0368_0386_bi_c07_te
3/13/06
7:26 PM
Page 369
15–1 The Puzzle of Life’s Diversity
Section 8–1
15–1
1 FOCUS
N
ature presents scientists with a puzzle. Humans share the
Earth with millions of other kinds of organisms of every
imaginable shape, size, and habitat. This variety of living things
is called biological diversity. How did all these different organisms arise? How are they related? These questions make up the
puzzle of life’s diversity.
What scientific explanation can account for the diversity of
life? The answer is a collection of scientific facts, observations,
and hypotheses known as evolutionary theory. Evolution, or
change over time, is the process by which modern organisms
have descended from ancient organisms. A scientific theory is
a well-supported testable explanation of phenomena that have
occurred in the natural world.
Objectives
Key Concepts
• What was Charles Darwin’s
contribution to science?
• What pattern did Darwin
observe among organisms of
the Galápagos Islands?
Vocabulary
The individual who contributed more to our understanding of
evolution than anyone was Charles Darwin. Darwin was born in
England on February 12, 1809—the same day as Abraham
Lincoln. Shortly after completing his college studies, Darwin
joined the crew of the H.M.S. Beagle. In 1831, he set sail from
England for a voyage around the world. His route is shown in
Figure 15–1. Although no one knew it at the time, this was to be
one of the most important voyages in the history of science.
During his travels, Darwin made numerous observations and collected evidence that led him to propose a
revolutionary hypothesis about the way life changes
over time. That hypothesis, now supported by a huge body of
evidence, has become the theory of evolution.
British
Isles
North
America
Vocabulary Preview
evolution
theory
fossil
Reading Strategy:
Using Visuals Before you
Voyage of the Beagle
read, examine Figure 15–1.
Find the British Isles, where
Darwin’s journey began, and
then trace his route. Write a
statement describing his
travels.
왔 Figure 15–1 On a five-year
voyage on the Beagle, Charles
Darwin visited several continents and
many remote islands.
Darwin’s
observations led to a revolutionary
theory about the way life changes
over time.
Challenge students to predict how
the three Vocabulary terms are related. Then, after they read the section,
have them check to see if they were
correct.
Reading Strategy
As students read, have them identify
questions Darwin asked during his
journey. Such questions include: Why
were there no rabbits in Australia
and no kangaroos in England? Why
had some species disappeared, and
how were they related to living
species? Had animals living on different Galápagos Islands once been
members of the same species?
2 INSTRUCT
Europe
Address Misconceptions
Asia
Atlantic
Ocean
Pacific
Ocean
Africa
South
America
Indian
Ocean
Australia
Galápagos
Islands
15.1.1 Describe the pattern Darwin
observed among organisms of the
Galápagos Islands.
Cape of
Good Hope
Cape
Horn
New
Zealand
H.M.S.
Beagle
Students may hold the misconception that because evolution is called
a theory, it is no more likely to be
true than any other explanation for
biological diversity. Point out that in
science a theory is a well-tested concept that is supported by evidence.
Explain that scientists do not dispute
the fact that evolution has occurred,
because so much evidence supports
it.
Voyage of the Beagle
Use Visuals
SECTION RESOURCES
Print:
Tim
r
• Teaching Resources, Lesson Plan 15–1,
Adapted Section Summary 15–1, Adapted
Sav15–1,
Worksheets 15–1, Section Summary
e
e
Worksheets 15–1, Section Review 15–1,
Enrichment
• Reading and Study Workbook A,
Section 15–1
• Adapted Reading and Study Workbook B,
Section 15–1
Figure 15–1 Have students use the
figure to trace Darwin’s voyage as
they read about it in the chapter.
Technology:
• iText, Section 15–1
• Transparencies Plus, Section 15–1
Darwin’sTheory
TheoryofofEvolution
Evolution 369
Darwin’s
0368_0386_bi_c07_te
3/13/06
7:26 PM
Page 370
Wherever the ship anchored, Darwin went ashore to collect
plant and animal specimens that he added to an ever-growing
collection. At sea, he studied his specimens, read the latest
scientific books, and filled many notebooks with his observations
and thoughts. Darwin was well educated and had a strong
interest in natural history. His curiosity and analytical nature
were ultimately the keys to his success as a scientist. During his
travels, Darwin came to view every new finding as a piece in an
extraordinary puzzle: a scientific explanation for the diversity of
life on this planet.
15–1 (continued)
Darwin’s
Observations
Use Community Resources
Invite the director of a local zoo or a
zoologist from a nearby university or
college to speak to the class about
similarities and differences between
Australian and North American animals that fill similar niches on the
two continents. Relate the speaker’s
presentation to Darwin’s observations, as described in the text.
Darwin’s Observations
Darwin knew a great deal about the plants and animals of his
native country. But he saw far more diversity during his travels.
For example, during a single day in a Brazilian forest, Darwin
collected 68 different beetle species—despite the fact that he
was not even searching for beetles! He began to realize that an
enormous number of species inhabit the Earth.
