Evolution 3e SB v5

ACTIVITY 8  HISTORY AND DIVERSITY OF LIFE

MATERIALS

For each student
1 Student Sheet 8.1, “Vertebrate Evolutionary Tree”

PROCEDURE

1. Read the following text.114115

2. Follow your teacher’s instructions for how to use the Stop to
Think questions.You will need Student Sheet 8.1, “Vertebrate
Evolutionary Tree,” for Stop to Think 1.

Evolutionary Trees116117118119

As you learned in the previous activity, “Origins of Species,” all
Galapagos finches evolved from one ancestral species that arrived on
the islands approximately 3 million years ago (abbreviated mya). An
ancestral species is the most recent species from which two or more
species evolved. That original Galapagos finch species evolved from
a previous ancestral species that also gave rise to other bird species.
If we go back far enough in time, all birds are descended from the
original bird species. Scientists estimated that this original bird species
evolved approximately 150 mya.

The ancestor, or common relative from the past, of the first bird A B C
species was also the ancestor of reptiles like crocodiles. While the

evolution of birds followed one branch of the evolutionary tree, the

evolution of crocodiles followed another. Another way to say this is D
that both birds and crocodiles descended from the same ancestor. If

we go back even further, the common ancestor of birds and croco-

diles is also the common ancestor of mammals, including humans. E

Evidence from fossils suggests that mammals split apart from birds/ D is the ancestral species for B
crocodiles about 200 mya.
and C. B and C descended from
In a similar way, we can keep going back in time to find evidence of
the ancestral species for all significant forks in the evolutionary tree. DLa. EbiAsitdhseSaEnPceUsPtraIAl sPpSeEciveoslution 3e
Amphibians, like frogs and salamanders, split apart approximately oFfigAuarned: EDv.oE3isetShBe c8o_m2mon
aMnycreisatdorPorof aRlleogf 9th.5e/o1t1her
species.

370 mya. Fish split apart about 530 mya. Fish, amphibians, reptiles,

birds, and mammals are all considered vertebrates, which means that

the first vertebrate diverged from invertebrates 530 mya. And these

events are actually quite recent! The evidence of the first life on Earth,

bacteria, goes back to at least 4.28 billion years ago (bya).

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40 EVOLUTION

HISTORY AND DIVERSITY OF LIFE  ACTIVITY 8

Timeline of Earth

rst rst
evidence multicellular
of life organisms

rst fossils of rst
Earth single-celled vertebrates rst
forms organisms humans

5.0bya 4bya 3bya 2bya 1bya present
900mya 530mya 6mya
4.6bya 4.28bya 3.77bya

LFiagbuASreidT: sEOvSoEPP3UeTPSBOIA8P_TS3EHvoIlNutiKon13e

Myriaa.d PUrosiRneggS9tu.5d/1e1nt Sheet 8.1, “Vertebrate Evolutionary Tree,” find the place in the
diagram, also shown below, that represents the point in time when birds and
crocodiles diverged, or split apart. When did this happen? Add the time in
millions of years ago (mya) to the diagram.

b. Find the place in the diagram that represents the point in time when mam-
mals diverged from birds/crocodiles. When did this happen? Add the time in
millions of years ago (mya) to the diagram.

Vertebrate Evolutionary Tree

sh amphibians mammals crocodiles birds

common ancestor
LabAids SEPUP IAPS Evolution 3e
Figure: Evo3e SB 8_4
MyriadPro Reg 9.5/11

EVOLUTION  41

ACTIVITY 8  HISTORY AND DIVERSITY OF LIFE

Diversity of Life120121122

You are probably familiar with a variety of mammal species or bird
species. Walk out your door and you might see one or more of the
following: pigeons, starlings, squirrels, dogs, or rats. But mammals
and birds together make up less than 0.2% of all species in existence
today. Most life on Earth is relatively unfamiliar to us because it is
small, lives underground, or lives in water. For example, scientists
estimate there are 350,000 species of beetles, a type of insect. That
is over 23 times the number of mammal and bird species combined!
The diagram below shows how many species exist today in each of the
major groups of organisms. The group labeled “chordates” includes all
vertebrates: fish, amphibians, reptiles, birds, and mammals.

Abundance of Known Species

KEY

insects 925,000

non-insect arthropods 123,000
(including spiders and crabs)

chordates 43,000

other animals 116,000
(including worms, clams, snails)

protists 30,000

fungi 69,000
(including mushrooms)

algae 27,000

vascular plants 248,000
(including trees and ferns)

SLFiaTgbuOAreidP:sEvSToEOP3Ue PSTBIHA8P_IS5NEvKolu2tion 3e

aM. yUriasdePtrhoeRkeegy9t.o5/c1a1lculate how many total known species there are.

b. Calculate the approximate percentage of insects.

c. The figure in the previous section shows the evolutionary tree for vertebrates.
Which “slice” in the figure above would be home to this tree?

111222021 NNNGGGCLLSSC44PAAA212

42 EVOLUTION

HISTORY AND DIVERSITY OF LIFE  ACTIVITY 8

Extinction123

Life has existed on Earth for over 4 billion years, and species keep
evolving into new and more varieties of life, so why doesn’t Earth
have billions of species? You estimated the total number of species
that have been documented (although scientists estimate there
may be 10 million species in existence, with most of them yet to be
discovered). Recall from the previous activity that separate species
are defined as populations whose members don’t interbreed. If every
species that has ever existed were still alive, scientists estimate that
there would be over 5 billion species on Earth today. This means that
over 99% of species that ever evolved have become extinct.

