Evolution__3rd_Edition

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Figure 22.2
Coevolution means that two
separate lineages mutually
influence each other’s
evolution. The two lineages
tend to (a) change together, and
(b) speciate together. Lineages
1 and 2 could be, for example,
an ant lineage and a lycaenid
butterfly lineage.
Interspecific relations can undergo
coevolution
Time
(a)
Lineage 1 Lineage 2
Character
(b)
CHAPTER 22 / Coevolution 615
Group A Group B
evolved in relation to each other. Natural selection probably favored mutually adapted
changes in each species, after the ancestors of the two species had become associated.
Changes in one species, such as to increase honeydew production, would favor changes
in the other (to increase protection) as the caterpillars became more beneficial to the
ants. This kind of reciprocal influence is what is meant by coevolution: each species
exerts selection pressures on, and evolves in response to, the other species. The two
lineages evolve together (Figure 22.2). In all examples of evolution, a species evolves
in relation to changes in its environment. Coevolution refers to the special case of
evolution in which the species’ environment is itself evolving.
We look at several examples of coevolution in this chapter. In some, like the ants and
caterpillars, coevolution promotes the mutual benefit of the coevolving lineages. In
others, such as parasite–host coevolution, the process is antagonistic. An improvement
in one party (such as improved host defense) is a deterioration in the environment of
the other party (the parasites).
Table 22.1
Caterpillars of the lycaenid butterfly Glaucopsyche lygdamus are more likely to be parasitized if
they are not tended by ants. Ants were experimentally excluded from some caterpillars, and the
rate of parasitism on these and untreated control caterpillars was measured. The two sites are
in Gunnis County, Colorado. The parasites were wasps and flies, and n is the sample size.
Reprinted, by permission, from Pierce & Mead (1981). © 1981 American Association for the
Advancement of Science.
Caterpillars without ants Caterpillars with ants
Site % parasitized n % parasitized n
Gold Basin 42 38 18 57
Naked Hills 48 27 23 39
1
2
3
4
5
i
ii
iii
iv
v