Encyclopedia of Evolution.pdf - Online Reading Center

can push their way into the flower. In these and many other
examples, coevolution has produced characteristics that benefit
both species: the pollinators have evolved the ability to
find and exploit these flowers as food sources, and the flowers
benefit from being pollinated.
In other cases, flowers attract pollinators by deception.
Many species of flowers have dark reddish petals and produce
the putrid scent of rotten meat, which attracts flies as pollinators.
A few species of orchids attract male wasps by imitating
the appearance and chemical characteristics of female wasps;
the male wasps attempt to mate with the flower, but succeed
only in either getting pollen stuck to it from stamens, or rubbing
pollen off onto the stigma. In these examples, the flowers
benefit, but the insects do not. It is questionable whether
these adaptations were produced by coevolution. Natural
selection favored flowers that most closely imitated dead
meat or female wasps; however, the evolution of the flies and
wasps was primarily influenced by the search for food and
mates, not by the flowers.
Whether or not the pollinator benefits from the relationships,
the pollination relationships just mentioned
are specific. Only certain wasp species will respond to the
orchid that fakes the female wasp. While the specificity is
not always absolute (hummingbirds sometimes drink nectar
from flowers that are not red, and bees sometimes visit red
flowers), it does assure that, in most cases, the animal carries
pollen from one flower to another flower of the same
species. An indiscriminate animal, carrying pollen from or to
a flower of a different species, would not benefit either of
the plants, in fact it would not even be a pollinator. Coevolution
that leads to specific pollination relationships, rather
than indiscriminate ones, helps the plants transfer their pollen
more efficiently and may help the pollinators find a more
reliable food source.
In other cases flowers are open to a wider variety of pollinators.
Many flowers are shallow, their stamens and pistils
open to any insect that can fly or crawl in. In these cases, the
plant population may be locally dense, so that pollen is seldom
carried to the wrong species, or the pollinators are relatively
faithful to one species even when not forced to be by
the flower structure.
Once coevolution had begun between flowering plants
and insects, 120 million years ago (see Cretaceous period),
it resulted in an astonishing diversification of flowering
plants. A floral innovation that allowed even a slight change
in which insects pollinated it would cause an almost immediate
isolation of the new form from the rest of the population
(see isolating mechanisms), starting it on the road to
becoming a new species of flowering plant. This is probably
the reason that each flowering plant family usually contains
a considerable variety of growth forms and environmental
adaptations, but all of its species have similar flowers: The
family itself began its evolution as a partner in a new coevolutionary
pollination relationship. Today there are nearly a
quarter million species of flowering plants. Hymenopterans
(bees and wasps) and lepidopterans (butterflies and moths)
had existed since the Paleozoic era, and their diversification
coevolution
was not stimulated as much by flowers as the diversification
of flowers was stimulated by them.
Coevolution Leading from Parasitism to Commensalism
The preceding discussion focused on how coevolution influenced
general relationships among groups of species. Coevolution
has also produced symbiotic species relationships in
which one species depends upon another.
Parasitism occurs when individuals of one species (the
parasite) obtains its living from individuals of another species
(the host). Some parasites are occasional visitors, such
as mosquitoes; some, like lice, may live on the host a long
time; some, like intestinal worms, live inside the host; microbial
parasites live among, even in, the host cells. Parasites
have a negative impact on the host, either by consuming it,
consuming some of its food (as when intestinal worms harm
the host’s nutrition), by producing toxic waste products (as
do many bacterial and viral parasites such as cholera and
bubonic plague), or by causing the host’s own immune system
to have harmful effects on the host. The evolution of parasites
has been strongly influenced by the hosts. For example,
the life cycles of fleas that live on rabbits are coordinated
with hormone levels in the rabbits’ blood, which ensures that
the young fleas hatch at the same time as the baby rabbits
are born. The evolution of the hosts has been influenced by
parasites; the very existence of immune systems may be considered
proof of this.
In contrast, a commensalistic relationship is one in which
the commensal obtains its living from the host but the host is
indifferent to its presence. (This is why, according to some
biologists, commensalism is not true symbiosis but represents
an intermediate condition between parasitism and mutualism.)
The human body contains trillions of commensalistic
microorganisms, primarily on the skin and in the digestive
tract; in fact, there are 10 times as many bacteria as there are
human cells in the human body. The commensals consume
materials that the body does not need; the worst thing they
do is to produce disagreeable odors. They are commensalistic
not out of goodwill toward humans, but because the body’s
defenses keep them in check. People with compromised
immune systems (such as from HIV) may be infected, and
killed, by microbes that would otherwise have been commensalistic
(see AIDS, evolution of).
Humans are surrounded by commensals. A mattress can
have two million microscopic mites eating the oils and skin
flakes produced by their human hosts. They are so small they
can go right through pillowcases and sheets. One-tenth of the
weight of a six-year-old pillow may consist of skin flakes, living
mites, dead mites, and mite dung. As shown in the photo
on page 82, mites are found in the hair follicles of nearly
every human being, no matter how thoroughly they wash.
Coevolution may select parasites that are more effective,
and hosts that have adaptations that will limit, resist,
or eliminate the parasite. If the host species’ evolution is successful
in causing the parasite to become harmless to it, the
relationship is no longer parasitism but commensalism. Similarly,
in many but not all cases, the parasites actually benefit