over time.
Very strong evidence for evolution
comes from the study of DNA—the
material that carries a living thing’s
genes. Genes pass along traits, or characteristics,
from one generation to the
next. The DNA of different species
shows that they developed from common
ancestors.
Natural Selection
Evolution results from a process called
natural selection. This process can happen
when an individual’s genes differ in
some way from the genes of others of its
kind. Variations, or differences, in genes
cause differences in an individual’s
traits—such as how it looks, its structure,
or its behavior.
These variations occur mostly by chance
as genes copy themselves to make new
Over millions of years horses grew in size.
At the same time they lost all but one of
their toes.
170 Evolution BRITANNICA STUDENT ENCYCLOPEDIA
cells. Mistakes, called mutations, sometimes
occur during the copying process.
Most changes in genes either harm the
individual or do not matter much. But
some changes help an individual to survive
and reproduce. For instance, an
animal may have better vision or faster
legs. These traits may help it find food
and avoid its enemies. A tree may have a
gene that causes it to make more seeds.
Such useful traits give the individual an
advantage over others of its kind. It will
be more likely to reproduce and pass its
genes along to the next generation. Over
time, more and more individuals will
have the helpful trait. In this way, living
things change over time.
Through natural selection species
become adapted, or better fit, to their
environments. For instance, a sandcolored
gerbil living in a sandy desert
blends in with its surroundings. Snakes
will be more likely to see and eat gerbils
of other colors. Over time the gerbils of
other colors will die out, leaving only
sand-colored ones.
History
The English scientist Charles Darwin
was the founder of the modern theory of
evolution. During a trip around the
world, he collected fossils and observed
the many variations among living things
on a group of islands. From these and
other studies, he came to believe that
new species developed from earlier
forms. In 1859 Darwin published his
theory of evolution in an important
book called On the Origin of Species. He
later wrote a book about the evolution
of humans.
In the early 1900s scientists used ideas
from the new science of genetics to
explain in detail how evolution happens.
Later advances have allowed scientists to
study DNA. These studies help scientists
determine how closely related various
species of living thing are to one
another.
#More to explore
Adaptation • Biology • Darwin, Charles
• DNA • Genetics • Human Origins
• Living Thing
Exercise
Exercise is a way of keeping the body
healthy through being active. Another
name for exercise is physical fitness.
Most people exercise in their spare time.
Others have jobs that involve exercise.
Sports are some of the most popular
ways to get exercise.
Longnose hawkfish have evolved to look
similar to the coral in their habitats. Their
color and shape help them to blend into
their surroundings.
Since Darwin’s
time many
more fossils
have been
collected,
including
ancestors of
modern
humans.
BRITANNICA STUDENT ENCYCLOPEDIA Exercise 171
Kinds of Exercise
The two main kinds of exercise are aerobic
exercise and anaerobic exercise.
Sports such as football and basketball are
great forms of exercise because they
combine both of these types.
Aerobic exercise is energetic movement
that lasts for a certain period of time.
During aerobic exercise the body combines
oxygen with sugar to get more
energy. The lungs and the heart work
harder to bring in extra oxygen. Swimming,
cycling, running, and rapid walking
are good examples of aerobic
exercise.
Anaerobic exercise is a powerful burst of
movement followed by a period of rest.
During anaerobic exercise the muscles
need more energy than the body can
make with its supply of oxygen. After
the exercise a person feels out of breath
because the muscles have worked without
oxygen.Weight lifting, jumping,
and sprinting are all types of anaerobic
exercise.
Health Benefits
Exercise affects every part of the body in
positive ways. It improves balance and
coordination. It also improves posture,
or the resting position of the body. Exercise
strengthens the heart and increases
the lungs’ ability to take in oxygen.
Through exercise the bones and muscles
get stronger, and the whole body
becomes trim and toned.
Exercise also gives people a greater feeling
of well-being. People who exercise
generally have a more positive, happy
attitude.
#More to explore
Sports
Exoskeleton
An exoskeleton is a hard covering that
supports and protects the bodies of
some types of animals. The word
exoskeleton means “outside skeleton.”
Many invertebrates, or animals without
backbones, have exoskeletons. Insects
are the largest group of invertebrates
that have exoskeletons.
Insects have exoskeletons made of a substance
called chitin. The exoskeletons of
crabs, lobsters, shrimp, spiders, ticks,
mites, scorpions, and related animals are
also made of chitin.
While exoskeletons are hard and stiff,
