Childrens Illustrated Encyclopedia

197

NATURAL SELECTION
Charles Darwin wrote a book called On the
Origin of Species, published in 1859, which
explained his theory of evolution.
Many people laughed at
Darwin’s idea that humans
were related to animals.
Above is a cartoon of
the time, picturing
Darwin as
a monkey.

AROUND 150 YEARS AGO, an English naturalist named Charles Darwin

shocked the world when he wrote a book suggesting that humans were related

to apes. Today, Darwin’s idea still forms the basis of what we call the theory of

evolution. The word evolution means “unfolding,” and it is used to describe the

way that all living things evolve, or change with time. There are three main parts

to the theory. The first is called variation. All living things vary in size, shape,

color, and strength. No two animals or plants are exactly the same. The second

part of the theory is that these variations affect whether a living thing can

survive and breed. Certain features, such as color, may mean that one animal or

plant has a better chance of surviving than another. Some animals and plants

have features that suit their surroundings. In other words, they are better

adapted, and these useful features are called adaptations. The third part of the

theory is inheritance. The adaptations that help a living thing survive, such as its

color or shape, may be passed on to its offspring. If the offspring inherit the

adaptations, they too will have a better chance of survival. Gradually, over

many generations, the better-adapted plants and

animals flourish, and those that are less well

adapted die out. Many people believe that

this process of evolution has led to

the millions of different

species that inhabit

Earth today.

EVIDENCE FROM THE PAST

Fossils—the remains of animals and plants
preserved in rocks—provide evidence for

evolution. They show how animals and
plants have gradually changed through time.

For example, each of the elephants shown
above lived for a certain amount of time, as

we know by the age of their fossilized bones.
Scientists cannot be certain that the first

type of elephant gradually evolved into the
next, but it is unlikely that each elephant

appeared completely separate from the
others. It is far more likely that these

elephants were related. As we find more
fossils, the relationships between various

kinds of animals and plants become clearer.

There are 28 species of honeycreepers on

the Hawaiian Islands. Scientists believe

they evolved from one species of bird.

Evolution 197-

EVIDENCE FROM THE PRESENT

Animals and plants alive today also

provide evidence for evolution.

In Hawaii, there are several

kinds of honeycreepers

that look similar. It is

unlikely that this is by chance.

More likely, these different

honeycreeper birds all evolved

from one kind of honeycreeper.

This first honeycreeper flew to

the islands five million years ago.

Since that time, natural selection

has produced several similar,

but separate, species.

Kona finch has

strong bill for

crushing seeds.

Maui parrotbill uses

lower bill for chiseling

into wood for insects.

Iiwi beak and tubular tongue

are suited to sipping nectar.

Kauai akialoa has

long beak for probing

for insects.

Trilophodon lived from 26

to three million

years ago.

Original species

of honeycreeper

Platybelodon lived

from 12 to 7 million

years ago.

Woolly mammoth

lived about two

million years ago.

Moenitherium lived about

38 million years ago.

Evolution of the elephant

African

elephant

of today

Akiapolaau

searches for

insects with

upper bill.

Apapane

has useful

all-around

beak.