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
Troutt Visual Services
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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