Evolution, 4th Edition

504 CHAPTER 19

significant correlation supports the hypothesis that the ability to eat plants has

promoted diversification. More elaborate subsequent analyses by other authors

supported Mitter and colleagues’ conclusion that herbivory has increased insect

diversity, in part because species diverge in diet, becoming specialized on differ-

ent plant species [22, 28, 77]. The diversity of many clades of plants, conversely,

has been enhanced by effective defenses against herbivores. Groups that evolved

rubbery latex (as in milkweeds) or resin (as in pines), both of which deter attack

by herbivorous insects, are more diverse than their sister groups [16], and diversi-

fication rates have been twice as high in the many plant families with extrafloral

nectaries than in families that lack this feature [76]. Extrafloral nectaries are sugar-

secreting organs that attract mutualistic arthropods, such as ants, that protect the

plant against herbivores (FIGURE 19.14).

In some cases, higher diversity seems to result simply from factors that increase

the probability or rate of speciation. For example, many groups of birds and other

terrestrial organisms are very diverse in mountainous regions, where allopatric spe-

ciation rates are elevated due to isolation of populations on different slopes [18]. The

nectar spurs on the petals of columbines are associated with greater diversification,

probably because they enabled different species to use morphologically different

insects and birds as pollinators, which would contribute to reproductive isolation

(FIGURE 19.15). Probably for the same reason, plant clades with bilaterally symmet-

rical flowers have had higher speciation rates than those with radially symmetrical

flowers (see Figure 16.4) [56]. Of course, different diversity-enhancing factors may

act in combination. The phenomenal diversification of cichlid fishes in the large

lakes of eastern Africa (see p. 213) is mirrored by smaller radiations in many other

African lakes. Comparing the cichlids in many lakes, Catherine Wagner and col-

leagues found that species numbers were enhanced both by the availability of habi-

tat (measured by lake depth) and by characteristics associated with sexual selection,

such as sexual dimorphism in coloration (FIGURE 19.16) [75].

In these examples, there is a correlation between a trait and the rate at which

the number of species increases or decreases. A consistent difference of this kind

is termed species selection, and results in certain characteristics becoming more

prevalent than others, among all species taken together.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_1914.ai Date 11-02-2016

(A) (B)

FIGURE 19.14 wo kinds of plant defenses against herbivores that have increased T

species richness. (A) Extrafloral nectaries (nectar-producing glands) on the leaf petioles

in some leguminous trees (here Acacia) attract bodyguards such as ants. (B) Milkweeds

(Asclepias) are among the many groups of plants that produce latex, a sticky liquid that

deters many herbivores (but not this milkweed leaf beetle, Labidomera clivicollis).

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