Evolution__3rd_Edition

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(a)
Time
Character
Plant
Insect
Figure 22.4
Sequential evolution means that change in one lineage selects
for change in the other lineage, but not vice versa. Sequential
evolution would apply to plants and insects if plant evolution
influences insects, but insect evolution has little influence
on plants. The pattern of change in (a) lineages, and
. . . with cospeciation ...
. . . or sequential evolution ...
(b)
CHAPTER 22 / Coevolution 619
Plant phylogeny Insect phylogeny
(b) phylogenies differs from strict coevolution (compare with
Figure 22.2). (a) Changes in plants coevolve with changes in
insects, but changes (for some reason other than changes in
plants) in insects do not cause changes in plants. (b) When
plants speciate, so do insects, but when insects speciate it has
no effect on plants.
become geographically separated from each other. Each subpopulation of milkweed
might well have its own subpopulation of Tetraopes.
The two milkweed populations might diverge as they coevolved with their local
insects. One population of milkweeds might evolve one set of cardenolides, while the
other population evolved a different set of poisons. Each local beetle population would
evolve detoxification mechanisms appropriate for the local milkweeds. Reproductive
isolation would probably evolve as a by-product, by the classic process of allopatric
speciation (Section 14.3, p. 383). After a while, the two forms of plant might meet up
but be reproductively isolated because of the genetic differences that had built up
between the two.
Alternatively, cophylogenies can arise by sequential evolution. In sequential evolution,
changes in one of the two taxa lead to changes in the other, but not the other way
round. For instance, some biologists have argued that plants influence the evolution
of insects, but insects have less effect on evolution in plants (Figure 22.4). This could be
for a number of reasons. One is that many insects eat only one type of plant, whereas
plants are eaten by many insects. When a plant changes, its insects will all have to
change to keep up; but when one insect species changes, it alone will exert only a small
selective pressure on its food plant. Sequential evolution may result in imperfect
matches in the phylogenies of the two taxa (Figure 22.4b). In principle, the shape of the
phylogenies can be used to distinguish sequential evolution from coevolution. However,
cophylogenies are rarely perfectly matched, and many factors can influence the
degree of match between the phylogenies of two taxa. It is statistically difficult to distinguish
real coevolution from sequential evolution. The extent to which insect–plant