Encyclopedia of Evolution.pdf - Online Reading Center

nesses of resource acquisition and war (see intelligence,
evolution of). In many cases, these explanations were
and are believable. Tools certainly have enhanced the ability
of humans to exploit natural resources, and complex social
interactions would be impossible without language (even
though chimpanzee societies are quite complex despite the
lack of true language). In other cases, the explanations sound
like just-so stories. Why should humans have evolved hairlessness,
when other hot-blooded mammals of the African
savanna have kept their hair?
Even the believable explanations seem insufficient. Consider,
for example, language, which some scientists consider
the major adaptation that distinguishes human from animal
intelligence (see language, evolution of). Theories proposed
early in the 20th century attempted to explain the origin
of language in terms of practical natural selection. What
should have been obvious even at that time is that languages
are far more complex than they need to be for such simple
practical requirements. Just how complex does language have
to be in order to say “Lion!” or “Time for lunch!”?
Perhaps the best explanation for the origin of language
is that it was an indicator of the quality of intelligence and
of the brain. People who could not just talk but speak with
complex grammar and poetical allusions obviously had good
brains. Many other characteristics of human intelligence,
such as the production of art and music, serve as clear indicators
of the quality of the brain and could therefore function
as sexual fitness indicators. Evolutionary biologist Geoffrey
Miller explains that the human brain itself is a fitness indicator,
since it is such an expensive organ (consuming tremendous
amounts of energy and oxygen), and at least half of the
genes are involved in brain development. Therefore practically
anything that the brain does can be a fitness indicator.
As such, it can be the subject of sexual selection. By choosing
intelligent mates, both men and women could be reasonably
certain that their mate had good genes.
Another example of a characteristic often considered to
have evolved for its practical benefits is hunting. The practical
explanation of hunting is that men bring home the meat
while women gather nuts, berries, and grains. While plant
materials may provide most of the calories, the meat brought
home by the men provides essential nutrients that would be
missing from a vegetarian diet. Even if it is true that a purely
vegetarian diet would be inadequate, this does not explain
why men hunt big game in nearly every tribe that has been
studied and continue to do so today in civilized nations. Men
could provide adequate meat by hunting small game. In fact,
they could provide more meat in this manner, and with considerably
less danger to themselves. Simply put, hunting is not
a practical way for a man to provide for his family. It appears
that hunting, in all societies, has been and is a sport. Its purpose
is to be a fitness indicator—only very strong males with
good brains can show off their qualities by undergoing the
physical exertion and having the intelligence to track down,
or trick, dangerous game animals, and then kill them. By
choosing good hunters, the women could be reasonably sure
that their mates were strong and intelligent.
sexual selection
Many human characteristics, not the least of which
is the large human brain, are part of a pattern called neoteny
in which juvenile characteristics are retained into adulthood.
This explains many unique human features as part of
a suite of neotenous traits but does not explain why neoteny
occurred in the first place. What, if anything, caused neoteny
to operate during human evolution? It is possible that the
retention of juvenile characteristics was selected because they
made older people look younger, thus deceptively enhancing
fitness indicators of youth in both sexes?
Sexual Selection in Plants
Mate choice, resulting in sexual selection, can occur in organisms
other than animals. Evolutionary biologists Mary Willson
and Nancy Burley pointed out that male plants, or the
male function of bisexual plants, compete with one another in
terms of sperm or pollen production, and that female plants,
or the female function of bisexual plants, may be able to discriminate
among sperm or pollen. Male competition in plants
occurs for the same reason that it does in animals: A plant
can produce more sperm, or more pollen, than a female plant
can produce eggs, or ovules; and the female plant, or female
function within the plant, not only produces eggs or ovules
but also nourishes the resulting embryo, often within a seed,
and provides for its dispersal, often within a fruit. Instances
of sexual selection in plants may include the following:
• Male competition can cause plants to undergo male reproduction
earlier than female reproduction, since the first
male plant to produce sperm or pollen can fertilize numerous
female plants. Botanist Stanley Rice and his students
have shown that in cottonwoods (Populus deltoides),
which have separate male and female trees, the male trees
open their flower buds earlier, on the average, than female
trees—although of course their flowering times must overlap
or else pollination would fail. Male trees that open earlier
may pollinate many female flowers before other male
trees get a chance to do so. This could be interpreted as
sexual selection among male trees.
• Plants do not copulate. Pollen must be carried from the
male parts of one plant to the female parts of another.
Plants rely on the wind, or on many species of pollinators,
to accomplish this. The plants that more effectively attract
pollinators are more effective at sexual reproduction. All
of the characteristics of plants related to pollination (see
coevolution) can be considered the products of sexual
selection. In animal-pollinated dioecious plant species, in
which some plants are male and others are female, male
flowers are usually larger and more colorful than female
flowers. This can be interpreted as male-male competition
for sperm delivery and therefore sexual selection.
• Once a flower has been pollinated, the pollen must grow a
tube down into the flower, making contact with an ovule,
which may be a great distance away. In maize, for example,
the tube from a pollen grain must grow up to eight
inches (20 cm), a thousand times the length of the pollen
grain, to reach the ovule. The sperm nucleus then descends