Evolution__3rd_Edition

634 PART 5 / Macroevolution
Carnivores and ungulates may have
evolved in an “arms” race
Evolution may be escalatory, ...
. . . as can be tested ...
. . . more, or less, powerfully ...
through time. In order to estimate encephalization in a fossil, it is necessary to estimate
body size, and the method used by Jerison has been criticized. The result is therefore
uncertain, but Jerison’s explanation is nevertheless interesting. He suggests that natural
selection has favored higher intelligence both in the prey, to escape predators, and in
predators, to catch prey. There has been a coevolutionary arms race between predator
and prey, leading to ever larger brain sizes in both. The selective forces might well be
truly coevolutionary, with each party exerting a reciprocal selective pressure on the
other: as predators become cleverer at catching prey, the prey are selected to avoid them
more intelligently, and vice versa. In this case, the evidence remains inconclusive.
However, it is difficult to demonstrate coevolutionary relations in fossils and Jerison’s
work is well worth noticing as a rare example in which coevolution is a plausible
explanation.
22.6.1 Coevolutionary arms races can result in evolutionary escalation
Vermeij (1987, 1999) has applied the same argument as was used by Jerison, but much
more generally. Vermeij suggests that predators and prey typically show an evolutionary
pattern that he calls escalation. By escalation, he means that life has become more
dangerous over evolutionary time: predators have evolved more powerful weapons and
prey have evolved more powerful defenses against them. Vermeij distinguishes escalation
from evolutionary progress. If evolution is progressive, organisms will become
better adapted to their surroundings through evolutionary time; if it is escalatory, the
improvement in predatory adaptations may be matched by improvements in prey
defenses, and neither ends up any better off. The two concepts are easy to distinguish by
a thought experiment. If evolution is progressive in predators (for example), then later
predators would be better at catching their prey than were earlier predators. If, however,
evolution is escalatory, later predators will be no better than their ancestors at
catching their contemporary prey types. But if transported in a time machine and set
loose on the prey hunted by ancestral predators, they should cut through them like a
modern jet fighter in a dog-fight with an early biplane.
Vermeij and his followers have identified both biogeographic and paleontological
evidence for escalation. Much of the evidence comes from mollusks in shallow-water
marine environments a mollusks are abundant as fossils, and the nature of the shell
itself can reveal how strongly adapted a species was, as a predator or a prey. More
strongly defended shells have properties such as general thickening, or thickening concentrated
around their apertures. Burrowing species, or those that cement themselves
down, are better defended that those that lie loose on the bottom surface.
Some simple indicators of escalation can be misleading, but advanced research is
needed to reveal the problems. Shell thickness, for instance, is usually a good indicator
of defensive adaptation. But Dietl et al. (2002) point out that two species with equal
shell thickness may differ in their degree of escalation if one grows faster than the other.
A species in which the shell grows more rapidly would be more escalated than a species
in which the shell grows more slowly. Dietl et al. estimated shell growth rates in fossils,
using the shell’s isotopic composition. However, information of this kind is rarely
available, and inferences about the broad patterns of escalation are based on more
..