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The Evolution of Cichlids
by Sabine Wilkins


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So, how did the cichlids in Lake Malawi and Lake Victoria evolve so rapidly in their respective lakes?

Genetic differences are known to exist among different colour forms of rocky-shore cichlids that live around the edge of the Lake Malawi and islands. Genetic differences were even found in populations living separated by a 700 m. wide sandy bay. This means that there are hundreds of geographically isolated populations, which are potentially able to diverge into new species. You could even see, that we have the privilege of observing the infinitely slow process of evolution. This is why it is important for us to take note of where our fish come from. Geographical variants do show genetic differences that should be preserved, because in nature they are kept apart as well.

Fin Fish Finger Watercourse Nail


Could a similar process have taken place in Lake Victoria? A recent study has revealed more than 130 species inhabiting the rocky shores. As in Lake Malawi many populations appear restricted to small islands or rocky outcrops separated by sandy or swampy bays. Rocky-shore cichlids are highly specialised to exploit their habitat, do not move around freely in the open water or along sandy shores and lack a dispersal phase in their lifecycle, such as migration. Their isolated lifestyle obviously plays an important role in their rapid diversification.

However, according to George Turner, a well known cichlid researcher, this is probably not the whole story. And it doesn't explain the vast number of species inhabiting sandy or muddy shores or the deep water. Lake Victoria as 300 such species, Lake Malawi 350. So, how did these evolve? There must be some other process driving speciation in these non-isolated population and accounting for an even faster rate of speciation than in the rocky shores.

According to George Turner, just feeding specialisations can't explain everything. Ole Seehausen, who researched Lake Victoria extensively, found that female mouthbrooders speciate faster than the rest, so we should be thinking about sexual selection, he says.

Sexual selection by females is most powerful, when males play no part in parental care and can mate with lots of females in one season. It can lead to the evolution of spectacular male courtship displays, colours and structures, all of which we see in cichlids. Of all the endemic cichlids of Malawi and Victoria, those of some of the most diverse groups in Tanganyika are maternal mouthbrooders. In these species, the males are larger, brighter and have longer fins than the females. Often researchers use male courtship colours for the identification of closely related species. Females perhaps use the same cues to identify their mates. If so, sexual selection might play a key role in speciation. This was tested in the laboratory (or better the aquarium), and these tests showed that females of Pseudotropheus auratus discriminate between males during courtship by the number of eggspots on the anal fin. This is strong evidence that sexual selection acts on male courting traits.

Water Vertebrate Fin Organism Underwater


Even clearer evidence came when Ole Seehausen observed females of a yet undescribed species discriminate between males of different colours in clear water conditions, but mated at random in turbid waters with limited visibility. Female choice can therefore prevent hybridisation between two species in their natural habitat. Computer simulations show that female selection can result in one species splitting in two in only a few generations. However, two conditions must be present for this to occur: females must see a large number of males to choose from, and relatives may not disperse very far between generations. Both conditions seem to be fulfilled in many African cichlids.

Until now I have talked only about African cichlids. But what of the American cichlids. How come that cichlids are present in an area that far away from Africa? How did they get there? In the early days before we had the knowledge we have now, one theory said that cichlids may have evolved at a time when the continents still formed a huge landmass called Pangea. However, Pangea broke up and the continents floated apart a long time before cichlids ever evolved. So, what did really happen?

Over the past millions of years the water level of the ocean fluctuated widely coinciding with Ice Ages, where a lot of the water was bound in the ice covering much of the earth. When the ice sheets melted, the sea level rose again. During the times of low sea levels the water may have receded far enough to expose peaks at the ocean floor and form islands, or maybe have receded even far enough to uncover the Atlantic ridge, a huge mountain range running across the floor of the Atlantic from Africa right up to the north of South America. Cichlids probably resembling those of the genera Hemichromis (our jewels) and Thylochromis moved along the coasts of this ridge, just like the monkeys and many other animals and plants did, and finally colonised South America. Again their nature of being a secondary freshwater fish would have helped them survive in the salty ocean water. By colonising one brackish river mouth after the other, eventually they would have spread to the northern tip of South America. Central America as we know it today did not exist at that time, rather it was a series of islands. Cuba and Hispaniola, the site of the oldest known American fossil cichlid, were closer to where Central America is today, before they moved eastwards to where they are today.

Fin Organism Underwater Fish Marine biology


And again we observe feeding specialisations and breeding specialisations amongst the many species that have evolved in the Americas. Within the genus Geophagus we see complete substrate spawners to mouthbrooders.

Ironically, now that we are starting to understand how such a large diversity was able to evolve our greatest challenge will be to preserve that diversity. Population explosion around the lakes and with it pollution and overfishing as well as habitat destruction in America are threatening our favourite fish. Our aquariums are increasingly becoming something of Noah's Arc, so we should treat our jewels with great care. They may yet become the last hope for the survival of these species.
Eye Plant Petal Tints and shades Font


This article was originally presented as a talk at the June 2001 meeting of the Cichlid Society of NSW, Australia.
 
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