Evolution__3rd_Edition

616 PART 5 / Macroevolution
Full evidence for coevolution is hard
to obtain
Insect–food plant relations may
result from biochemical evolution
22.2 Coadaptation suggests, but is not conclusive
evidence of, coevolution
Coadaptations, such as those between an ant and a caterpillar, likely arise by coevolution
between the lineages leading to the two modern species. However, the observation
of coadaptation between two species is not by itself enough to confirm that the two
have coevolved together. Janzen (1980) pointed out that the two lineages could have
been evolving independently, and at some stage the two forms just happened to be
mutually adapted to each other. The ancestors of Glaucopsyche lygdamus might have
evolved their Newcomer’s organs for some other reason than feeding Formica and the
ants might have evolved antiwasp behavior patterns for some other reason than defending
caterpillars; when the two came together they were already coadapted. To demonstrate
coevolution requires showing not only that two forms are coadapted now but
also that their ancestors evolved together, exerting selective forces on each other.
That is a tall order. In practice, biologists tend to assume that interspecific coadaptations
are due to a long history of coevolution unless a convincing alternative hypothesis can
be put forward. Janzen’s stricture is logically correct, but difficult to live up to in practical
biology. Further evidence that a coadapted system arose by coevolution can come from
comparison with related species. The relation between G. lygdamus and Formica is not
unique. Lycaenids and ants have evolved a large number of relationships in different
species and this suggests the two groups have been evolving together for some time.
22.3 Insect–plant coevolution
22.3.1 Coevolution between insects and plants may have driven the
diversification of both taxa
In a paper that is perhaps the most influential modern discussion of coevolution,
Ehrlich & Raven (1964) listed the food plants of the main butterfly taxa. Each family of
butterflies feeds on a restricted range of plants, but these plants are in many cases not
phylogenetically closely related. Ehrlich and Raven explained the diet patterns mainly
in terms of plant biochemistry. Plants produce natural insecticides a chemicals like
alkaloids that can poison herbivorous (phytophagous) insects. Insects, in the manner
of pest species evolving resistance to artificial pesticides (Section 5.8, p. 115), may
evolve resistance to these chemicals, for instance by means of detoxifying mechanisms.
When a new detoxifying mechanism arises, it will open up a new array of food supplies,
consisting of all those plants that produce the now harmless chemical. The insects can
feed on them, and will diversify to exploit the resource. The result will be that each
insect group can feed on a range of food plants, the range being set by the capabilities of
the insect’s detoxifying mechanisms. The range of food plants will form a biochemical
group, but need not form a phylogenetic group because unrelated plants could use the
same defensive chemicals. Ehrlich and Raven’s pattern of butterfly–plant relationships
could arise as a result.
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