Encyclopedia of Evolution.pdf - Online Reading Center

coevolution
Coevolution of Organisms and Predators
Predators are animals that kill entire organisms for food.
Usually their prey is other animals, but animals that eat seeds
are usually called seed predators because a seed contains an
entire plant.
The evolution of predators has been overwhelmingly influenced
by their search for prey. They tend to have greater intelligence,
sensory acuteness, and agility than prey. Some predators
have evolved pack behavior. The evolution of the prey has
been just as strongly influenced by the need to avoid or resist
predators. In some cases, this involves sensory acuteness, agility,
and herd behavior just as it does with predators. Some prey
form large herds in which the members can watch out for one
another; in some flocks of birds and schools of fish, the coordinated
and sudden movements of the members can disorient
and confuse the predators. In some cases, the prey animals
have evolved to simply become too big for most predators to
be able to handle. The evolution of predators constantly influences
the evolution of prey, and vice versa, because individuals
that are slower, less intelligent, or less able to function as part
of the group are more likely to be eaten or to not eat.
Seed predators have evolved the ability to find, and
sometimes store and retrieve, seeds. The fact that seed predators,
such as squirrels, fail to retrieve all the seeds they store
is what makes them effective dispersers of seeds to new locations;
as such they often constitute a net benefit to the plant
some of whose seeds they eat. Seeds are a rich source of food,
for predators as well as for the embryonic plants inside of
them. Seed plants have evolved, in turn, to resist seed predators,
at least the ones like weevils that offer them no benefit.
Many seeds are mildly or strongly toxic. For example, while
the apple fruit is delicious and nutritious, the seeds contain
cyanide. This encourages the evolution of animals that will
eat the apple but pass the seeds unharmed through their
digestive tracts, obtaining nutrition from the fruit but not
from the seeds. Many fruits have a laxative effect on animals,
which encourages the animals to evacuate the seeds before
they die in the intestines. Perhaps the most interesting way
in which plants have evolved in response to seed predators
is mast seeding. Many trees produce large seed crops every
few years, and few seeds at other times. In this way, the seed
predator populations do not build up; when the trees produce
a huge (mast) crop, the predator populations cannot consume
them all. Not only has coevolution occurred between plants
and the animals that both eat and disperse their seeds, but it
has produced a radiation of species within each. Many pines
have seeds with wings that blow in the wind, but some have
wingless seeds that are stored, and eaten, by crows and jays.
A proliferation of both pine and jay species occurred in the
Miocene epoch (see Tertiary period), which some evolutionary
scientists attribute to coevolution between them.
Coevolution of Flowering Plants and Pollinators
Pollination is necessary for sexual reproduction in seed plants
(see sex, evolution of). The pollen must be carried from the
male organs (male cones of conifers, or stamens of flowers) to
the female organs (female cones of conifers, or the pistils of
flowers) (see gymnosperms, evolution of; angiosperms,
evolution of). The pollen lands upon the surface of the
female cone, or the stigma of the flower’s pistil, and grows a
tube down to the immature seed.
In the ancestors of flowering plants, pollen was carried
from one plant to another primarily by the wind. Modern
conifers continue to rely on wind pollination. The earliest
flowering plants relied upon animals (usually insects) to carry
pollen from one plant to another. Very soon after the evolution
of these early insect-pollinated flowers, some flowering
plants reverted to wind pollination, and many flowering
plants today rely on wind pollination. The evolution of wind
pollination is not coevolution, since the wind is not an organism.
Flowers that rely on wind pollination cannot control
where their pollen goes, and they have characteristics such as
the following:
• Their petals are reduced or absent, as petals would only get
in the way of the transportation of pollen by the wind from
stamens of one flower to pistils of other flowers.
• They produce massive amounts of pollen, since most of the
pollen misses its target.
• The stigmatic surfaces are large, to increase the chance of
the pollen hitting its target.
• The flowers of many wind-pollinated trees, such as oaks,
open in the early spring, before leaves of deciduous plants
emerge; otherwise, leaves would slow down the wind, and
pollen might stick, uselessly, to the leaves.
In contrast, flowers that are pollinated by animals have
characteristics such as the following:
• Their petals and nectar attract the animals.
• They produce less pollen, since animals can carry the pollen
directly to another flower.
• The stigmatic surfaces are not usually large.
• Their flowers may open at a later time during the growing
season.
Flowers often attract pollinators with a variety of
rewards: nectar is high in calories; pollen is a high-protein
food; in some cases, flowers in cold climates offer a warm
spot (by concentrating sunlight and blocking wind) for the
pollinators to get in from the cold. In some cases, flowers
produce extra pollen, which is sterile and intended specifically
to feed the pollinators.
In many cases, flower characteristics attract and offer
rewards to specific kinds of pollinators. Red tubular flowers
(such as those of trumpet creepers, Campsis radicans) attract
hummingbirds, whose long beaks and tongues fit into the
tube, while they exclude animals that are too large to crawl
into the tube and do not have long tongues (for example,
bumblebees). Hummingbirds may have evolved a preference
for red because it helped them find flowers, and the flowers
may have evolved red pigment production because it brought
pollinators to them. In contrast, red flowers do not strongly
attract bees, which cannot see red. Moths and butterflies
have long tongues and often visit tubular flowers. Some flowers
have closed petals and a landing platform; only large bees