Extinction is the complete and permanent disappearance of a species,
while speciation is the formation of a new species. Extinction, like
speciation, is part of evolution.Through chance, or by being poorly
adapted to a changing environment, a species may go extinct through
natural selection. Sometimes major groups of organisms go extinct.
Trilobites were common until they went extinct 250 mya. Large
dinosaurs went extinct 65 mya. At other times, smaller groups became
extinct, and sometimes only certain species disappeared. For example,
approximately 12,000 years ago, the saber-toothed cat went extinct.
As recently as 350 years ago, the dodo bird went extinct.You will learn
more about extinction in the activity, “The Sixth Extinction?”

STOP TO THINK 3 EVOLUTION  43

What are some reasons why a species might go extinct?

Life Over Time124125

Scientists know that life evolved in the ocean. For the first 3.75
billion years of life on Earth, life existed only in water. Life on land is
relatively recent, beginning around 550 million years ago at what is
the start of an event that scientists have named the Cambrian explo-
sion. This “explosion” was not an actual explosion. The Cambrian
explosion was the rapid evolution of a wide variety of life forms and
groups of organisms over a relatively short period of time. Scientists
think that the Cambrian explosion lasted around 20 million years.
That may seem like a long period of time to most people, but it is
a short period of time to scientists studying evolution! During this
time, most of the major groups of animals that still exist today in
some form began to evolve.

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ACTIVITY 8  HISTORY AND DIVERSITY OF LIFE

The diagram below shows when different life forms evolved, beginning
with the Cambrian explosion and continuing through today. Note that
some life forms went extinct.

Life Over Time First appearance Last appearance
Present rst humans

last dinosaurs
last ammonites

100mya rst primates

rst owering plants
rst birds

200mya rst mammals last trilobites
300mya rst dinosaurs last scale trees
rst ammonites

rst reptiles
rst ying insects
rst scale trees

rst amphibians

400mya rst ightless insects
rst vascular land plants

500mya rst vertebrates ( shes)
rst molluscs
550mya rst trilobites
Cambrian
explosion

44 ELVabOALUidTsISOENPUP IAPS Evolution 3e
Figure: Evo3e SB 8_7
MyriadPro Reg 9.5/11

HISTORY AND DIVERSITY OF LIFE  ACTIVITY 8

STOP TO THINK 4

a. Which life forms no longer exist today?
b. When did flowering plants first evolve?
c. When did the first humans, Homo sapiens, evolve?

ANALYSIS

1. Why do scientists who study evolution think of 20 million years
as a short time?126

2. What does the statement “extinction and speciation are both
parts of evolution” mean?

3. How did there get to be such a great variety of life?127
4. Write a detailed caption for the diagram shown below.128129

Dinosaur-Bird Evolutionary Tree

lizards and crocodiles most all meat-eating birds
snakes plant-eating and very large

dinosaurs dinosaurs

common ancestor
LabAids SEPUP IAPS Evolution 3e
Figure: Evo3e SB 8_6
MyriadPro Reg 9.5/11

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EVOLUTION  45



9 Fossil Evidence
l a b o r at o r y

You learned in the previous activity that many species have
become extinct during the history of Earth. How can you know
that these creatures ever existed? Some of the evidence is right under
your nose—or your feet, to be more precise.
Our planet’s thin outer layer, the crust, can be up to 40 kilometers
(25 miles) thick. The crust is made up of many layers of rock that
have been forming for over 4 billion years, and are still forming
today. These rock layers can form when a volcanic eruption covers
the land with lava, or when a flood spreads out a layer of mud. Lava,
mud, or even sand can eventually harden into solid rock. Any new
layer or rock can seal off the layer below it. Organisms trapped within
these sealed-off layers can become part of the rock itself. These
trapped organisms can become fossils.
Any remains of life preserved in a rock is called a fossil. It can be an
entire organism, a part of an organism, a footprint, a piece of feces,
or a piece of shell, bone, or tooth. The scientists who study fossils are
paleontologists. In this activity, you will examine real fossils as a
paleontologist would.130131

GUIDING QUESTION

What kind of evidence do fossils provide about evolution?

113301 NNGGLLSS44AA12

When and where did this
fossil vertebrate live?

EVOLUTION  47

ACTIVITY 9  FOSSIL EVIDENCE

MATERIALS

For each group of four students
1 set of 4 fossils
1 set of 4 simulated drill cores
1 pair of scissors
1 metric ruler
colored pencils

For each pair of students
1 hand lens

For each student
1 Student Sheet 9.1, “Stratigraphic Columns from Drill Cores”

PROCEDURE

Part A: Examining Fossils
1. One pair in the group should begin by examining one of

the fossils, while the other pair begins by examining another
fossil.132133134135

2. Work with your partner to identify the unique features of the
fossil. Use the hand lens to help you.

3. In your science notebook, sketch the general shape and unique
features of this type of fossil. Then record additional observations
that are difficult to show in your sketch, such as color or size.

4. Record anything else you know about the fossil that you may have
learned in previous lessons.

5. Repeat Steps 1–4 until you have examined all four fossil types.

6. Read the section below on finding fossils.

How Do Scientists Find and Date Fossils?

Scientists look for fossils where several layers of rock are visible, one
on top of the other. One common place where these layers are visible
is in the walls of a deep river canyon. As each rock layer forms, it is
deposited on top of an existing layer. When you observe a sequence of
rock layers, the top layer, along with any fossils it contains, is younger
than any other layer in that sequence, and the bottom layer, along
with any fossils it contains, is the oldest layer in that sequence. This is
called the law of superposition.

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48 EVOLUTION

FOSSIL EVIDENCE  ACTIVITY 9

A diagram representing a series of rock layers, such as
the one on the right, is called a stratigraphic column.
Stratigraphic columns can be made by looking at the sides of
cliffs, or by looking at drill cores. A drill core is a cylindrical
piece of rock removed from Earth by a large drill, similar
to the drills that are used to make oil wells. Drill cores can
provide samples from many miles beneath Earth’s surface.

