Evolution, 4th Edition

498 CHAPTER 19

Futuyma Kirkpatrick Evolution, 4e

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(A)

(B)

Total extinction rate (families per My)

20

15

10

5

0

0

600 500 400 300

Time (Mya)

200 100 0

0.2

0.4

Rate of genus origination

0.6

0.8

1 (C)

0

600 500 400 300

Time (Mya)

200 100 0

0.2

0.4

Rate of genus origination

0.6

0.8

1

(D)

0

350 300 250200 150

Time (Mya)

100 50 0

0.2

0.4

Rate of family origination

0.6

0.8

(^1) (E)
0
450 375 300 225
Time (Mya)
150 75 0
0.2
0.4
Rate of family origination
0.6
0.8
1
C O S D C P Tr J K Pg Ng
500 400 300 200 100 0
Time (Mya)
Paleozoic Mesozoic Cenozoic
FIGURE 19.7 Rates of extinction and
origination of lineages have declined over
geological time, as shown by two stud-
ies. (A) Extinction rates of marine animal
families, expressed as the number of
families per My. The solid regression line
fits the blue points, which represent fewer
than 8 extinctions per My, and which are
interpreted as background extinction. The
red points mark the five major mass extinc-
tion events, at the end of the Ordovician
(O), Devonian (D), Permian (P), Triassic (Tr),
and Cretaceous (K). The extinction rates
are given as absolute numbers of extinc-
tions, not per capita rates, and thus do
not control for differences in diversity at
different times. (B–E) Rates of origination of
genera in two phyla of animals in the sea,
Mollusca (B) and Echinodermata (C), and of
families in two terrestrial groups, insects (D)
and nonflowering vascular plants (E). The
origination rates are expressed as the pro-
portion of lineages in a geological stage
that originated (are first seen) in that stage.
(A after [58]; B–E after [12b].)
Rates of extinction (E) and origination (S) are correlated (FIGURE 19.8); trilobites, for
example, were more volatile than gastropods, turning over more rapidly.
Why are extinction and origination rates correlated? Steven Stanley suggested
that certain features of organisms influence both rates [70]. First, ecologically spe-
cialized species of mammals and other groups show higher extinction rates than
generalized species [43, 68] because they are more vulnerable to changes in their
environment (see Chapter 11) [35]. They may also be more likely to speciate because
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