Make Connections
Patterns of Diversity Darwin was intrigued by the fact
Earth Science Explain to students
that climate on the Galápagos
Islands varies with elevation. Ask:
Why do you think the islands are
drier at lower elevations and wetter at higher elevations? (At higher
elevations, the air is cooler and cannot
hold as much moisture, which causes
clouds and precipitation.)
that so many plants and animals seemed remarkably well
suited to whatever environment they inhabited. He was
impressed by the many ways in which organisms survived and
produced offspring. He wondered if there was some process that
led to such a variety of ways of reproducing.
Darwin was also puzzled by where different species lived—
and did not live. He visited Argentina and Australia, for example, which had similar grassland ecosystems. Yet, those
grasslands were inhabited by very different animals. Also,
neither Argentina nor Australia was home to the sorts of animals
that lived in European grasslands. For Darwin, these patterns
posed challenging questions. Why were there no rabbits in
Australia, despite the presence of habitats that seemed perfect
for them? Similarly, why were there no kangaroos in England?
Figure 15–2 Many of the fossils that
Darwin discovered resembled living
organisms but were not identical to
them. The glyptodon, an extinct
animal known only from fossil remains,
is an ancient relative of the armadillo
of South America. Comparing and
Contrasting What are some similarities and differences between these two
types of animals?
UNIVERSAL ACCESS
Inclusion/Special Needs
Tell at-risk students that the most important thing
Darwin learned from his voyage was that organisms vary through time and space. Provide visual
examples of each type of variation by showing
students pictures of extinct organisms and their
living relatives and pictures of related organisms
that differ geographically. In each case, point out
the similarities as well as the differences in the
organisms being compared.
370
Chapter 15
Advanced Learners
Have students obtain a copy of Darwin’s book
On the Origin of Species and find passages in
which Darwin describes his observations of plants
and animals on the Galápagos Islands. Urge students to put a few of Darwin’s observations in
their own words and share them with the class in
a written or an oral report. If possible, students
should illustrate their reports with copies of
Darwin’s original drawings.
0368_0386_bi_c07_te
3/13/06
7:26 PM
Page 371
Build Science Skills
Pinta
Tower
Marchena
Pinta Island
Intermediate shell
James
Fernandina
Santa Cruz
Isabela
Santa Fe
Hood Island
Saddle-backed shell
Floreana
Hood
Isabela Island
Dome-shaped shell
Living Organisms and Fossils Darwin soon realized that
living animals represented just part of the puzzle posed by the
natural world. In many places during his voyage, Darwin
collected the preserved remains of ancient organisms, called
fossils. Some of those fossils resembled organisms that were
still alive, as shown in Figure 15–2. Others looked completely
unlike any creature he had ever seen. As Darwin studied fossils,
new questions arose. Why had so many of these species disappeared? How were they related to living species?
The Galápagos Islands Of all the Beagle’s ports of call, the
one that influenced Darwin the most was a group of small
islands located 1000 km west of South America. These are the
Galápagos Islands. Darwin noted that although they were close
together, the islands had very different climates. The smallest,
lowest islands were hot, dry, and nearly barren. Hood Island, for
example, had sparse vegetation. The higher islands had greater
rainfall and a different assortment of plants and animals.
Isabela Island had rich vegetation.
Darwin was fascinated in particular by the land tortoises
and marine iguanas in the Galápagos. He learned that the giant
tortoises varied in predictable ways from one island to another,
as shown in Figure 15–3. The shape of a tortoise’s shell could be
used to identify which island a particular tortoise inhabited.
Darwin later admitted in his notes that he “did not for some
time pay sufficient attention to this statement.”
How did the fossils Darwin observed compare with the
living organisms he studied?
왖 Figure 15–3
Darwin
observed that the characteristics
of many animals and plants
varied noticeably among the
different Galápagos Islands.
Among the tortoises, the shape of
the shell corresponds to different
habitats. The Hood Island tortoise
(right) has a long neck and a shell
that is curved and open around the
neck and legs, allowing the tortoise
to reach the sparse vegetation on
Hood Island. The tortoise from
Isabela Island (lower left) has a
dome-shaped shell and a shorter
neck. Vegetation on this island is
more abundant and closer to the
ground. The tortoise from Pinta
Island has a shell that is intermediate between these two forms.
N S TA
N S TA
For: Links on evolution
Visit: www.SciLinks.org
Web Code: cbn-5151
FACTS AND FIGURES
Galápagos Islands
The Galápagos Islands lie almost 1000 km off the
coast of South America. They have a total land
area of about 8000 square km, or about three
quarters the land area of the Hawaiian Islands.
The Galápagos were formed by volcanoes that
rise out of the ocean at different elevations, from
barely above sea level to 1500 m above the sea.
About 15,000 people live on the islands, which
are best known for their unusual animals,
Inferring Help students appreciate
why the Galápagos Islands were such
an important influence on Darwin.
Discuss why islands make good
places for studying evolution. Point
out how different some of the
Galápagos are from one another in
climate and other environmental features, despite their close proximity. If
possible, show students pictures
taken on different islands that illustrate these differences. Ask: What
traits do you think an animal
might need to survive on a hot,
dry, rocky island? (Students might
describe traits that help conserve
water, provide protection from the sun,
or make use of scarce food resources.)
