Evolution__3rd_Edition

636 PART 5 / Macroevolution
Figure 22.14
(a) Total number of gastropod
subfamilies through time.
(b) Proportion of subfamilies
with members that have
evolved internally thickened
or narrowed apertures, a
character that probably evolved
as a defense against predators.
Because the total number of
subfamilies has increased, it is
necessary (as here) to plot the
proportion, not the total number
of subfamilies, in order to show
a trend. Dots marked with
numbers represent that number
of subfamilies. Redrawn, by
permission of the publisher,
from Vermeij (1987).
. . . and fossil plant–insect relations
Percentage Number
200
150
100
50
0
30
20
10
0
(a) Gastropod subfamilies
(b) Gastropoda with internally thickened or narrowed apertures
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(4)
(3)
(2)
(4)
(11) (14)
(42)
(38)
(35)
(43)
L. Camb. Ord. Sil. Dev. Carb. Perm. Tri. Jur. E. Cret. L. Cret. Cenozoic
Time interval
proportionally increased through time (Figure 22.14). In all, more recent mollusks
appear to be more strongly defended than were their earlier ancestors.
Some trends that look like escalation may be due to other factors. For example, Wilf
& Labandeira (1999) counted the frequency of damage inferred to be caused by insects
in fossil leaves. Paleocene leaves have lower levels of insect damage than Eocene leaves.
In the samples, 29% of Paleocene leaves had insect damage, increasing to 36% in Eocene
leaves. Plant–herbivore relations look more “escalated” in the Eocene. However, Wilf
and Labandeira attribute the trend to warming temperatures. The Eocene was warmer
than the Paleocene, and plants suffer more from herbivory when it is warmer. (Leaf
damage is greater at the tropics than the poles today.) Although plant–herbivore relations
came to be more dangerous in the Eocene, the reason may have been external climatic
change rather than coevolutionary escalation between plants and insects.
Vermeij’s evidence would not be persuasive enough to convince a skeptic. The
evidence is not abundant; it is noisy; the patterns are not all consistent; alternative
interpretations are sometimes possible; and the fossil record may be biased. However,
in the absence of any particular argument that the trends are due to sampling biases, we
can give the evidence the benefit of the doubt. It does then suggest that some escalation
of predator–prey relations has occurred during evolution.
Escalation is a widely influential idea about macroevolution. It features again in the
next section of this chapter. It also underlies many hypotheses about the Cambrian
explosion (Section 18.4, p. 535). The diversification of animals with skeletons during
the Cambrian has been attributed to the origin of predators, or more dangerously
armed predators. This is an example of a hypothesis that invokes escalation. In the next
chapter, we see how escalation may help us understand taxonomic replacements
(Section 23.7.2, p. 670) and trends in species diversity (Section 23.8, p. 674).
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(19)
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..