in the adaptive radiation of the Cichlidae. In contrast to much of the theory relating to terrestrial taxa, the speciation of cichlids was not found to be restricted by habitat area. The best explanatory model of cichlid diversification was generated when both lineage-specific and environmental traits were used together, suggesting that interaction between these two types of predictors is of great importance in the adaptive radiation of the Cichlidae.

Wagner et al. have used the evidence from this paper to demonstrate that adaptive radiations are predictable. Though this study is undoubtedly a step towards predicting adaptive radiations in the African Lake Cichlid system, no true predictions have been made. The next logical step would be to apply our knowledge to predict radiations for lakes with no prior information to establish any patterns that exist across groups of organisms.

Wagner CE, Harmon LJ, & Seehausen O (2012). Ecological opportunity and sexual selection together predict adaptive radiation. Nature, 487 (7407), 366-9 PMID: 22722840

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