What traits might an animal need
to survive on an island with a lot
of rainfall and vegetation? (Students
might describe traits that help the animal move around in trees or tolerate
dim, damp conditions.) Conclude by
pointing out that the sharply contrasting environments on the Galápagos
Islands and the different animals
found on them helped Darwin surmise
how evolution could occur.
Download a worksheet on evolution
for students to complete, and find
additional teacher support from
NSTA SciLinks.
Answers to . . .
including the giant tortoises for which the islands
were named. The largest species weigh more
than 230 kg. The Galápagos also have an unusual history. They were once known as the
Enchanted Isles, and pirates buried their treasure
there. Ships also abandoned mutineers on the
islands. During World War II, the United States
established a military base on the Galápagos to
guard the Panama Canal.
Some fossils resembled
organisms that were still alive, while
others looked completely unlike any
organism Darwin had ever seen.
Figure 15–2 Both animals have a
type of shell that covers their bodies.
The armadillo’s covering consists of
overlapping plates, whereas the
glyptodon’s covering consists of one
entire piece.
Darwin’s Theory of Evolution
371
0368_0386_bi_c07_te
3/13/06
7:26 PM
Page 372
Darwin also saw several types of small, ordinarylooking brown birds hopping around, looking for seeds.
As an eager naturalist, he collected many specimens,
several of which are shown in Figure 15–4. However,
he did not find them particularly unusual or important. As Darwin examined the birds, he noted that
they had differently shaped beaks. He thought that
some of the birds were wrens, some were warblers, and
some were blackbirds. But he came to no other
conclusions—at first.
15–1 (continued)
The Journey Home
Build Science Skills
Inferring Challenge students to
make inferences about what a particular species of animal needs to
survive. Point out that not only do
the physical characteristics of an
organism help it to survive, but also
its behaviors and relationship with its
environment. Have each student
choose a particular organism and
explain how its characteristics help it
survive in its particular environment.
The Journey Home
3 ASSESS
Evaluate Understanding
Call on students at random to
explain in their own words what the
Vocabulary terms mean.
Reteach
Work with students to develop a list
of examples of the biological diversity that Darwin observed.
왖 Figure 15–4 Darwin’s notebooks and some
of the finch specimens he collected have been
preserved for today’s scientists to study.
Inferring What might modern scientists learn from
examining evidence collected by earlier investigators?
While heading home, Darwin spent a great deal of
time thinking about his findings. Examining different
mockingbirds from the Galápagos, Darwin noticed
that individual birds collected from the island of
Floreana looked different from those collected on
James Island. They also looked different from individuals collected on other islands. Darwin also remembered that the tortoises differed from island to island.
Although Darwin did not immediately understand
the reason for these patterns of diversity, he had
stumbled across an important finding.
Darwin
observed that the characteristics of many animals and plants varied noticeably among the
different islands of the Galápagos. After returning
to England, Darwin began to wonder if animals living
on different islands had once been members of the same
species. According to this hypothesis, these separate
species would have evolved from an original South
American ancestor species after becoming isolated from
one another. Was this possible? If so, it would turn
people’s view of the natural world upside down.
15–1 Section Assessment
Students should note that biotic
factors are those that are living,
such as predators and vegetation,
and abiotic factors are those that
are not living, such as climate and
topography. Students’ examples
will vary. One possible example is
that sparse vegetation (biotic factor) led to the evolution of
long-necked tortoises.
If your class subscribes to the iText,
use it to review the Key Concepts in
Section 15–1.
Answer to . . .
Figure 15– 4 They can learn how
the earlier investigators formed their
ideas based on the evidence they had.
They might also interpret the evidence
in new ways.
372
Chapter 15
1.
Key Concept What did
Darwin’s travels reveal to him
about the number and variety of
living species?
Key Concept How did
2.
tortoises and birds differ among
the islands of the Galápagos?
3. What is evolution? Why is
evolution referred to as a theory?
4. What is a fossil?
5. Critical Thinking Inferring
Darwin found fossils of many
organisms that were different
from any living species. How
would this finding have
affected his understanding
of life’s diversity?
Interdependence in
Nature In Chapter 5, you
learned that both biotic and
abiotic factors affect ecosystems. Distinguish between
these two factors, give some
examples of each, and explain
how they might have affected
the tortoises that Darwin
observed on the Galápagos
Islands.
15–1 Section Assessment
1. Darwin’s travels showed him that the diversity of living species was far greater than he
had previously known.
2. Each Galápagos island had its own type of
tortoises and birds that were clearly different
from the tortoises and birds on other islands.
3. Evolution, or change over time, is the
process by which modern organisms have
descended from ancient ones. Evolution is
referred to as a theory because it is a wellsupported explanation of phenomena that
have occurred in the natural world.
4. A fossil is the preserved remains of an
ancient organism.
5. It would have greatly increased his estimates
of biological diversity.