No single location contains a complete set of all the rock
layers or fossils that exist on Earth.To study a particular
fossil organism or find out which organisms lived during
which geologic era, paleontologists must compare rocks from
different places throughout the world.

Scientists must also figure out how old the rocks are. With
the help of radioactive dating technology, scientists have
made good estimates of how old each layer is and how many
years each layer took to form.

Part B: Figuring Out Fossils Rock layers in the Grand
Canyon (top) and a
How can you determine which fossils are older, which are younger, schematic diagram of fossils
and which are likely to be from extinct species? You will examine in rock layers, also known
and compare four different drill cores, each representing the rock as a stratigraphic column
layers found on different fictional continents.136137138139 (bottom)

7. Each pair should examine one of the drill cores. The top of each
drill core is marked with its number.

8. Create a stratigraphic column by sketching in the boundaries of
the layers and the fossils found within each layer in the appro-
priate place on Student Sheet 9.1, “Stratigraphic Columns from
Drill Cores.”

9. Based on the evidence within the layers of this drill core, list the
fossils in order from youngest to oldest.

10. Repeat Steps 7–9 for one of the remaining two drill cores.
(The other pair in your group will examine the other
remaining drill core.)

11. Combine all four columns, and look at them side by side.

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EVOLUTION  49

ACTIVITY 9  FOSSIL EVIDENCE

12. Based on the appearance of the rock layers and the fossils found
within each layer, match (or correlate) the layers from each core
as best as you can. Make a chart, similar to the one below, that
shows your correlation of the rock layers from the four different
drill cores.

Hint 1: You may want to cut out each column from the Student
Sheet so that you can move them around as you try to match up
the layers.

Hint 2: Layers don’t have to be exactly the same to correlate.

Sample Correlation of Stratigraphic Columns

Column Y

Column X

Layer 1 Column Z

Layer 1 Layer 2 Layer 1
Layer 2
Layer 2 Layer 3 Layer 3
Layer 4
Layer 3 Layer 4

Layer 4

13. Use your correlation chart to list all four of the fossils in order
from youngest to oldest.

Hint: If you think a layer found in one drill core is the same as a
layer found in another drill core, you can infer that those layers,
and the fossils in them, are the same age.140

140 NGSPCE6

50 EVOLUTION

FOSSIL EVIDENCE  ACTIVITY 9

ANALYSIS

1. Imagine that scientists have discovered two new fossils. Describe
two ways that scientists can figure out the ages of the fossils and
whether the two organisms existed at the same time.141

2. Based on evidence from all four drill cores, answer the following
questions:142143144145146147

a. Which, if any, of the organisms represented by the fossils may
be from species that are now extinct? Explain.

b. Which of the fossils that you examined in Part A is or are
from a group of organisms that may have gone extinct, and
how long ago did that happen?

c. Which fossil species could have lived at the same time?
Explain.

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EVOLUTION  51



10 Fossilized Footprints
i n v e s t i g at i o n

In the previous activity, you examined some fossils that were the
preserved remains of whole or partial organisms. But in some cases,
the only evidence left by an organism is its tracks. Footprints and
other types of animal tracks can be fossilized in the same way as
actual body parts. These are called trace fossils. But what can you
find out from just footprints? Like detectives, paleontologists can use
the information from fossil footprints to determine how an organism
moved, how fast it traveled, what type of environment it lived in,
and what it might have been doing when its footprint was formed.
They might also be able to draw some conclusions about the social
behavior of the organism. 148149150151

GUIDING QUESTION

What other kinds of information can we get from fossils?

111155448190 NNNNGGGGCSLLPSSCN44NAASS2113

Few fossil remains are as complete as this 10-million-year-old rhinoceros in Nebraska.

EVOLUTION  53

ACTIVITY 10  FOSSILIZED FOOTPRINTS

MATERIALS

For each group of four students
1 set of 3 Fossil Footprint Cards
1 metric ruler

Evidence Comes in Steps

A fossil footprint site has just been discovered! You take a helicopter to the
location with hope that your expertise will be useful. The rest of the team is
slowly brushing away layers of sediment to carefully uncover the footprints.
Your task is to make observations of fossil footprints and use them to
draw inferences about what happened in the past. An observation is any
description or measurement gathered by the senses or by instruments. An
inference is a conclusion based on observations or what is already known.
Based on your inferences, you will develop a hypothesisa tentative expla-
nationabout what happened to form the footprints. As the footprints are
uncovered, there will be more evidence to examine. Remain open to new
possibilities as the investigation continues.

PROCEDURE152153

Part A: Initial Data
1. Examine Fossil Footprint Card 1, which shows what the team has

uncovered so far.154155
2. In your group, discuss what you think was happening while these

footprints were being created.You do not have to agree, but
• if you disagree with others in your group about what happened,

explain to the rest of the group why you disagree.
• listen to and consider other people’s explanations and ideas.
3. Record your ideas in your science notebook. Separate your ideas
into observations and inferences.
Note: Even though some of your inferences may conflict with
other inferences, consider as many ideas as possible.
4. Time passes and more footprints are uncovered. Obtain Fossil
Footprint Card 2.

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54 EVOLUTION

FOSSILIZED FOOTPRINTS  ACTIVITY 10

5. Repeat Step 2. Record your additional observations and infer-
ences in your science notebook. However, do not change what
you wrote for Card 1!

6. Time passes and a third section of footprints is uncovered.
Obtain Fossil Footprint Card 3.

7. Repeat Step 2. Record your additional observations and infer-
ences in your science notebook. Remember, do not change what
you wrote for Cards 1 and 2!

8. Look back at all your observations and inferences.Try to think of
the best possible explanation for how the footprints were formed.
Record your strongest hypothesis, or tentative explanation, in your
science notebook. If you have two or more explanations in mind,
record them all, but rank them from most likely to least likely.

