Where two fields meet

A teacher of mine once simplified his complex family history by saying that he, like all of us, originated from Olduvai Gorge in Tanzania (the 'cradle of mankind'). Tropical Africa has been a cauldron of diversity not only for our own species. It is, to take one example, surprisingly fishy. The Great Lakes of East Africa (Figure 1.1), and surrounding rivers, contain a whopping 1500 species in just one fish family, the cichlids, familiar to freshwater aquarium enthusiasts. This makes cichlids the most species-rich family of vertebrates, beating such diverse and familiar groups as songbirds and mice. They are so diverse that many still await proper scientific description, and many more are doubtless completely undiscovered. Lakes Victoria and Malawi each contain about 500 species, and about 250 species are found in Lake Tanganyika. Diversity of this sort is what makes our planet such an interesting place, and of course, we have to find out what caused it.

The cichlid species of the East African lakes have not each immigrated there from the surrounding habitat; they were born there, and in most cases they are endemics, being found in just one of the lakes (Fryer and Iles 1972). They are a 'radiation' of species. This radiation is all the more remarkable when the ages of the lakes are considered. Lake Tanganyika is the oldest (but has fewest species) at about 10 million years. Lake Malawi, the second oldest is a mere 1-2 million years old. Lake Victoria, amazingly, may have been completely dry around 14,500 years ago, the end of the last ice age. Since then, 500 cichlid species have been born. If species arose in a clockwork linear fashion, that would mean one new species of fish every 29 years!

The varied lifestyles of the fish are equally impressive. In Lake Victoria, for example, have been found cichlids with the following diets: adult fish, fish larvae, fish scales, fish parasites, freshwater snails, insect and other invertebrate larvae, plant and animal plankton, algae growing on rocks, and vascular plants, all with specialized jaws to match (Figure 1.2). The most impressive radiations have occurred among the 'haplochromine' cichlids living on rocky shores in Lakes Victoria and Malawi (Kocher 2004). Clearly, we need to know how so many species could have formed in such a short time span, why it happened here, why cichlids, and why haplochromines most of all? At stake is our understanding of species richness itself.

Fig. 1.1 Seen from space, the Great Lakes of the East African Rift Valley are major landscape features.

The two largest ones here are Lake Victoria (right) and Lake Tanganyika (bottom)—Lake Malawi is off the bottom of the picture. Lake Victoria is about 300 km across and its northern tip is on the equator. Photo from the NASA Visible Earth image archive. Black lines indicate national boundaries.

Fig. 1.1 Seen from space, the Great Lakes of the East African Rift Valley are major landscape features.

The two largest ones here are Lake Victoria (right) and Lake Tanganyika (bottom)—Lake Malawi is off the bottom of the picture. Lake Victoria is about 300 km across and its northern tip is on the equator. Photo from the NASA Visible Earth image archive. Black lines indicate national boundaries.

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