In the Rift Valley of eastern Africa, just south of the equator, lie three great
lakes and many smaller ones. Lakes Tanganyika and Malawi are deep and old,
having been formed by the separation (rifting) of two continental plates. Lake
Victoria, in contrast, is broad and shallow, lying in a basin that was dry only
15,000 years ago. These lakes harbor a few species of catfishes, spiny eels,
and other fish families, but more than 90 percent of all the fish species are
cichlids, a family that includes species well known to tropical fish hobbyists.
Lake Tanganyika has at least 250 species of cichlids, Lake Victoria between
450 and 530 species, and Lake Malawi at least 480 species [89]. (The American
Great Lakes, in comparison, have only about 175 species of fishes, of all kinds.)
These cichlid fishes are extraordinarily diverse in coloration, form, feeding
habits, and habitat use (FIGURE 9.1). Different species eat insects, snails, detri-
tus, rock-encrusting algae, aquatic plants, phytoplankton, zooplankton, baby
fishes, and larger fishes. Some species are specialized to feed on the scales
of other fishes, and one has the gruesome habit of plucking out other fishes’
eyes. The teeth of some closely related species differ more than do those of
some whole families of fishes. Many of these habits and morphologies have
evolved convergently in the different lakes [45]. Phylogenetic analyses show
that the 250 cichlids in Lake Tanganyika have evolved from at most 16 original
species. The cichlids of Lake Victoria have multiplied faster than any other
group of vertebrates on Earth: the 450-plus species evolved from just 5 origi-
nal ancestral species in perhaps only 15,000 years [95, 105].9
A male gray tree frog inflates his vocal sac as he calls to attract females. Female frogs re-
spond almost exclusively to their own species’ calls, which are a barrier to interbreeding.
Male calls differ between two morphologically indistinguishable species of gray tree
frogs in eastern North America. Hyla chrysoscelis has 12 pairs of chromosomes, whereas
H. versicolor is a tetraploid, with 24 pairs.Species and Speciation
Allopatric speciation
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