Evolution__3rd_Edition

620 PART 5 / Macroevolution
. . . or cospeciation without
coevolution
A beetle group and its food plants
do not form cophylogenies
The insects colonize plants
according to biochemical similarity
evolution is sequential, or fully coevolutionary, is an active research topic that has
reached no generally accepted conclusion.
Finally, cophylogenies may arise if two taxa have no evolutionary influence on each
other, but some independent factor leads to speciation in both. For example, allopatric
speciation could occur in several non-interacting taxa if they occupy much the same
range and something splits all their ranges. A river might cut through and divide the
ranges of several species in an area, for instance. All the taxa might speciate at the same
time, but not because of coevolution.
In Section 22.5.2 below we look some more at cophylogenies, for parasite–host relationships,
and meet a fourth process that can lead to cophylogenies.
22.3.3 Cophylogenies are not found when phytophagous insects
undergo host shifts to exploit phylogenetically unrelated but
chemically similar plants
Insects and plants can coevolve without producing mirror-image phylogenies. Becerra
(1997), for example, studied coevolution between plants in the genus Bursera and the
specialist chrysomelid beetles of the genus Blepharida that feed on them. Bursera is a New
World genus of the family Bursaraceae, a tropical family of trees and shrubs that is famous
for its aromatic resins, which are used in perfume and incense. Frankincense and myrrh
are two Old World relatives of Bursera. Figure 22.5a shows the phylogenies of some
associated Bursera and Blepharida. They are not cophylogenies. Becerra also analyzed
the chemical defenses of Bursera and found that they fall into four main groups; but
species with the four systems are found scattered around the phylogeny (Figure 22.5b).
The relations between the plant and its insects becomes clear when we look at the
chemical defenses of the plants in relation to the beetle phylogeny (Figure 22.5c). Now
the beetle phylogeny has a clear pattern, with four distinct regions, corresponding to
the four chemical defenses of the plants. The caption explains further how the information
in Figure 22.5c can be used to make sense of the initially confusing patterns in
Figure 22.5a.
Probably what has happened is that during evolution a beetle species evolves a
defense against a certain set of plant chemicals. The beetles can then colonize other
plants that have similar chemical defenses. These colonizations, in which a beetle shifts
from one host plant to another, are called host shifts. If chemically similar plants are not
phylogenetically closely related, the result is that the beetles evolutionarily jump
around the plant phylogeny. Although Bursera and Blepharida are coevolving, they do
not have cophylogenies. The pattern in Figure 22.5 illustrates Ehrlich & Raven’s (1964)
ideas discussed in Section 22.3.1 above.
In summary, the phylogenies of two interacting taxa, such as flowering plants and
insects, can be used to study coevolution. A cophylogeny alone is not strong evidence of
coevolution, because coevolution may or may not produce cophylogenies and coevolution
is not the only factor that can cause cophylogenies. However, cophylogenies and
the deviations from them can be used (in combination with other forms of evidence) to
tease apart the coevolutionary forces in the history of two taxa.
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