Childrens Illustrated Encyclopedia

198

The ichthyosaur is an extinct reptile. Its paddle-like
front limb had many small bones.

The dolphin is a mammal. Its paddle has the typical bones
of the mammal
arm and hand.

EVOLUTIONARY TREE

Scientists believe that all living things are related and that

they have evolved from the same ancestors over millions

of years. This chart is called an evolutionary tree. It has

lines between the main groups of animals and plants alive

today, showing which ones are most closely related.

CONVERGENT EVOLUTION

Evolution sometimes makes different animals and plants look similar. This is called
convergent evolution. It means that different animals or plants that live in the same

environment, such as the sea, gradually take on the same adaptations, such as body
shape. All the animals shown above have evolved, or developed, the same streamlined

body form, because this is the best shape for moving speedily through water.

CHANGING ENVIRONMENTS

As the environment changes, living

things evolve. About 200 years ago

in Britain, peppered moths had

mostly light-colored wings that

matched the light-coloured tree

trunks where they rested, so birds

could not see them easily. During

the Industrial Revolution, smoke

from factory chimneys made the

tree trunks darker in some areas.

Light-colored moths became easier

to see. Gradually, more dark-colored

moths evolved, which were better

camouflaged on the dark tree trunks.

HOW EVOLUTION OCCURS

Imagine some green frogs,
living and breeding in green

surroundings. Most of the young
inherit the green coloring of their

parents. They are well camouflaged,
and predators do not notice them

in the grass. Their green colour is
an adaptation that helps them

survive. A few of the young have
different colours, because of

variation. Predators can see
them in the grass, and these

frogs are soon eaten—this is
natural selection at work. Then

the environment slowly changes
to yellow as the grass dies. Now

the green frogs show up on
the sand, and predators eat

them. Gradually, the following
generations of frogs change from

mainly green to mainly yellow.
A new species has evolved.

Green, grassy

environment

Predators easily

spy frogs that

are not

camouflaged.

Yellow, sandy

environment

New yellow

species of frog

has evolved.

Green frogs survive

because they are

well camouflaged.

Other frogs get

eaten because they

show up so much.

Birds

Amphibians

Bony fish

Echinoderms

Coelenterates

Sponges

Bacteria

Protists

First simple organisms

Brown

algae

Red

algae

Mosses

Ferns

Conifers

Flowering

plants

Reptiles

Mammals

Segmented

worms

Mollusks

Brachiopods

Fungi

Green

algae

Flatworms

Arthropods

The penguin is a bird that cannot fly. It has the typical bird’s wing bones in its paddle. Find out more

Animals

Darwin, charles

Dinosaurs

Fossils

Geology

Prehistoric life

Prehistoric peoples

EVOLUTION