Explosive speciation: evolution puzzle solved
No fewer than 500 new cichlid species have evolved in the East African Lake Victoria over the past 15,000 years, a record in the animal and plant kingdoms. Researchers at Eawag and the University of Bern have been able to prove that an earlier interbreeding of two distantly related cichlid species enabled this explosive speciation.
The complete mixing of two cichlid species apparently made possible a great many new combinations of gene variants that can otherwise hardly exist in a single population. "This is similar to how a new combination of Lego bricks for a tractor and an airplane can create quite a few different companions," says first author Dr. Joana Meier.
In fact, the newly formed species come in countless color combinations and are adapted to different habitats, such as sandy bottoms, rock formations or the open water, ranging from the sunlit shallows to the permanent darkness of the great depths. Depending on the species, they scrape algae from rocks, eat plankton, break open snail shells, search for insect larvae or hunt for other fish, their eggs or scales. Experts speak of "adaptive radiation," the rapid emergence of a variety of new species accompanied by adaptations to different ecological niches.
Mixing 150'000 years ago
The intermingling of the two species occurred about 150,000 years ago when, during a rainy period, former tributaries of the Congo River crossed over to the Nile River basin with what later became Lake Victoria. In all larger lakes of the region, this hybrid population then formed a variety of species by adaptive radiation.
Researchers have not yet reconstructed what exactly happened in the primordial Lake Victoria. But it is clear: After a dry phase, it filled up again about 15,000 years ago. Descendants of the hybrid population with its very high genetic diversity migrated into the lake. There, in the - evolutionarily speaking - short time of a few thousand years, they formed a wealth of ecological specializations and split into around 500 new cichlid species that are found exclusively in Lake Victoria. The fact that the cichlids in Lake Victoria had very special facilities and adapted particularly well to the new ecological niches is proven by over 40 other fish species: they colonized the lake at the same time, but have hardly changed since then.
Complex food web created in record time
For their findings, the researchers sequenced more than three million positions in the genome of 100 cichlid species - until recently an impossible task. As a result, the group led by Ole Seehausen (Head of Eawag's Fish Ecology and Evolution department and Professor of Aquatic Ecology & Evolution at the University of Bern) was able to prove for the first time his theory that, under certain circumstances, mixing of species can lead to explosive speciation and adaptive radiation. In Lake Victoria, this process has created a complex food web in a few thousand years, as the new species have in turn shaped their environment.
In the last 50 years or so, land use and sewage have led to increasing fertilization (eutrophication) of the lake. The consequences of this are turbid water and a lack of oxygen in the deeper areas. Thus, different species merged into a few hybrid populations as the bright courtship colors by which females could recognize their conspecifics lost their effect. Deep-water habitats even became completely uninhabitable. Part of the species diversity and ecological diversity was thus lost again.
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Source: Eawag
