Environmental context for understanding the iconic adaptive radiation of cichlid fishes in Lake Malawi.

Biologists rarely have access to reliable and detailed historical records of environmental conditions, and changes thereof, over extended periods of time. Where such data exist, they can be instrumental for understanding evolutionary processes by providing a framework for interpreting patterns of organismal diversification, supplementing biogeography, molecular evidence, and fossils. In PNAS, Ivory et al. (1) present a long and fully timecalibrated paleoecological record for Lake Malawi in Africa, which is home to the most species-rich extant adaptive radiation on Earth, consisting of ∼800 species of cichlid fishes. The ecological and morphological diversity, as well as the rapidity of species formation, is extraordinary in Malawi cichlids, paralleled only by the cichlid radiations in two other African Great Lakes: Victoria and Tanganyika (2). The reconstruction of the history of Lake Malawi covering the past 1.2 My (1) illuminates the links between external factors shaping the lake’s environment and the evolution of its biota, in particular the cichlids. Adaptive radiation— the rapid origin of an array of species from a common ancestor as a consequence of adaptation to distinct ecological niches—is thought to be responsible for much of the biological diversity on Earth (3, 4). The most well-known and best-studied— including genomic examinations—examples of adaptive radiation are Darwin’s finches, Anolis lizards, threespine stickleback fish, Heliconius butterflies, and cichlid fishes in East Africa (5). Such outbursts of life may be triggered by: (i) external, often abiotic, factors creating ecological opportunity (e.g., “empty” or underoccupied ecological niches in newly created environments or “liberated” niches resulting from extinction); and/or (ii) internal or biotic features of the radiating groups (i.e., evolutionary “key-innovations” providing access to previously underused resources) (4).