Natural Selection

•A species is a group of organisms that can mate
with one another to produce fertile offspring. A
characteristic that helps an organism survive and
reproduce in its environment is called an
adaptation.

•Scientists observe that species have changed over
time. The process in which populations gradually
change over time is called evolution.

•The remains or imprints of once-living organisms
found in layers of rock called fossils.

•By studying fossils, scientists have made a timeline
of life known as the fossil record. The fossil record
organizes fossils by their estimated ages and
physical similarities.

 Use
the words adaptations, evolution, and
population in a sentence.

 How do adaptations work?

 Can
you tell whether two organisms are the
same species just by looking at them?

Upright walking, hair, fingerprints, binocular vision,
and speech are all traits that almost all humans
have in common.

 List the advantages and disadvantages of each
trait.

 Do you think the advantages are greater than the
disadvantages? Why or why not?

Record your responses in your notebook.

• List four sources of Charles Darwin’s ideas about
evolution.

• Describe the four parts of Darwin’s theory of
evolution by natural that gradual processes had
changed the Earth’s surface over selection.

• Relate genetics to evolution.

After Charles Darwin graduated from college, he

served as naturalist on a ship called the HMS
Beagle. During a voyage around the
world, Darwin collected thousands of plant and
animal samples.

Darwin noticed that the finches of the Galápagos

Islands were a little different from the finches in
Ecuador. And the finches on each island differed
from the finches on the other islands

• Ideas About Breeding The process in which
humans select which plants or animals to reproduce
based on certain desired traits is called selective
breeding.

• Ideas About Population Only a limited number of
individuals survive to reproduce. Thus, there is
something special about the offspring of the
survivors.

 Thomas Malthus: wrote a book stating that
humans have the potential to reproduce
rapidly, eventually overrunning the food
supply

 Thisled Darwin to realize that all species can
reproduce rapidly, those who survive must
pass on traits to their offspring to help them
survive

 Charles Lyell: Earth is old, allowed evolution
to be a slow gradual process

• What Is Natural Selection? Darwin proposed the
theory that evolution happens through a process
that he called natural selection. Individuals that are
better adapted to their environment survive and
reproduce more successfully than less well adapted
individuals.

• Genetics and Evolution Today, scientists have
found most of the evidence that Darwin lacked.
They know that variation happens as a result of
differences in genes.

Write the four parts of natural selection, and create
a mnemonic device to remember each part by using
the first letter of the words.

Write your response in your notebook.

• Give three examples of natural selection in action.

• Outline the process of speciation.

• Adaptation to Hunting People hunt elephants for
their tusks. As a result, fewer of the elephants that
have tusks survive to reproduce, and more of the
tuskless elephants survive.

• Insecticide Resistance A few insects in a
population may be naturally resistant to a chemical
insecticide. These insects pass their resistance trait
to their offspring, and an insect population gradually
becomes resistant to the insecticide.

Many species have so much competition for

mates that interesting adaptations result. For
example, the females of many bird species prefer
to mate with males that have colorful feathers.

• Sometimes, drastic changes that can form a new
species take place. The formation of a new species
as a result of evolution is called speciation.

• Separation Speciation often begins when a part
of a population becomes separated from the rest.

• Adaptation Populations constantly undergo
natural selection. After two groups have separated,
natural selection may act on each group in different
ways.

Division Over many generations, two separated

groups of a population may become very different
until the point when they can no longer mate with
one another. At this point, the two groups are no
longer the same species

Standardized Test Preparation
Chapter C5

Passage 2 You know from experience that individuals
in a population are not exactly the same. If you look
around the room, you will see a lot of differences
among your classmates. You may have even noticed
that no two dogs or two cats are exactly the same. No
two individuals have exactly the same adaptations. For
example, one cat may be better at catching mice, and
another is better at running away from dogs.

Continued on the next slide

Chapter menu Resources

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Chapter C5

Passage 2, continued Observations such as these
form the basis of the theory of natural selection.
Because adaptations help organisms survive to
reproduce, the individuals that are better adapted to
their environment are more likely to pass their traits
to future generations.



Chapter C5

1. In the passage, what does population mean?
A a school
B some cats and dogs
C a group of the same type of organism
D a group of individuals that are the same



Chapter C5

2. In this passage, which of the following are given as
examples of adaptations?
F differences among classmates
G differences among cats
H differences between cats and dogs
I differences among environments



Chapter C5

3. Which of the following statements about the
individuals in a population that survive to reproduce
is true?
A They have the same adaptations.
B They are likely to pass on adaptations to the next
generation.
C They form the basis of the theory of natural
selection.
D They are always better hunters.



Chapter C5

Interpreting Graphics
The graph below shows average beak sizes of a group
of finches on one island over several years. Use the
graph to answer the questions that follow.



Chapter C5

1. In which of the years
studied was average beak
size the largest?
A 1977
B 1980
C 1982
D 1984



Chapter C5

2. If beak size in this group of
birds is linked to the amount of
rainfall, what can you infer about
the year 1976 on this island?
F The year 1976 was drier than
1977.
G The year 1976 was drier than
1980.
H The year 1976 was wetter
than 1977.
I The year 1976 was wetter than
1984.


Chapter C5

3. During which year(s) was
rainfall probably the lowest on
the island?
A 1978, 1980, and 1982
B 1977, 1980, 1982, and 1984
C 1982
D 1984



Chapter C5

4. Which of the following statements best summarizes
this data?
F Average beak size stayed about the same except
during wet years.
G Average beak size decreased during dry years and
increased during wet years.
H Average beak size increased during dry years and
decreased during wet years.
I Average beak size changed randomly.



Chapter C5

Math
Read each question, and choose the best answer.

Average Beak Measurements of Birds
of the Colores Islands
Island Ave. beak Ave. beak No. of unique
length (mm) width (mm) species
Verde 9.7 6.5 5
Azul 8.9 8.7 15
Rosa 5.2 8.0 10



Chapter C5

1. What is the ratio of the number of species on Verde
Island to the total number of species on all three of the
Colores Islands?
A 1:2 Average Beak Measurements of Birds
of the Colores Islands
B 1:5 Island Ave. beak Ave. beak No. of unique
length (mm) width (mm) species
C 1:6
Verde 9.7 6.5 5
D 5:15
Azul 8.9 8.7 15
Rosa 5.2 8.0 10



Chapter C5

2. What percentage of all bird species on the Colores
Islands are on Rosa Island?
F approximately 15% Island No. of unique
species
G approximately 30%
Verde 5
H approximately 50% Azul 15
Rosa 10
I approximately 80%



Chapter C5

3. On which of the islands is the ratio of average beak
length to average beak width closest to 1:1?
A Verde Island Island Ave. beak Ave. beak
length (mm) width (mm)
B Azul Island
Verde 9.7 6.5
C Rosa Island
Azul 8.9 8.7
D There is not enough Rosa 5.2 8.0
information to
determine the answer.



Chapter C5

4. On which island does the bird with the smallest
beak length live?
F Verde Island Island Ave. beak Ave. beak
length (mm) width (mm)
G Azul Island
Verde 9.7 6.5
H Rosa Island
Azul 8.9 8.7
I There is not enough Rosa 5.2 8.0
information to
determine the answer.



Natural Selection

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