Evolution__3rd_Edition

..
Pollinator relationships have led
to the evolution of specialized
adaptations ...
. . . that may have promoted
insect–angiosperm diversity
CHAPTER 22 / Coevolution 617
In turn, natural selection on the plants favors the evolution of improved insecticides.
Plant–insect coevolution should therefore consist of cycles, as plant groups are drawn
into, and removed from, the diets of insect groups, and the insects evolutionarily
“move” between plant types according to their biochemical abilities. The biochemical
arms race between plants and insects should persistently favor new mechanisms on
both sides, and might therefore have promoted the diversification of insects and
angiosperms (Section 14.10.2, p. 412, and Figure 18.8, p. 539). In Ehrlich & Raven’s
(1964) words “the fantastic diversification of modern insects has developed in large
measure as the result of a stepwise pattern of coevolutionary stages superimposed on
the changing pattern of angiosperm variation.”
Coevolution between plant poisons and insect detoxification mechanisms is only
one way in which insects and flowering plants may have influenced each other’s evolution.
Pollination is another example. A few gymnosperms are pollinated by insects, but
insect pollination really took off with the evolution of flowers in angiosperms. Plants
without flowers are mainly pollinated by abiotic mechanisms, such as the wind.
Once insect pollination had evolved, natural selection could favor increasingly specialized
pollinator relations. In any one flower species, natural selection favors those
flowers whose pollen is transported only to other flowers of the same species. If the
insect flies to another flower species, the pollen is more likely to be wasted. A flower
may put its nectar reward in a place that can only be reached by insects with a specialized
organ, such as a long tongue. Only insects with long tongues can then obtain the
reward a and those insects will be well rewarded because they have little competition
from other insects. The insects with the specialized adaptation will probably next fly to
another flower of the same type, because it will be well rewarded there too. The process
can continue, as the plant places its nectar deeper and deeper, and the insects evolve
longer and longer tongues. The final result could be something like the Madagascan
orchid Angraecum sesquipedale, which puts its nectar in long spurs, up to 45 cm in
length. Darwin knew of this species and predicted that a specialist pollinator would be
discovered with a long tongue. Wasserthal (1997) recently confirmed that several
hawkmoth species, with exceptionally long tongues, are able to obtain nectar from and
pollinate this orchid (see the cover illustration of this book). 1
As natural selection favors specialized pollinator relationships, it will tend to
increase the diversity of both plants and insects. Plants that are pollinated by a single
insect species have an advantage, because less of their pollen is wasted. Insects that
specialize on one plant species will make more efficient use of their specialized feeding
adaptations. Other factors can also operate in the coevolution of plants and insects,
but the two factors we have looked at here, diet and pollination, illustrate the general
subject. The theoretical ideas have been tested in many ways, but one particularly active
method at present is to use phylogenies.
1 The evolutionary story may be more complex. Wasserthal (1997) found that pollinators with long tongues
are less vulnerable to predation. They can feed on a flower without landing, and thereby avoid predation from
spiders that sit on the flower and catch insects that land there. The hawkmoths could have evolved long
tongues as an antipredator adaptation, perhaps while feeding on unrelated plants. The orchids may have
evolved their long spurs to make use of hawkmoths that had already evolved long tonges pollinating other
species. Wasserthal’s argument illustrates how hard it is to distnguish coadaptation due to coevolution from
coadaptation following unrelated histories in the two lineages.