9. Answer Analysis items 1 and 2.

Fossil footprints

Part B: Additional Data

10. Hypotheses change as scientists gather new data. The informa-
tion in the table below has just come in from the fossil site!

Average Depth of Footprints (Scenario 1)

CORE CARD 1 CARD 2 CARD 3
Larger footprints 6.0 cm 6.2 cm 8.3 cm
Smaller footprints 2.5 cm 2.6 cm ___

a. What hypothesis would the data in the table above support?
b. Develop an argument for which hypothesis is strongest.

EVOLUTION  55

ACTIVITY 10  FOSSILIZED FOOTPRINTS

11. Instead of the data in Scenario 1, imagine you just received the
data in Scenario 2, below. 156157158

Average Depth of Footprints (Scenario 2)

CORE CARD 1 CARD 2 CARD 3
Larger footprints 6.0 cm 6.2 cm 6.1 cm
Smaller footprints 2.5 cm 2.6 cm ___

a. What hypothesis would the data in Scenario 2 support?

b. Develop an argument for which hypothesis is strongest.

c. What factor(s) might explain the difference in the depth of the
footprints in the different scenarios?

ANALYSIS

1. What kind of information might be obtained from trace fossils
that cannot be obtained from the fossilized remains of the
organism itself?159

2. Why is it important for scientists—and people in general—to
distinguish between observations and inferences when they
develop hypotheses?160161

3. Imagine that the team uncovered a fourth section of footprints.
Draw what you predict this fourth section might look like. Explain
how it would provide more support for the argument you favor.162

4. How does studying all kinds of fossils help us understand
evolution?163164

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56 EVOLUTION

11 Family Histories
i n v e s t i g at i o n

Sasha walked into the kitchen and saw a drawing she had made in
kindergarten on the refrigerator door. She exclaimed, “Where did you find
that? It is so old! I made that a million years ago!”

Sasha’s dad replied, “I was going through a box of keepsakes and found it.
It was always one of my favorites. But it was only seven year ago! If you’d
made it a million years ago, there would have been saber-toothed cats
around.”

“Well, it seems like a million year ago,” said Sasha. “But how do we know
that there were saber-toothed cats back then?”

“That’s a great question, Sasha!” declared her dad.

Fossils have been found in rocks 3.77 billion years old! But most
have been found in rocks that are all less than 550 million years old,
after the Cambrian explosion you learned about in the “History and
Diversity of Life” activity. The types of organisms found in different
rocks can provide important information about the history of life on
Earth. The term fossil record refers to all of the fossils that have been
found on Earth.165166167168

The fossil record has been used to classify fossils into different
groups of organisms, depending on how similar they are. One kind
of group is a family. For example, among mammals, dogs are in
the family Canidae, which also contains foxes, jackals, coyotes, and
wolves. Lions are also related to dogs, but not as closely. They are
more related to mammals in the family Felidae, which includes
leopards, tigers, cheetahs, house cats, and extinct species such as the
saber-toothed cat.You will investigate how the numbers of families of
fish, mammals, and reptiles have changed over geological time.

GUIDING QUESTION

What can you learn about evolution by comparing the fossil
records of fish, mammals, and reptiles?

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EVOLUTION  57

ACTIVITY 11  FAMILY HISTORIES

Classifying Carnivores DOGS LIONS
Animalia Animalia
CLASSIFICATION LEVEL Chordata Chordata
Kingdom Mammalia Mammalia
Phylum Carnivora Carnivora
Class Canidae Felidae
Order Canis Panthera
Family familiaris leo
Genus
Species

MATERIALS

For each student
colored pencils
1 Student Sheet 11.1, “Graphs of Fossil Families”

PROCEDURE

1. The “History of Fossil Fish Families” table below provides
the history of all the families of fish currently known from the
fossil record. When a fossil is found that does not belong to any
family found in earlier geologic time periods, we call it a first
appearance. It is the first appearance of that family in the fossil
record. When a fossil is found that does not belong to any family
found in later geologic time periods, we call it a last appearance.
It is the last appearance of that family in the fossil record. Look
at the table below and discuss the following questions with your
partner:169170171

• Between which years did the greatest number of fish families
appear in the fossil record?

• Between which years did the greatest number of fish families
disappear from the fossil record?

History of Fossil Fish Families

111767910 TIME (MNNNYAGGGSCS)PP CACPDEA612 >545 485 425 365 305 245 185 125 65 0

Number of first 0 25 43 162 67 13 52 33 84 299
appearances
0 9 31 158 49 48 36 20 44 34
Number of last
appearances

58 EVOLUTION

FAMILY HISTORIES  ACTIVITY 11

2. The double bar graph below is based on the data shown in the
“History of Fossil Fish Families” table. Look at the graph, and
discuss with your partner in what ways the graph makes the data
easier to interpret.

Graph of Fossil Fish Families Over Time 299
65 0
200 rst appearances
last appearances
Number of Families 150
365 305 245 185 125
100 Millions of years ago

50

0 >545 485 425

3. Use the informatLiaobnAiidns tShEPeU“PHIAiPstSoErvyoloutfioFno3sesil Reptile Families”
table to make a dFoiguubrele: Ebvoa3regSrBa1p1h_3for families of reptiles, similar
to the one for fishMysrhiaodwPrno Raebgo9v.e5./1S1ince you will be comparing
graphs, be sure to use the same scale on the y-axis. 172

History of Fossil Reptile Families

TIME (MYA) >545 485 425 365 305 245 185 125 65 0

Number of first 0 0 0 0 3 67 95 68 97 35
appearances
0 0 0 0 1 57 93 46 84 26
Number of last
appearances

A familiar example of a fossilized reptile

172 MASP6B5 EVOLUTION  59

ACTIVITY 11  FAMILY HISTORIES

4. Use the information in the “History of Fossil Mammal Families”
table to make a double bar graph for families of mammals. Since
you will be comparing graphs, be sure to use the same scale on
the y-axis.173

History of Fossil Mammal Families

TIME (MYA) >545 485 425 365 305 245 185 125 65 0

Number of first 0 0 0 0 0 0 6 14 33 404
appearances
0 0 0 0 0 0 2 8 33 262
Number of last
appearances

ANALYSIS

1. Describe the patterns in the three graphs and what these
patterns tell you about the evolution of the three groups of
organisms. Be sure to include how they are similar and how they
are different.174175176177178179

2. Scientists sometimes label periods of time by what organisms were
common at the time.

a. The period of time from 65 mya until today is often referred
to as the age of mammals. Using evidence from this activity,
explain why.

b. Based on evidence from this activity, what would you call
the period of time from 305 mya to 65 mya? Explain your
reasoning.

