Worldwide, there are more than 1,700 species of cichlids, about 90 percent of which swim in the lakes of East Africa. As early as the 19th century, biologists traveled to Lake Victoria, Lake Malawi and Lake Tangayika to study these fish. In 1935, German Richard Woltereck introduced the term ‘Artexplosion’ to describe the situation, an explosion of sorts. Walter Salzburger (University of Basel) puts the diversity of African Cichlids in perspective, “During the evolutionary period of our own species, starting with the split between humans and chimpanzees about 5 to 7 million years ago, about 2000 species of cichlids evolved in East Africa.” But what caused this explosive evolution? Evolutionary biologists are now trying to find out.Some recent papers indicate that they are slowly but surely solving the mystery of rapid evolution.
Researchers from the University of Konstanz studied the diversity of cichlids in Lake Tangayika. They managed to collect DNA from almost all species in this lake and reconstructed the evolutionary history of these fish. The analyzes showed that the species explosion started about 12 million years ago, a time that corresponds to the emergence of Lake Tangayika (between 9 and 12 million years ago). The researchers discovered that there were many crossings just before the start of this explosion. Through these crosses or hybrids genetic material was exchanged between different types of cichlids. This resulted in an increase in genetic diversity that fueled the lightning-fast speciation.
Did You Know
Cichlids are exemplary parents? They guard their eggs and are also ready for their young if they have already hatched. Some species even feed their free-swimming offspring for weeks or months.
Hybridization may therefore be the driving force behind the species explosion in Lake Tangayika. This conclusion is supported by further analyzes of certain genes. Among other things, the researchers compared the opsin genes of various cichlids. These genes contain the codes to produce proteins that determine what colors animals can perceive. In cichlids that eat plankton, the opsin gene sws1 shows a rapid evolution. This gene allows the fish to see ultraviolet. Plankton is then more visible because the ultraviolet reflects. In fish scraping green algae from the rocks, another opsin gene, rh2α-α, has experienced rapid evolution. This gene allows the cichlids you guessed it to distinguish green colors better. Both genes were exchanged between a handful of species just before the species explosion 12 million years ago.
A similar process took place in a small lake in Cameroon. Jelmer Poelstra (University of North Carolina) and his colleagues studied four cichlid species of the genus Coptodon. Genetic analyzes indicated hybridization just before the emergence of two new species. When the researchers looked at which genes had been exchanged, a cluster of eight odor receptors appeared to be among them. It is known that some fish recognize their peers by various odors. This package of ‘fragrance genes’ may have contributed to the rapid speciation in this lake.