In just 16,000 years, more than 500 cichlid species, distributed throughout the entire food web, have evolved in Lake Victoria. This explosion of biodiversity was made possible by repeated cycles of fusion and diversification in evolutionary lineages, as researchers from Eawag and the University of Bern have described in the “Science” and “Nature” journals. The results underscore that it is not just species that need protection, but entire “species swarms”.
A good part of the world’s biodiversity has evolved through adaptive radiation. This means that when new habitats are colonised, many new, closely related species emerge from a parent species within a few hundred thousand or even ten thousand years, occupying different ecological niches. Examples of this are Darwin’s finches on the Galapagos Islands, chars in Greenland, amphipods in European groundwater streams, or whitefish in the deep lakes near the Alps.
But why do some families tend to speciate quickly while others do not? Two teams led by Ole Seehausen from the Swiss aquatic research institute Eawag and the University of Bern investigated this question on the basis of the fastest and largest known radiation of cichlids in Lake Victoria. They discovered answers in the genomes of the fish and in fish fossils from the lake sediments. These finding have just been published in the “Science” and “Nature” journals.
Starting with the survivors of a disaster
The history of today’s cichlid abundance in Lake Victoria began with a disaster. Towards the end of the last ice age, about 20,000 years ago, Lake Victoria dried up. Remnants of its previously presumably rich fish life survived the almost 4,000-year drought phase in isolated swamps that were left over from Africa’s largest lake.
When the lake basin filled up with water once again, the survivors also returned. Using over 7,000 fish teeth from sediments dating back almost 17,000 years, Nare Ngoepe, lead author of the “Nature” study, was able to identify five different families of carp-like catfish and cichlids.
Only one of these families, however, the cichlids, has since rapidly split into different species with as many as 500 today. According to the “Science” study, which compared the genomes of present-day species, these 500 species are descended from a hybrid population that arose at the same time as the lake was reforming. Hybrids are formed when two individuals of different species mate. In Lake Victoria, which regained size after the drought phase, three different cichlid lineages, themselves the product of hybridisation more than 300,000 years earlier, came together, intermingled and thus combined their genomes.
Field work on Lake Victoria (Photos: Eawag, Ole Seehausen, Anna Mahulu, Nare Ngoepe, Moritz Muschick)