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60 EVOLUTION

FAMILY HISTORIES  ACTIVITY 11

3. Look at your answer for Analysis item 1 and the following
table and graph. Where do you think scientists have placed the
amphibian family? Explain your answer.

History of Fossil Amphibian Families

TIME (MYA) >545 485 425 365 305 245 185 125 65 0
0 0 0 3 35 33 19 11 5 15
Number of first
appearances 0 0 0 3 16 53 18 5 1 5

Number of last
appearances

Graph of Fossil Amphibian Families Over Time

200 rst appearances
last appearances
Number of Families 150
185 125
100

50

0 >545 485 425 365 305 245 65 0

Millions of years ago

4. Does the evidencFLeiagbuiAnreid:tsEhvSioEsP3aUecPStBiIAv1Pi1tS_yE8svoulputpioonrt3ethe figure of the
vertebrate evolutMioynriadrPyrotrReeegi9n.5/t1h1e activity, “History and
Diversity of Life”? 180

180 NGSPCE6

EVOLUTION  61



12 A Whale of a Tale
i n v e s t i g at i o n

Whales, dolphins, and porpoises are mammals that live in
the sea. Like all mammals, they are warm-blooded animals
that give birth to live young and need air to breathe. DNA evidence
shows that whales are closely related to hoofed land mammals such
as hippopotamuses, pigs, cows, and sheep. DNA is the molecule in
our genes that codes for traits in all organisms. All of these mammals
are thought to have descended from a single species that lived
millions of years ago and is now extinct. But how did this take place?
Did sea mammals appear first and then evolve into land mammals,
or did it happen the other way around?

GUIDING QUESTION

How did whales evolve?

EVOLUTION  63

ACTIVITY 12  A WHALE OF A TALE

MATERIALS

For each pair of students
1 set of 5 Whale Skeleton Cards

For each student
1 Student Sheet 12.1, “Whale Fossil Chart”

The Fossil Exhibit

You’ve just been hired as the assistant curator of
the fossil collection of a museum. On your first day,
you discover that the skeletons in the exhibit on the
evolution of whales have all been moved to a new room
and need to be arranged. Unfortunately, you are not a
whale expert and the skeletons are not clearly labeled.

A local middle school has scheduled a field trip to the
museum. It is very important that you arrange the
skeletons properly before the students arrive. You decide
to examine them to see if you can figure out how they
should be arranged.

P R O C E D U R E 181182183184185 Skeleton of a modern blue whale

1. Compare the five Skeleton Cards. Based on similarities you
observe, group the skeletons into two sets, each containing two or
three cards. The set of skeletons containing Skeleton A should be
called “Group 1.” The other set of skeletons will be “Group 2.”

2. Create a Venn diagram like the one shown below in your science
notebook.186187188189

group 1 group 2

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64 EVOLUTION

A WHALE OF A TALE  ACTIVITY 12

3. Compare Group 1 skeletons with those of Group 2. In the Venn
diagram, describe and record as many similarities and differ-
ences as you can.

4. It’s time to figure out how to arrange the exhibit! Use similarities
and differences in the skeletons to arrange the cards in order.
(While all five skeletons can be in a single line, they don’t have to
be.) Record the order in which you have arranged the skeletons.

Hint: Place the two least similar skeletons on either side of your
desk. Then arrange the other three skeletons between them.

5. Your colleague reminds you that the museum has a collection
of whale embryos, and you wonder if you could use them in the
exhibit. When you inspect the embryos, which are organisms in
the early stages of development before they are born or hatched,
you discover two things that surprise you:190
• The embryos have some hair on their bodies. Adult whales
don’t have any hair.
• The embryos have small hind limb buds. Limb buds are the
structures that develop into limbs in animals. Adult whales
have no external hind limbs.

6. You wonder if these findings might be important for your
exhibit. Look at the order of your Skelton Cards, and make any
changes that you think are necessary. 191

7. You’re in luck! You discover a chart with information about the
relative ages of the five skeletons. Obtain Student Sheet 12.1,
“Whale Fossil Chart,” from your teacher.

8. Compare the age data from Student Sheet 12.1 with the order in
which you placed the skeletons in Step 4. If necessary, rearrange
your Skeleton Cards. Record your final reconstruction of the
museum exhibit in your science notebook.192

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EVOLUTION  65

ACTIVITY 12  A WHALE OF A TALE

ANALYSIS

1. During the evolution of whales,
a. what kinds of skeletal changes have occurred?
b. what changes in habitat might have occurred at the same time? 193

2. Use natural selection to explain how these changes (or one of
these changes) could have occurred.194

3. How does the observation that whale embryos have hair and hind
limb buds help you understand whale evolution?195

4. Answer the question in the introduction: Did sea mammals
appear first and then evolve into land mammals, or did it happen
the other way around? Explain using evidence and scientific
reasoning.196197198199200

5. In this activity, you examined extinct and modern whale skeletons.
How does the study of these skeletons provide evidence about
how species are related?

