Evolution__3rd_Edition

644 PART 5 / Macroevolution
Adaptive radiations ...
. . . occur at all levels, ...
. . . and many examples are known
They occur in several circumstances
23.1 The number of species in a taxon increases during
phases of adaptive radiation
The diversity of life through time reflects the rates of loss and gain of new life forms.
The loss of species is by extinction, and the gain of species is by speciation. When the
speciation rate exceeds the extinction rate for a taxon, its diversity increase. When the
extinction rate exceeds the speciation rate, its diversity decreases. We can begin by
looking at periods when the number of species in a taxon increases during adaptive
radiations. An adaptive radiation (often just called a “radiation”) means that a small
number of ancestral species in one taxon diversifies into a larger number of descendant
species, occupying a broader range of ecological niches. Adaptive radiations can occur
at all taxonomic levels, and on all geographic scales. In a sense, the proliferation of life
on Earth from the origin of life to the present is an adaptive radiation on the largest
scale. However, adaptive radiations are particularly clear when they occur in a relatively
small taxonomic group, in a confined geographic area.
We have seen several examples of local adaptive radiations earlier in this book. On a
small scale, we saw in Section 17.5 (p. 502) how lizards have undergone radiations on
Caribbean islands, with a similar set of ecological forms evolving independently many
times as different islands have been colonized. Darwin’s finches on the Galápagos are
another example where a single ancestral species has evolved into 13–14 species with a
range of ecological adaptations (Figure 21.2, p. 595). On a larger scale, the fruitflies
have radiated on the Hawaiian islands into hundreds of species (Figure 15.27, p. 465).
In the East African lakes, the cichlid fish have also evolved into hundreds of species
(Section 13.3.3, p. 357). Many other examples exist. In Lake Baikal in Russia, aquatic
invertebrates, and particularly crustaceans, have radiated into unknown hundreds of
species. On Madagascar, there has been a radiation of lemurs a a taxon of primates that
differs from the primates elsewhere on Earth.
Adaptive radiations can also be seen over wider geographic areas. Figure 23.1a shows
an increase in the number of species of mammals in North America over the past
80 million years. The number increased rapidly after 65 million years ago. This is the
early Tertiary radiation of mammals, which occurred after the extinction of the
dinosaurs. The mammals in Figure 23.1a are eutherians, but the marsupial mammals
also underwent a distinct radiation, across the land masses of Gondwanaland. The
marsupial mammals in the south, and the eutherian mammals in the north, radiated
into a similar set of ecological forms.
Radiations occur in a number of circumstances.
1. Colonization of a new area where there are no competitors. The radiation of fruitflies,
lizards, and finches on island chains, and of cichlid fishes in African lakes, all occurred
after ancestral species colonized the respective areas. Probably no competitors were
present, because the islands had only recently emerged from the sea, or the lakes had
only recently been flooded. The areas all contained unexploited resources, and the
ancestral species radiated into a range of forms that could exploit those resources.
2. Extinction of competitors. The radiation of mammals followed the extinction of the
dinosaurs. The dinosaur extinction vacated ecological space that was then occupied
by mammals. We return to this topic in Section 23.7.3 later in the chapter.
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