As the hybrid population spread, parts of it began to specialise. These sub-populations increasingly reproduced in isolation from each other due to different behavioural patterns. Ecological differentiation and the associated selection of different genomes in different ecological niches quickly led to the formation of different species. These, in turn, were able to successfully expand into new, even more extreme ecological niches.

“The new cichlid species tapped into the entire food web,” says Joana Meier, lead author of the “Science” study. “As a result, several genera developed that specialised on very specific algae or insects, on waste or plant parts, while others sought out fish eggs, filtered plankton from the water, or developed special hunting techniques for hatchlings, young fish and adults.”

Since the fish were closely related, new hybridisations were possible once again, tightening the food web with new combinations of genetic material. At common spawning sites of large hunters and small plankton filter feeders, for instance, several species of dwarf hunters evolved at once. This ecotype, which only exists in Lake Victoria, combines anatomical characteristics of the head of large hunters with the size and body shape of plankton eaters, which has made it possible to take advantage of new resources.

All fish species had the same starting point

The success of cichlids is not due to the fact that they were in the new Lake Victoria before the other fish species and monopolised resources for themselves,” noted Nare Ngoepe on the basis of fish fossils. “On the contrary, carp-like fish dominated for the first thousand years or so after recolonisation.”

But as the water levels rose and a deep lake developed, the species that specialised in shallow water and small streams remained tied to the areas near the shore and marshes. “The cichlid fossils, on the other hand, are common in the sediments of these new habitats,” says Ngoepe. Evidently, the cichlids immediately rushed into the open water or into the depths of the lake.
 

The fact that the cichlids were able to conquer the new lake so quickly, and efficiently use the entire food web, was only possible thanks to the diverse genomes that came together in hybrid swarms. “The few genetic mutations that arose in the short evolutionary time frame of the last 16,000 years did not play a role,” says Joana Meier. “The decisive factor was rather that different genomes that had survived in the various cichlid lineages only had to be ‘recomposed’ after the drought phase.” For example, one parent line brought the genetic information relevant to hunters, while the genomes of the other two lines enabled the specialisations of algae eaters and herbivores.

Thanks to the cycles of hybridisation and specialisation, countless variants of hereditary traits that have accumulated over millions of years within the cichlid populations were preserved despite the region’s turbulent climate history and repeated species extinctions,” says Ole Seehausen. After all, even if some of the specialised species were lost in a disaster, a large part of their genomes survived, distributed among the few surviving populations. When these populations came together again, the lost specialisations could be quickly “reconstructed” and new ones “invented”.

Protecting evolutionary potential with species swarms

“These are important findings for the conservation of biodiversity,” says evolutionary and fish biologist Seehausen. Attempts to protect individual species fall short, especially in times when humans are increasingly triggering ecological disasters with over-fertilisation and construction, and with the introduction of invasive species and climate change. “We must also protect the evolutionary potential of entire ‘species swarms’ because they can help ecosystems recover more quickly, and with them their vital functions for humans. Prerequisites for this are contiguous habitats that allow animal migrations, a large variety of different ecological niches so that diverse genomes are maintained through diverse selection, and an understanding of evolution that does not view occasional hybridisation across species boundaries as a dead end.”

Cover picture: Lake Victoria in East Africa was completely dry for almost 4,000 years before it filled up with water again 16,000 years ago (Photo: Eawag, Nare Ngoepe).
 

• Eawag
• SNSF
• Universität Bern