6. Look again at Skeleton A. This is known as an ambulocetid
(am-byoo-low-SEE-tid). The word ambulocetid means “walking
whale.” Where do you think the ambulocetids lived? Describe how
you think they lived.201

EXTENSION

Visit the SEPUP Third Edition Evolution page of the SEPUP website
at www.sepuplhs.org/middle/third-edition and go the to the whale DNA
link to learn how DNA evidence is used to investigate the evolutionary
history of whales. How does this evidence compare to the evidence in
this activity?

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66 EVOLUTION

13 Embryology
i n v e s t i g at i o n

In “a whale of a Tale,” you compared skeletons of five whale species
to determine their evolutionary relationships. These skeletons
came from mature, or fully grown, animals. Although it is possible
to identify similarities and differences between mature animals,
scientists also look at developing animals for evidence of evolu-
tionary relationships. In this activity, you will look at developing
animal embryos and try to identify evolutionary relationships that
might not be obvious in the mature animals.202203

GUIDING QUESTION

How can embryos provide evidence about evolutionary
relationships?

220032 NNGGLCSC4PAA32

This is an embryonic pig transitioning between early and middle stage development. It is
approximately 16 millimeters in length. Structures, such as the eye, nose, mouth, forelimb,
hindlimb, and ribs, can be identified.

EVOLUTION  67

ACTIVITY 13  EMBRYOLOGY

MATERIALS

For each group of four students
colored pencils

For each pair of students
1 set of 12 Embryonic Limb Cards
1 set of 20 Whole Embryo Cards

For each student
1 Student Sheet 13.1, “Comparison of Vertebrate Forelimbs”

PROCEDURE

Part A: Skeletal Forelimb Comparison
1. With your partner, examine the forelimb skeletons of six animals on

Student Sheet 13.1, “Comparison of Vertebrate Forelimbs.” 204205
2. Compare the bones in the skeletons and identify the following:

• hand/foot
• wrist
• forearm
• upper arm
3. Use colored pencils to color bones that you think might function
the same way. (Use the same color for any structure that performs
the same function.)
4. Compare your Student Sheet with the scientifically accepted
color-coded diagram provided by your teacher.
5. With your partner, discuss how the structures and functions of
the forelimbs you labeled provide evidence about evolutionary
relationships between the six animals.206

Part B: Embryonic Limb Comparison
6. Obtain a set of 12 Embryonic Limb Cards from your teacher.

They show three different limbsa bat forelimb, a bat hindlimb,
and a mouse hindlimbat three different stages of development:
early, middle, and late.
7. For each limb, compare the images on the cards. Sort the cards
into three groups: early, middle, and late development. Then see if
you can identify which of the cards corresponds to each limb.

222000465 NNNGGGCLCSCC4SPAAF212

68 EVOLUTION

EMBRYOLOGY ACTIVITY 13

8. As you sort, be sure to discuss what information you are using
to decide the order. Record this information in your science
notebook. Each card has an identifying letter in the corner. Use
these letters to record your sorting in your notebook.

9. Based on your observations and final sorting, discuss with your
partner whether you see evidence that these animals are related.
Be sure to record your ideas in your science notebook.207

Part C: Whole Embyro Comparison
10. Now you will look at the embryological development of five

whole animalsa human, a snake, a bat, a chicken, and a
salmon.You will examine the five embryos one stage at a time.
a. Obtain the early-stage Whole Embryo Cards from your

teacher. There are five embryos along with the names of the
five animals.
b. Make and record your observations in your science notebook.
c. Identify which animal you think matches each embryo. Each
Whole Embryo Card has an identifying letter in the corner.
Use these letters to record your decision and rationale in your
science notebook.
11. Repeat Step 10 with the middle-stage Whole Embryo Cards.
This set of cards shows the next stage of development for the
same five animals.
a. Identify which early-stage embryo you think matches each
middle-stage embryo. Adjust the cards as your thinking
changes.
b. Record your new sorting and reasoning in your science
notebook.
12. Repeat Step 10 with the late-stage Whole Embryo Cards. This
third set of cards shows the last stage of development for the
same five animals.
13. Share your sorting and reasoning with another pair of students.
As a small group, revise your sorting based on your discussion, if
necessary.

207 NGLS4A3

EVOLUTION  69

ACTIVITY 13  EMBRYOLOGY

14. Record your final sorting and reasoning in your science
notebook.208

15. Based on your observations and final sorting, discuss with your
partner what embryological evidence suggests about whether and
how the five animals are related. Be sure to record your ideas in
your science notebook.

EXTENSION

Visit the SEPUP Third Edition Evolution page of the SEPUP website at
www.sepuplhs.org/middle/third-edition, and go the link for chick embryo
development to watch the video of chick embryo development. Try to
identify the points in the video that correspond to images on the cards
in Part C.

ANALYSIS

1. Was it easy to identify the type of animal when looking at embryo-
logical images? Why or why not?

2. Review the observations you recorded in your science notebook.209
a. What patterns did you observe?
Hint: A pattern is something that happens in a repeated and
predictable way.
b. What structures appeared and when?
c. What structures disappeared and when?

3. What relationships across different animal species can you see
in embryological data that you cannot observe by comparing
mature animals? Use data from your investigation to support your
answer.210211212213214215

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70 EVOLUTION

14 The Sixth Extinction?
talking it over

If every species that has ever existed were still alive, scientists estimate
that there would be over 5 billion species on Earth today. But most
scientists agree that there are approximately 10 million species in
existence today. This means that over 99% of species that ever evolved
eventually went extinct. Scientists have concluded from the fossil
evidence that extinctions have not occurred evenly over time but have
come in waves. In this activity, you will see if you can identify patterns
in the rate of extinction in the past. You will also compare the distant
past to the rate of extinction since the year 1500.216

GUIDING QUESTION

Is the current rate of extinction typical?

