for National Geographic News
The cichlids, a family of tropical freshwater fish, have evolved an astonishing array of forms in Africa. Their diversity is perhaps most impressive in the Great Lakes of East Africa, where a handful of colonizing fish species have multiplied over thousands of years into many hundreds of species found nowhere else in the world.
Scientists say they have now uncovered a second evolutionary hot spot for these fish. The experts have traced the origins of cichlids in rivers across southern Africa to a single lake that no longer exists.
Writing in the latest issue of the science journal Nature, researchers say their discovery shows how evolutionary processes, even when operating in one location and for a limited period, can have a huge and lasting impact on a continent's wildlife.
The study team found that cichlids found today in rivers such as the Congo, Zambezi, Okavango, and Limpopo originated from a massive lake that dried up some 2,000 years ago. Known as Lake Makgadikgadi, it once covered an area larger than Switzerland and was centered north of the Kalahari Desert in present-day Botswana.
Researchers used genetic analysis to reveal the extent of the evolutionary dispersal of cichlid species from Lake Makgadikgadi. The scientists' finding is "truly impressive," said Thomas D. Kocher, a professor of zoology and genetics at the University of New Hampshire who was not involved in the study.
"The high rates of speciation [evolution of new species] observed in these African cichlids are almost beyond belief, but the evidence is clear," he added.
Kocher said Lake Makgadikgadi's remnant cichlids, which now swim the Congo, Zambezi, Okavango, and Limpopo Rivers, are almost as varied in appearance as species from the East African Great Lakes of Victoria and Malawi.
These river populations include "ambush predators with long heads and jaws, and snail crushers with deep heads and massive teeth." All that's missing, Kocher added, are a few specialized cichlids, such as plankton-sippers and algae-scrapers. He speculates that conditions preferred by these species aren't available in rivers.
The study was led by Domino Joyce of Hull University in England and the University of Berne in Switzerland. Her team plotted the geographical origins of southern African river cichlids through their mitochondrial DNA, which is inherited through the mother.
The researchers examined a set of closely linked genes called a haplotype, which tend to be inherited together. The haplotype revealed that the fish share a common ancestor, quite separate from that which first colonized East Africa's existing Great Lakes.
The findings appear to solve the riddle of why southern Africa's rivers contain such a range of cichlids when the rest of the continent supports far fewer species.
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