LIVING THINGS
ARE MADE OF CELLS.

THE CELLS CONTAIN
A SET OF INSTRUCTIONS

THAT DETERMINE HOW THE PLANT
OR ANIMAL WILL LIVE AND GROW.

THESE INSTRUCTIONS
ARE INSIDE EACH CELL

IN BITS OF MATERIAL
CALLED GENES.

THE YOUNG OF MOST PLANTS
AND ANIMALS

RECEIVE HALF OF THEIR GENES
FROM THE MOTHER

AND HALF FROM THE FATHER.

THIS MEANS EACH NEW PLANT
OR ANIMAL RESEMBLES ITS PARENTS

BECAUSE IT INHERITS
THEIR CHARACTERISTICS

FROM THEIR GENES.

BUT BECAUSE THE YOUNGSTER
HAS A COMBINATION OF GENES

FROM BOTH PARENTS,

IT IS NOT EXACTLY LIKE
ITS FATHER OR ITS MOTHER.

 

NOTICE HOW THE EARS
ON THIS CAT CURL BACKWARD.

SOMETIMES, WHEN THE GENES
ARE BEING FORMED,

THEY ARE ALTERED.

THIS CAN CAUSE A CHANGE
IN THE NEW PLANT OR ANIMAL.

THAT'S WHAT CAUSED
THE CURLED EARS OF THIS CAT.

THIS TYPE OF CHANGE
IS CALLED MUTATION.

IF PEOPLE BELIEVE
A SUDDEN CHANGE SUCH AS THIS

IS AN IMPROVEMENT,
THEY MAY BREED THOSE ANIMALS

TO CONTINUE THE MUTATION
IN FUTURE GENERATIONS;

HOWEVER, MUTATIONS DO NOT
USUALLY PRODUCE AN IMPROVEMENT.

 

CHANGE CAUSED BY A MUTATION
IS MORE OFTEN HARMFUL

THAN IT IS HELPFUL.

OCCASIONALLY, HOWEVER,

MUTATIONS
CAN BE VERY BENEFICIAL.

RED DELICIOUS APPLES
ARE THE RESULT OF A MUTATION.

 

SO PLANTS AND ANIMALS
ARE DIFFERENT THAN THEIR PARENTS

BECAUSE OF THE COMBINATION
OF THEIR PARENTS' GENES

AND BECAUSE OF ANY MUTATIONS
THAT MAY HAVE HAPPENED

BECAUSE OF ALTERED
OR DAMAGED GENES.

 

A PERSON WHO WANTS TO DEVELOP
A NEW KIND OF PLANT OR ANIMAL

WILL BREED ONLY
THOSE INDIVIDUALS

THAT HAVE THE CHARACTERISTICS
THEY WANT.

TO GET A SMALLER HORSE,
FOR EXAMPLE,

PEOPLE WILL MATE ONLY
THE SMALLEST HORSES AVAILABLE.

THIS WILL, OVER TIME, PRODUCE
SMALLER AND SMALLER OFFSPRING.

MAKING CHANGE
HAPPEN IN THIS WAY

IS ARTIFICIAL SELECTION.

PEOPLE HAVE USED
ARTIFICIAL SELECTION

TO DEVELOP
NEW KINDS OF PETS,

FARM ANIMALS,

PLANTS,
AND CROPS.

THE CHANGES MADE
BY ARTIFICIAL SELECTION

OVER THE LAST FEW THOUSAND YEARS
HAVE BEEN VERY SMALL

COMPARED TO THE CHANGES
MADE BY EVOLUTION.