MATERIALS

For each group of four students
1 set of 5 Extinction Event Cards
1 Student Sheet 14.1, “Past Extinction Rates”

216 NGLS4a1

The dodo was a flightless bird
that went extinct in the 1600s.

EVOLUTION  71

ACTIVITY 14  THE SIXTH EXTINCTION?

Extinction rate PROCEDURE
(families per million years)
Part A: Rates of Extinction in the Past
1. Look at the graph on Student Sheet 14.1, “Past Extinction 100 0

Rates,” also shown below. What does it show? Do you detect any
patterns? Explain.217218219220

Past Extinction Rates
20

15

10

5

0600 500 400 300 200
Millions of years ago

Lab Aid2s. SEOPbUtPaIiAnPSaEsveotluotifoEn 3xetinction Event Cards from your teacher, and
Figure: Evosp3ereSaBd14t_h2em out so that all group members can read them. Each
MyriadProoRfetgh9e.s5e/1c1ards represents a wave of extinction identified by scien-

tists in the graph above.
3. Work with your group to match each of these five Extinction

Event Cards to the peaks in the graph above.
4. With your group, identify any similarities and differences among

the possible causes for the five mass extinction events identified by
scientists, and be prepared to share.221222
5. The green area on the graph represents the background rate of
extinction. This is the average rate of extinction over time. How
many times greater than the background rate was the second mass
extinction? the fifth mass extinction?

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72 EVOLUTION

THE SIXTH EXTINCTION?  ACTIVITY 14

Part B: Rates of Extinction, 1500–Present223224225226Cumulative percentage of extinctions
6. Read the text below about extinction in modern vertebrates.

Recent Extinctions

While the extinctions you have investigated so far all happened a very long
time ago—tens or hundreds of millions of years agoextinction happens all
the time. Some scientists study fossils and their living relatives to understand
extinctions that are taking place today, or have taken place in the recent past—
hundreds to a few thousand years ago. This allows scientists to determine
the current rate of extinction and compare that with the background rate. A
group of scientists examined the number of vertebrate species that have gone
extinct since the year 1500, and below are their results.

2.50

2.00 Mammals

1.50 Birds
Other vertebrates

1.00

0.50

Background
0.00

1500–1600 1600–1700 1700–1800 1800–1900 1900–2010
Time period

Extinction in Modern Vertebrates
Scientists calculate that the observed extinctions in just the past 114 years would have taken
FLiaagbpuAprreido:xsEimvSoEaP3tUeelPySB5IA,01P04S0_yE3evaorlsuatitothne3beackground rate of extinction.
MyriadPro Reg 9.5/11

7. With your partner, discuss the pattern you see in the above
graph and what might be causing this pattern.

222222225346 ENNELLGGWREESHSS63368CC87912

EVOLUTION  73

ACTIVITY 14  THE SIXTH EXTINCTION?

Part C: Evidence About Causes of Extinction, 1500–Present

8. With your partner, read the text below about extinction in birds. 227228

222278 NNGGLLSS44BD21

Bird Extinctions

Birds have long fascinated scientists and the frequently. Scientists studying both fossil birds
average person alike. Because of this, their and their modern relatives have been able to
distribution and abundance over recent time determine the actual or probable causes of
have been better documented than those of extinction for species that have gone extinct
other organisms. Because they are vertebrates, since 1500. Below are their findings:
with hard bones, they form fossils relatively

Causes of Bird Extinctions Since 1500

1500–1525 1775–1800 KEY
invasive species
1875–1900 1975–2000 hunting
LabAids SEPUP IAPS Evolution 3e agriculture
Figure: Evo3e SB 14_4 logging
74M yErViaOdLPUroTIROeNg 9.5/11 residential and
commercial
development
climate change and
severe weather
other: energy
production, mining,
transportation,
harvesting plants
and aquatic
resources, water
use, pollution, etc.

THE SIXTH EXTINCTION?  ACTIVITY 14

9. Discuss with your partner what patterns you see in the data
about bird extinctions and what conclusions you might make
from them.

10. What similarities and differences do you notice when comparing
extinctions from long ago with more recent extinctions?

11. Discuss with your group and be prepared to share with the class
the following question: Do you think we are experiencing a
sixth mass extinction? Be sure to cite evidence to support your
answer.229

ANALYSIS

1. In the activity “Fossil Evidence,” you examined fossils of
organisms that have gone extinct. Did any of them go extinct
during the five extinction events? If so, during which events did
the extinctions occur? If not, when on the timeline did they go
extinct?

2. Are we experiencing a sixth extinction? Be sure to use scientific
evidence from this activity (and elsewhere in the unit) to support
your claim.230231

3. How might people be affected if the current rate of extinction
continues into the future?

4. Should we do anything to reduce the current rate of extinction?
Support your answer with evidence, and provide the trade-off(s)
of your decision.232233

EXTENSION

Visit the SEPUP Third Edition Evolution website at www.sepuplhs.org/
middle/third-edition, and go to the link for this activity to learn more
about how humans might be affected by extinction of species and
loss of biodiversity. Reflect on some of the ways that some of the
ways you benefit from biodiversity.

222223333291320 SSNNNEEGGGAASSCSSPPCAEEENTRAA11S224

EVOLUTION  75



15 Bacteria and Bugs:
Evolution of Resistance

reading

In the previous activity, you explored how humans affect evolution
by causing an increase in the rate of extinction. In this activity, you
will revisit a phenomenon you explored in the first activity when
you explored resistance of bacteria to antibiotics. By now, you know
more about evolution by natural selection and how the presence
of a change in the environment can affect evolution. In the case of
bacteria, the presence of antibiotics in their environment can lead
to selection for traits that increase resistance. But does resistance
to chemicals evolve in other situations? In this activity, you will
explore some similar situations to decide if the evolution of resistance
happens elsewhere.234235236

GUIDING QUESTION

What is the evidence that resistance to chemical control
methods is evolving in other types of organisms?

