Encyclopedia of Evolution.pdf - Online Reading Center

to detoxify them, because the toxins and the ability to detoxify
them are both often specific to certain groups of plants
and herbivores.
In coevolution, natural selection favors those individuals
within a population that most effectively resist, or
cooperate beneficially with, individuals of another species.
In many cases, coevolution has produced symbiotic relationships
between species (symbiosis), in which the life processes
of individuals within one species are closely linked
to those of individuals of another species (see table). The
following are examples, rather than a thorough analysis, of
coevolution.
Coevolution of Plants and Herbivores
Around the 1950s, ecologists such as Gottfried Fraenkel
began to ask, why is the world green? Insects are capable of
massive population growth, and by now they should have
eaten every bit of green leaf on the planet. Part of the reason
that this has not happened is that many insects die during the
winter, and their populations must grow again the following
spring. But even the winterless tropical forests are green.
Why? The answer lies with coevolution.
Plants produce a great variety of secondary compounds
in their tissues, especially their leaves. Secondary compounds
have no primary metabolic role in organisms; they usually
serve a defensive role instead. Many secondary plant compounds
are famous for the uses to which humans put them.
The active ingredients of spices, of many pharmaceuticals, as
well as many miscellaneous compounds such as caffeine are
all secondary compounds of plants. Some of the chemicals,
such as tannins, are digestion inhibitors, while many others
are mildly to strongly poisonous. In some cases, plant secondary
compounds can work in ways other than as toxins. Compounds
in the needles of some conifers function as inhibitors
of insect growth, so that insects that eat them cannot reach
maturity. Certain members of the nightshade family of plants
produce compounds that aphids mistake for predator alarm
signals; this scares the aphids away. Most plants have evolved
chemical defenses against herbivores. Because of coevolution,
the world is not a big salad bowl.
Plants usually produce the greatest amounts of secondary
compounds in their leaves, as the stems are tougher (and
often defended by thorns or spines), while roots are underground
where herbivorous animals are fewer. Leaves need
to be relatively free of defensive spines, and to be somewhat
soft, because they have to release water and oxygen into, and
absorb carbon dioxide and light from, their environment.
Some plants have relatively little toxic defense against
herbivores. These plants are usually the ones that can grow
quickly after being eaten. Grasses, for example, have few
toxic compounds in their leaves, but they grow back quickly
from underground structures after being grazed. Grass leaves
have high levels of silicon dioxide, which makes them tough.
Only specialized grazing animals can chew and digest them.
Other plants get by without secondary compounds because
they live for only a short time and vanish from the scene after
reproducing.
Types of Symbiotic Interactions
coevolution
Effect Effect on
Type of interaction on host other species
Parasitism Harmful Beneficial
Commensalism Unaffected Beneficial
Mutualism Beneficial Beneficial
Chemical defenses are costly. Some plants may produce
fewer chemical defenses in years of abundant rainfall, when
they can more easily afford to grow new leaves to replace
those lost to herbivores. In some cases, plants wait until
the herbivores begin to eat them, then they undergo a rapid
response and produce toxins. In some cases, plants respond to
drought by producing chemicals that make them more toxic.
They may respond to drought and to herbivory by some of
the same reactions. In all these and other ways, herbivores
have influenced plant evolution.
Herbivores have evolved ways of eating plant tissue
despite these defenses. Some animals consume massive quantities
of herbage, get what little nutrition they can from it,
and evacuate the rest. These animals (such as caterpillars,
koalas, and sloths), deriving little energy from their food, are
generally lethargic. Other animals can tolerate the toxins of
specific plants, and even specialize upon them. The caterpillar
of the monarch butterfly (Danaus plexippus) eats milkweeds
(genus Asclepias), which have chemicals that interfere
with heartbeats, but they sequester the poisons in special
sacs. They even use the chemical for their own defense: Having
stored up the milkweed poison, they are now themselves
poisonous to their predators. Caterpillars of Heliconius
butterflies similarly use toxic compounds from the passionflower
vines (genus Passiflora) upon which they feed.
Humans eat tissues of plants as spices. The fact that spices
contain potent chemicals prevents humans from eating very
much plant tissue. The human attraction to spices may have
evolved because some spices have antimicrobial activity. In
all these and other ways, plants have influenced herbivore
evolution.
Plants and herbivores are participating in what has
been called an arms race, in which increased plant defense
selects for increased ability of the herbivore to tolerate or
to avoid the plant defenses; and increased herbivore success
selects for increased plant defense. Since plant-herbivore
coevolution occurs over a long time period, the only
thing similar to experimental confirmation is that on some
islands plants do not have strong defenses. On Hawaii,
which has no native browsing mammals, there is a stingless
bush nettle.
Evolution of defense against herbivores has occurred separately
in every evolutionary lineage of plants. Therefore hardly
any two plant defenses are the same. So vast is the diversity
of plant chemical defenses that these chemicals are sometimes
used to assist in the evolutionary classification of plants.