In the great African lakes, cichlid fish are ten times more species rich than the best represented of the other fish families. That requires an explanation. We found that it is thanks to the fussy sexual behavior of cichlids and the flexible build of their heads that many species could emerge in a short time that managed to survive side by side.
The Greater Part of Sea Creatures
Perhaps three million species of animals live on Earth, perhaps many more. These species are unevenly distributed among the different families. The beetles, for example, are very species-rich. When the English evolutionary biologist Haldane was once asked what he had learned about creation in his study of nature, he replied that God must have had an extraordinary love for beetles, as 300,000 species are known to exist. Among the vertebrates there are also strikingly species-rich groups, such as the songbirds among the birds and the so-called Cichlids among the fish.
- Species formation took place extremely quickly and frequently in a group of cichlids, the so-called haplochromines. More than five hundred species have developed in less than thirteen thousand years. This is incredibly fast for speciation in vertebrates. We know that it must have happened so fast because Lake Victoria had dried up completely before then and the species only occur in this lake. In addition, genetic research has made it clear that Lake Victoria cichlids as species are extremely young and probably all descend from one species (or some species) that swam from the rivers into the reformed lake.
- The question is how this speciation could have progressed at such a rapid pace. You can easily imagine that two species can emerge from a species when the original species is split into two groups by a barrier (this is called allopatric speciation in technical terms). This has also happened several times in the African lakes.
But the number of species of cichlids is so great that this allopatric speciation alone cannot explain the diversity of species. Many species must have split into new species without any geographical barriers. This can happen, for example, if individuals within a population no longer randomly mate with each other: if individuals of certain variants mate with each other more often than with individuals of other variants, the difference in behavior forms the barrier that causes the groups to grow apart. called sympatric speciation).
We often encountered such behavioral barriers in cichlid fish. Hundreds of cichlid species live side by side in Lake Victoria, and many closely related species spawn at the same place and time. There are no genetic barriers that prevent hybridization (crossbreeding of species): crosses that we carried out in the aquarium between different species yielded viable and fertile offspring. Nevertheless, under most circumstances each individual mates in the lake with a peer.
How can behavioral changes within a population occur that are large enough to separate and separate groups in terms of their reproduction? One possibility is that there is variation in sensory powers that leads to variation in sexual preferences. If individuals then mostly mate with a preferred individual, the variants may become isolated and develop into separate species. The first step is to distinguish variants within a species (so-called polymorphism or hereditary multiformity).