African Cichlid Fishes

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African Cichlid Fishes

The African cichlid fishes are well-known in the field of evolutionary biology as perhaps the most spectacular example of an adaptive radiation . An adaptive radiation describes a group of species that have rapidly diverged, or speciated, from a single ancestor, each of which occupies and is adapted to a distinct ecological niche. Other well-known adaptive radiations include those of two birds, the honeycreepers of Hawaii and Charles Darwin's finches, which occupy several islands in the Galapagos.

The African cichlids are found in the Great Lakes of the Rift Valley of Africa, including Lakes Victoria, Malawi, Tanganyika, Nabugabo, and others. In these lakes, cichlid fishes have undergone an unparalleled radiation. In all, over 1,500 species have evolved over the course of only ten million years, suggesting a truly remarkable rate of speciation.

Four separate genera and over 200 species occur in Lake Victoria alone. This is particularly impressive given that Lake Victoria is less than one million years old, having made a comparatively recent geological transition from flowing river to lake. When the Lake Victorian cichlids were initially discovered, the remarkably diverse species was initially confusing to biologists because the lake appeared to provide little opportunity for the ecological isolation of populations . Isolation is one of the factors generally believed to promote speciation.

The Reasons for Their Success

Why, then, have cichlid fishes been so successful in their African lake habitats ? One reason is that the lakes provided a novel, underexploited habitat with few competing species. However, this cannot be the entire answer because several other species had the same opportunity to diversify in the new habitat soon after it formed. Why, among all the species present, was it the cichlids who underwent a remarkable radiation?

Feeding habits.

The diversity of feeding habits is at the heart of the African cichlid radiation. The ancestral cichlid species was likely a generalist and an insect eater. However, extreme specialization now exists among the cichlid species, with each species specializing on a distinct, fairly narrow food niche.

There are, for example, cichlid species that specialize on algae, plankton , other fish species, mollusks, and insects, and there are even some species that eat only fish scales, or even fish eyes. This way of dividing up the available resources in a habitat is called trophic specialization ("trophic" refers to feeding). Because increased specialization allows for the coexistence of larger number of species, trophic specialization in cichlids is partly responsible for the great diversity of species.

Morphological specialization.

In addition to using different food resources, the various species have also evolved different morphological specializations, or specializations of form, that are appropriate to their diet. For example, species exhibit a wide range of tooth shapes and sizes, as well as many different mouth and lip morphologies .

But how are individual cichlid species able to exploit such narrow food niches? One hypothesis is that cichlid success is related to their highly unusual anatomic characteristic of having two pairs of jaws. The outer jaws, also called the oral jaws, are homologous to the jaws of other vertebrates, and appear externally in the usual position.

The second pair of jaws are invisible externally and lie within the throat area. These are known as the pharyngeal jaws. Because the pharyngeal jaws are able to chew and process food, the outer jaws are free to become highly specialized for obtaining food.

Interestingly, trophic specialization is implicated in other instances of adaptive radiation, including those of the Hawaiian honeycreepers and of Darwin's finches. In both these radiations of birds, there is a great diversity in beak morphology, which is in turn related to differences in feeding. Among the Hawaiian honeycreepers, there are species that feed on seeds, insects, and nectar. Among Darwin's finches, as well, different species are specialized on different seed sizes.

Pharyngeal jaws, which characterize cichlids as well as a few other related families of fishes, can be described as the key innovation in the African cichlids that has allowed for the evolutionary success of the group. A key innovation is defined as a novel adaptation that allows a lineage to exploit resources that were previously unavailable. Key innovations are frequently associated with impressive species diversification. Other examples of key innovations include flight in birds, direct development in certain lineages of frogs (which bypasses the aquatic larval stage and allows for the occupation of more terrestrial habitats), incisors that grow constantly in rodents, and insect pollination by angiosperms .

In addition to the possession of pharyngeal jaws, cichlids appear to be highly adaptable both behaviorally and morphologically. Certain cichlid species are able to alter their tooth and skull morphology depending on available food resources. The ability to change morphologically in response to cues from the environment is known as phenotypic plasticity.

Plasticity allows cichlid species to take advantage of whatever resources become available. One cichlid species found in Lake Victoria, for example, specializes on snails and has the robust skull and teeth necessary to deal with snail shells. However, when introduced into a different habitat, the species switched to an insect-based diet and no longer developed the robust dentition.

Sexual selection.

Another factor that may have played a role in the evolution of the African cichlids is sexual selection. Sexual selection describes a situation in which different individuals in a population have different reproductive success. Different reproductive success can result from any of several factors. In many animal species, however, including cichlids, sexual selection arises because the reproductive success of males depends on the number of female mates they are able to attract and convince to mate with them. Sexual selection can lead to the rapid evolution of male traits for the attraction of females and of female preferences for these traits.

Sexual selection has been hypothesized to have played a role in the rapid diversification of many other species-rich lineages, including the songbirds, where both plumage coloration and song are objects of selection; Hawaiian fruitflies of the genus Drosophila; ducks; and hummingbirds. All these groups are characterized by elaborate courtship displays. Cichlid fishes are also characterized by a complex courtship behavior in which males build elaborate nests, known as bowers, and females inspect them in order to decide which males to mate with.