Scientists have solved the puzzle of the rapid evolution rate among cichlids in Lake Victoria. Image courtesy of EAWAG: Swiss Federal Institute of Aquatic Science and Technology.
No less than 500 new species of African cichlids evolved in Lake Victoria over the past 15,000 years — a record in the animal and plant world. This rapid evolution was facilitated by earlier hybridisation between two distantly related cichlid species from the Upper Nile and Congo drainage systems, scientists have discovered.
This hybridisation enabled genetic variants to be recombined on a scale which would not otherwise be possible in a single population, say scientists from Eawag and Bern University, who published their study in Nature Communications.
According to Dr Joana Meier, first author of the study: “It’s similar to the way the recombination of parts from Lego tractor and aeroplane kits could generate a wide variety of vehicles.” Indeed, the species which evolved exhibit innumerable combinations of colours and are adapted to different habitats, such as sandy bottoms, rocky shores or open waters — ranging from the clear shallows to the permanent darkness of the turbid depths. Depending on the species, cichlids may scrape algae from rocks, feed on plankton, crack open snail shells, forage for insect larvae, or prey on other fish, including their eggs or scales.
The hybridisation event probably took place around 150,000 years ago, when — during a wet period — a Congolese lineage colonised the Lake Victoria region and encountered representatives of the Upper Nile lineage. Across the large lakes of this region, the hybrid population then diversified in a process known as “adaptive radiation” (evolution of multiple new species adapted to different ecological niches). While the precise course of events in ancestral Lake Victoria has yet to be reconstructed, it is clear that, after a dry period, it filled up again about 15,000 years ago. Descendants of the genetically diverse hybrid population colonised the lake and, within the evolutionarily short period of several thousand years, diverged to form around 500 new (endemic) cichlid species, with a wide variety of ecological specialisations. The particular genetic diversity and adaptive capacity of Lake Victoria’s cichlids is demonstrated by the fact that more than 40 other fish species which colonised the lake at the same time have barely changed since then.
The study involved sequencing over three million sites in the genome of 100 cichlid species — a task which until recently would not have been feasible. This allowed the group led by Ole Seehausen to provide strong evidence for his theory that hybridisation between divergent species, in conjunction with ecological opportunity, can facilitate rapid adaptive radiation. Over a few thousand years, this process gave rise to a complex food web in Lake Victoria, as the new species, in turn, influenced their environment.
Over the past 50 years, changes in land use and wastewater inputs have led to eutrophication of the lake, with increased turbidity and oxygen depletion in deeper waters. Consequently, various species have merged into hybrid populations, as the male nuptial coloration which attracts females of the same species has become less effective; certain deep-water habitats are no longer viable. Some of the lake’s biodiversity and ecological diversity has therefore been lost.