Evolution, 4th Edition

508 CHAPTER 19

diversity increased. In a similar analysis, John Alroy found evidence that extinction

rates (E) were higher if diversity at the start of an interval was higher [1]. Moreover,

a high extinction rate in one interval was correlated with a high origination rate

in the following interval. These and other such analyses imply that the diversity of

taxa tends to be stabilized and approach an equilibrium.

Likewise, the tendency of LTT plots from phylogenies to level off with time,

as seen in the wood-warbler data (see Figure 19.17B), is generally interpreted to

mean that the earliest species in a new clade rapidly adapt to different resources or

environments (i.e., adaptive radiation), and that fewer subsequently formed spe-

cies can persist because fewer vacant ecological opportunities remain available.

Nevertheless, both the fossil record and some phylogenetic studies suggest that

diversity is still increasing, even though the rate slows down over time [48]. One

reason is that throughout the history of life, as we have seen, evolutionary innova-

tions have enabled clades to break through into new ecological modes of resource

use. And diversity promotes diversity. For example, an entire family of fishes (the

pearl fishes, Carapidae) lives inside sea squirts and sea cucumbers. Many plant lin-

eages in the American tropics have adapted to hummingbird pollination by evolv-

ing long, tubular flowers—but these are a resource for the flowerpiercers (Diglossa)

that “rob nectar” by biting through the base of the flower (see Fig ure 11.17). On

a global scale, moreover, diversification has undoubtedly been augmented by the

separation of Pangaea into separate land masses and by the greater temperature

gradient between low and high latitudes that developed during the Cenozoic [72].

Competition is generally thought to be the chief brake on increasing diversity.

The fossil record provides many instances in which the reduction or extinction of

one group of organisms has been followed or accompanied by the proliferation

of an ecologically similar group. For example, the diversity of rodents in North

America increased as that of the ecologically similar multituberculate mammals

declined (FIGURE 19.19) [36].

Two major hypotheses that involve competition can account for these patterns [7,

65]. One possibility is that the later clade caused the extinction of the earlier clade by

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_19.19.ai Date 12-12-2016

Taeniolabis Paramys

(A)

(B)

Multituberculate genera Rodent genera

1

1

1

3

5

50

30

29

28

4

Late Cret.

Paleo.

Eocene

Olig.

12

10

7

6

7

FIGURE 19.19 A likely case of competitive displacement in

the fossil record. (A) Many species of Multituberculata (left,

Taeniolabis), a nonplacental group of mammals that extended

from the Cretaceous to the Oligocene, were convergently

very similar to squirrels and other rodents (right, the Eocene

rodent Paramys). (B) The diversity of rodents in North America

increased as the diversity of multituberculates dwindled. The

width of a bar represents the number of species, indicated by

numerals. (A after [60]; B after [36].)

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