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74 Nature

Evolution

Living things gradually change over many generations in a process called

evolution. This allows organisms to fully adapt to their surroundings, and

gives rise to new species, while others may become extinct. The driving

force for evolution is. NATURAL SELECTION.

NATURAL SELECTION

Living things produce more offspring than they need to replace

themselves. Only the few that are well adapted to their particular

environment will survive. Thanks to this natural selection, the

features needed to survive are passed on to their offspring and

become more widespread, so a species gradually evolves.

HOW DOES A WELL-ADAPTED INDIVIDUAL SURVIVE?

Certain individuals have features that let them compete

better for food, water, shelter, or mates. These fitter

individuals are more likely to survive, breed, and pass

on their advantageous features to their offspring.

In this way, natural selection makes it possible for

only the fittest to survive. Naturalist Charles Darwin

believed that this is how species evolve over time.

1 RED CRAB SURVIVAL RACE

This forest-dweller releases many

tiny eggs into the sea. After a

month in the water, they race

to the forest. Many die en route.

WHAT IS THE EVIDENCE FOR EVOLUTION?

Evolutionary clues are found in fossils, the ancient

remains of organisms. Creatures alive today share

some characteristics with fossil ancestors but have

evolved new ones as well. Fossils also show that

millions of living and extinct species evolved from a

few simple organisms that lived billions of years ago.

HOW DO NEW SPECIES EVOLVE?

A species is a group of similar living things, such

as lions, that can breed together. If a group of

individuals within a species becomes separated from

all the other groups of that species, they can no longer

breed with them. In time, the group evolves separately

and becomes increasingly different. Eventually, it is so

different that it becomes a new species.

SPURGE CACTUS

HOW LONG DOES EVOLUTION TAKE?

Evolution is a continuous process of change. In a stable

environment, evolution can cause change, which can

be very rapid in small organisms, while in others it

may take thousands of years.. HUMAN EVOLUTION

from an ape-like ancestor took millions of years and

gave rise to several different species, not just our own.

1 CONVERGENT EVOLUTION

This is the evolution of similar

features in unrelated species living

in similar conditions. Spurge and

cactus are unrelated plants with

thick, spiny water-holding stems

to help them survive in dry places.

Phiomia was as big as a modern

horse and had pillar-like legs

and a short trunk. Sharp

tusks were used for

fighting and

gathering food

Moeritherium, an early

elephant, had a bulky body,

short legs, and a long upper lip.

It lived in Africa, and is thought

to have bathed in lakes and

rivers and fed on water plants

MOERITHERIUM

50 MILLION YEARS AGO

PHIOMIA

35 MILLION YEARS AGO

CHARLES DARWIN

English, 1809–1882

Between 1831 and 1836

naturalist Charles Darwin

sailed around the world,

describing the animals and

plants he saw. This trip

convinced him that living

things evolved, even though

most scientists believed that

they remained the same. In

1859, he published his theories

in On the Origin of Species.

evolution