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Changes Over Time ■ Adapted Reading and Study
Darwin’s Theory (pages 172–179)
Darwin’s Observations (page 173)
Key Concept: Charles Darwin’s important observations
included the diversity of living things and the remains of
ancient organisms.
• In 1831, Charles Darwin left from England on a ship that
made many stops along the coast of South America.
Darwin’s job was to learn about the living things he saw.
• Darwin saw many plants and animals that he had never
seen before. The living things he saw in South America
were all very different from those in England.
• A species is a group of similar living things that can mate
with each other and produce offspring.
• Darwin saw fossils in South America. A fossil is the
preserved remains of an organism that lived long ago.
Answer the following questions. Use your textbook and the
ideas above.
1. Is the following sentence true or false? Darwin saw that
living things in South America were just like those in
England.
2. Read each word in the box. In each sentence below, fill
in the correct word or words.
fossil
living thing
species
a. The preserved remains of an organism that lived
long ago is a
b. A group of similar living things that can mate
and produce offspring is a
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Changes Over Time ■ Adapted Reading and Study
Galápagos Organisms (pages 174–175)
Key Concept: Darwin’s important observations
included the characteristics of organisms on the
Galápagos Islands.
• Darwin saw that the plants and animals on the
Galápagos Islands were much like those on the
mainland of South America.
• The Galápagos plants and animals did have some very
important differences from those on the mainland. For
example, Galápagos iguanas had larger claws than
mainland iguanas.
• Darwin thought that some plants and animals came to
the Galápagos Islands from the mainland. Eventually, the
offspring of these plants and animals became different
from their mainland relatives.
• Darwin also saw that tortoises and finches were
different from one Galápagos island to the next.
Finches, for example, had different beak shapes.
• Beak shape is an example of an adaptation. An
adaptation is a trait that helps an organism survive and
reproduce.
Answer the following questions. Use your textbook and the
ideas above.
3. Is the following sentence true or false? Darwin thought
that some plants and animals came to the Galápagos
Islands from the mainland.
4. What is an adaptation? Circle the letter of the correct
answer.
a. a difference between two animals
b. a trait that helps an organism survive and
reproduce
c. a way for plants and animals to get to an island
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Changes Over Time ■ Adapted Reading and Study
Evolution (page 176)
Key Concept: Darwin reasoned that plants or animals that
arrived on the Galápagos Islands faced conditions that
were different from those on the mainland. Perhaps,
Darwin hypothesized, the species gradually changed over
many generations and became better adapted to the new
conditions.
• The gradual change in a species over time is called
evolution.
• Darwin concluded that the living things that came to the
Galápagos Islands from the mainland had changed over
time. The living things changed so that they could live better
in the island environment.
• Darwin’s ideas are often called the theory of evolution. A
scientific theory is a well-tested idea.
Answer the following questions. Use your textbook and the
ideas above.
5. Is the following sentence true or false? The Galápagos
plants and animals changed over time so that they
could live better in the island environment.
6. Read each word in the box. In each sentence below, fill in the
correct word or words.
adaptation
evolution
scientific theory
a. A well-tested idea is a(an)
b. The gradual change in a species over time is
called
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Changes Over Time ■ Adapted Reading and Study
Natural Selection (pages 177–179)
Key Concept: Darwin proposed that, over a long time,
natural selection can lead to change. Helpful variations
may gradually accumulate in a species, while unfavorable
ones may disappear.
• Darwin suggested that evolution happens because of natural
selection. In natural selection, individuals that are better
adapted to their environment are more likely to survive and
reproduce.
• Factors that affect natural selection are overproduction,
competition, and variation.
• Most species produce far more offspring than can possibly
survive. Overproduction makes it more likely that some
offspring will survive.
• Food and other resources are limited. Members of a species
must compete with each other for these resources. Some
members of a species may not find enough to eat, so they
do not survive.
• Any difference between individuals of the same species is
called a variation.
• Some variations make individuals better adapted to their
environment. Individuals that are better adapted are more
likely to live and produce more offspring. Their offspring may
inherit these helpful variations. After many generations, more
members of the species will have the helpful variations.
• Variations in characteristics are caused by mutations and the
shuffling of alleles during meiosis.
Answer the following questions. Use your textbook and the
ideas above.
7. Evolution happens because of
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83
Name
Date
Class
Changes Over Time ■ Adapted Reading and Study
8. Look at the picture of the turtle eggs above. Circle the
letter of the factor affecting natural selection that the
picture shows.
a. competition
b. overproduction
c. variation
9. Draw a line from each factor that affects natural
selection to its meaning.
Factor
Meaning
competition
overproduction
variation
a. difference between
individuals of the same
species
b. caused by limited food
and other resources
c. produce more offspring
than can survive
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