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Evolution__3rd_Edition

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644 PART 5 / Macroevolution<br />

Adaptive radiations ...<br />

. . . occur at all levels, ...<br />

. . . and many examples are known<br />

They occur in several circumstances<br />

23.1 The number of species in a taxon increases during<br />

phases of adaptive radiation<br />

The diversity of life through time reflects the rates of loss and gain of new life forms.<br />

The loss of species is by extinction, and the gain of species is by speciation. When the<br />

speciation rate exceeds the extinction rate for a taxon, its diversity increase. When the<br />

extinction rate exceeds the speciation rate, its diversity decreases. We can begin by<br />

looking at periods when the number of species in a taxon increases during adaptive<br />

radiations. An adaptive radiation (often just called a “radiation”) means that a small<br />

number of ancestral species in one taxon diversifies into a larger number of descendant<br />

species, occupying a broader range of ecological niches. Adaptive radiations can occur<br />

at all taxonomic levels, and on all geographic scales. In a sense, the proliferation of life<br />

on Earth from the origin of life to the present is an adaptive radiation on the largest<br />

scale. However, adaptive radiations are particularly clear when they occur in a relatively<br />

small taxonomic group, in a confined geographic area.<br />

We have seen several examples of local adaptive radiations earlier in this book. On a<br />

small scale, we saw in Section 17.5 (p. 502) how lizards have undergone radiations on<br />

Caribbean islands, with a similar set of ecological forms evolving independently many<br />

times as different islands have been colonized. Darwin’s finches on the Galápagos are<br />

another example where a single ancestral species has evolved into 13–14 species with a<br />

range of ecological adaptations (Figure 21.2, p. 595). On a larger scale, the fruitflies<br />

have radiated on the Hawaiian islands into hundreds of species (Figure 15.27, p. 465).<br />

In the East African lakes, the cichlid fish have also evolved into hundreds of species<br />

(Section 13.3.3, p. 357). Many other examples exist. In Lake Baikal in Russia, aquatic<br />

invertebrates, and particularly crustaceans, have radiated into unknown hundreds of<br />

species. On Madagascar, there has been a radiation of lemurs a a taxon of primates that<br />

differs from the primates elsewhere on Earth.<br />

Adaptive radiations can also be seen over wider geographic areas. Figure 23.1a shows<br />

an increase in the number of species of mammals in North America over the past<br />

80 million years. The number increased rapidly after 65 million years ago. This is the<br />

early Tertiary radiation of mammals, which occurred after the extinction of the<br />

dinosaurs. The mammals in Figure 23.1a are eutherians, but the marsupial mammals<br />

also underwent a distinct radiation, across the land masses of Gondwanaland. The<br />

marsupial mammals in the south, and the eutherian mammals in the north, radiated<br />

into a similar set of ecological forms.<br />

Radiations occur in a number of circumstances.<br />

1. Colonization of a new area where there are no competitors. The radiation of fruitflies,<br />

lizards, and finches on island chains, and of cichlid fishes in African lakes, all occurred<br />

after ancestral species colonized the respective areas. Probably no competitors were<br />

present, because the islands had only recently emerged from the sea, or the lakes had<br />

only recently been flooded. The areas all contained unexploited resources, and the<br />

ancestral species radiated into a range of forms that could exploit those resources.<br />

2. Extinction of competitors. The radiation of mammals followed the extinction of the<br />

dinosaurs. The dinosaur extinction vacated ecological space that was then occupied<br />

by mammals. We return to this topic in Section 23.7.3 later in the chapter.<br />

..

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