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when applied to cichlid fishes it reliably causes spectacular evolutionary radiations of hundreds of new species. Generalist riverine cichlids have given rise to flocks of highly specialized fishes in each of the major lakes in East Africa. Now, on page 90 of this issue, Joyce and colleagues describe a previously unrecognized radiation of cichlids in the extinct Lake Makgadikgadi, whose descendants still haunt the rivers of southern Africa today. Joyce and colleagues’evidence consists of mitochondrial DNA sequences determined from fishes collected from rivers throughout southern Africa. Rivers in East Africa typically hold just one haplochromine cichlid species (haplochromine is the name given to one of the major lineages of cichlid fishes in Africa). But the Okavango, Cunene, Limpopo and upper sections of the Congo and Zambezi rivers in southern Africa each harbour several species of haplochromines. Evolutionary analyses show that these fishes share a common ancestor quite separate from that which gave rise to the better known radiations in lakes Malawi and Victoria (Fig. 1). The diversity of these riverine cichlids peaks in the Okavango delta, which geological evidence suggests was, until about 2,000 years ago,part of a lake larger than Switzerland. The extent of this newly recognized adaptive radiation is truly impressive. The remnants of the Lake Makgadikgadi flock display a morphological diversity nearly as great as that of the extant cichlids in lakes Malawi and Victoria. The flock includes ambush predators with long heads and jaws, and snail crushers with deep heads and massive teeth. Only a few specialized morphologies are missing — such as those of pelagic plankton eaters and benthic algal scrapers — probably because these niches are not available in the remaining river habitat. What is it about lakes that facilitates these rapid radiations? Certainly, large lakes provide a range of new ecological habitats not present in rivers. The still waters of lakes probably also reduce rates of gene flow between different groups of fish, enabling them to diverge genetically from each other and eventually to speciate. But other groups of fishes have not responded to these physical factors with similar bursts of speciation. It seems there is also something special about haplochromine cichlids that causes them to diversify in these large lakes.What are the key innovations that make them particularly prone to rapid adaptive radiation? A long-standing hypothesis is that cichlids have a malleable feeding apparatus that can be quickly altered to take advantage of new feeding opportunities. Cichlids have a second set of jaws in the back of the throat — the pharyngeal jaws — that process food before it enters the gut. The oral jaws are therefore relatively free to evolve specializations for acquiring food. This decoupling of functions has clearly contributed to the rapid evolution of feeding specializations in each of the species flocks that inhabit lakes. Another key innovation is maternal mouth-brooding. Most non-haplochromine cichlids lay large numbers of small eggs on the lake or river bottom, and both parents cooperate in the care of the eggs and fry. In contrast, female haplochromines lay small numbers of large eggs, and carry the eggs in their mouths for several weeks until the young are released to fend for themselves. Haplochromine males provide no postspawning care of their offspring. news and views