Three of the largest lakes in the world reside in the 3,700 mile long Rift Valleys of Africa. These lakes, Lakes Victoria (located in the countries of Kenya, Uganda and Tanzania), Tanganyika (split between Burundi, the Democratic Republic of the Congo [DRC], Tanzania, and Zambia), and Malawi (located between Malawi, Mozambique and Tanzania), rank among the top ten largest lakes in the world; third, seventh, and ninth, respectively. (For some reason, these rankings differ upon where you look. Lake Victoria as the third largest lake seems pretty universal, but the ranks of Tanganyika and Malawi differ for whatever reason. You'd think that it would be pretty universal, but I suppose not.) Besides being such large lakes, these lakes are important for other reasons, perhaps the most important reason (for biologists, at least) being their isolation.
You see, the three great lakes are islands, of a sort. According to Websters, the definition of an island is "a land mass smaller than a continent and surrounded by water." The Rift Valley lakes are essentially the opposite; "a body of water smaller than an ocean and surrounded by land." For our purposes, an island is just something that has been isolated for a time, and allowed its flora and fauna to flourish in new and interesting ways.
And flourish it did in the Rift Valley lakes. If you were to visit these lakes, snorkel or scuba in their waters, you would most likely notice a wide variety of fish. You would not be wrong in this assessment; however, you might be surprised to find that most of these types of fish are belonging to a group called the cichlids (SICK-lids), and that all 1,650 plus species of these cichlids descended from a common ancestor. While the dates of when the common ancestor of the various cichlids came to be trapped in their respective lakes, it has been estimated that the cichlids of Lake Malawi all evolved from a common ancestor trapped 700,000 years ago, and those of Lake Victoria around 12,000 years ago. Trapped in these growing lakes while they were being formed, this small group of fish quickly came to dominate their new home, exploding in biodiversity to adapt to the wide variety of niches left open to them.
Tenuous can this biodiversity be, as recent logger-based erosion has shown. Logging nearby to one of the lakes resulted in rapid erosion. This erosion caused a great deal of silt to build up in one of the lakes. This, of course, caused the water to become quite murky; think about your average beach, and how murky the water often is near the shore. Many of the cichlids that lived in this area relied upon visual identification to recognize members of the same species for mating purposes. With the cloudy, muddled water, this became quite difficult. As a result, many fish from closely related species ended up mating with each other and, in the case of many of the couplings, resulted in viable offspring. These viable offspring in turn bred with other species, lowering, at least temporarily, the biodiversity of the cichlids in this particular corner of the lake.