222333654 NNNGGGLLLSSS444BBC211

Crop duster spraying
chemical herbicide on crops

EVOLUTION  77

ACTIVITY 15  BACTERIA AND BUGS: EVOLUTION OF RESISTANCE

MATERIALS

For each student
1 Student Sheet 15.1, “Evolution of Resistance”

READING

1. Read the following text.237238

2. After reading about each type of organism, complete the
corresponding section of Student Sheet 15.1, “Evolution of
Resistance.” 239240

House Mice
House mice, Mus musculus, can pose problems for people because
they eat and contaminate human food. They also occasionally spread
certain diseases. For centuries, people have used traps and poisons
to get rid of house mice. In 1948, people started using warfarin, a
chemical that at high doses causes deadly internal bleeding.

At first, warfarin was very effective in controlling mouse populations.
But by the 1960s, warfarin began to be less effective. Today, some
house mice are no longer affected by warfarin. A new chemical, nick-
named “super-warfarin,” was developed. This newer chemical was also
highly effective—at first. But now, some populations of house mice are
resistant to that chemical as well.

Weeds
Weeds are any plants that are growing where people do not want them.
While rarely harmful to people, they can interfere with human activity.
For example, weeds are a problem for farmers because they compete
with crop plants for space and nutrients. Scientists estimate that weeds
caused an average of 50% reduction in corn production in the United
States and Canada over the period from 2007–2013. This loss equates
to 148 million metric tons (more than 326 billion pounds) of corn. The
cost of this loss was over $26.7 billion each year.

Much effort and money has been put into developing methods for
weed control. These methods include the use of chemicals. One
commonly used chemical, glyphosate, was shown to kill weeds in
1970. One common brand is Roundup©. It works against a wide
variety of plants. For the first 20 years of its use, it remained highly

222243338079 ENSEELLGWLRSTSHPD6O68A8E1191

78 EVOLUTION

BACTERIA AND BUGS: EVOLUTION OF RESISTANCE  ACTIVITY 15

effective at killing weeds. But
in the 1990s, some weeds
began to develop resistance to
glyphosate. To date, over 30
important weed species are no
longer affected by glyphosate.

Mosquitoes This Colorado potato beetle,
an agricultural pest, is
Mosquitoes spread serious resistant to insecticides.
diseases like the Zika virus,
West Nile virus, Dengue fever,
and malaria. Because of the
risk to humans, public health
efforts have gone toward reducing adult mosquito populations and
reducing mosquito breeding grounds. There are several classes of
chemicals commonly used as insecticides.

The World Health Organization has reported that since 2010, a total
of 60 countries have observed mosquito resistance to at least one
class of insecticide, with the first resistance to DDT (one of these
insecticides) documented in 1959 in India. In 49 of those countries,
mosquitoes are resistant to two or more classes of insecticides.
Experts are concerned that this will cause an increase in the number
of cases of mosquito-borne diseases and the deaths they cause.

Plasmodium

As you learned earlier in the unit, mosquitoes spread malaria. A
mosquito transmits the parasite (Plasmodium) to a human when
taking its blood meal. Malaria is one of the deadliest mosquito-borne
diseases in the world. More than 250 million cases are reported each
year, leading to more than one million deaths. Globally, it costs about
$12 billion a year to treat people with malaria. Another $12 billion a
year is lost because of decreased productivity due to work absences.
In countries where malaria is common, the disease accounts for 60%
of visits to health clinics.

The original treatment for malaria was the drug chloroquine, which
was developed in 1934. This drug, which requires taking pills once
a week, worked well for about 30 years. In the 1960s, resistance of
Plasmodium to chloroquine was detected in Southeast Asia. Since

EVOLUTION  79

ACTIVITY 15  BACTERIA AND BUGS: EVOLUTION OF RESISTANCE

then, resistant populations of Plasmodium have spread to nearly all
parts of the world where malaria exists. Newer drugs were developed
to treat the strains resistant to chloroquine. But eventually, Plasmodium
developed resistance to the new drugs, too. Currently, only one class
of drugs remains effective against all malarial strains.

ANALYSIS

1. When you compare all four examples, what similarities and
differences do you notice?241242243

2. In the first activity, “The Full Course,” you explored why it is
important to finish an entire course of antibiotics even if you start
to feel better. Use information from this activity and what you
now know about evolution to provide a scientific explanation for
why this is true.244245246

3. Can the evolution of resistance by pests or harmful organisms be
avoided, or is it inevitable? Explain.

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80 EVOLUTION

16 Manipulating Genes
i n v e s t i g at i o n

In the past two activities, you learned about ways that people have
unintentionally affected evolution. You have also learned that these
effects can, in turn, affect people. In this activity, you will gather
information about two ways that people are intentionally affecting
evolution to produce plants and animals that have traits that are
desirable to us. One method, called selective breeding, involves
selecting and breeding parent organisms with desirable traits. This
method has been used for thousands of years to improve animals and
plants used for food. The second method, genetic modification,
involves modern biotechnology techniques to change one or a small
number of genes in an organism. Genetic modification has been
developed and improved over the past 20–30 years.247248

GUIDING QUESTION

How have humans manipulated genes in other organisms?

224487 NNGGLLSS44DB21

These Holstein cows are bred
for milk production.

EVOLUTION  81

ACTIVITY 16  MANIPULATING GENES

This “golden” rice has been
genetically engineered to
produce beta carotene, a
precursor to Vitamin A.

MATERIALS

For each pair of students
1 computer with Internet access
1 Student Sheet 16.1, “Manipulating Genes Research”

PROCEDURE

1. Listen as your teacher explains how you and the other members in
your group of four will select the topic for your group’s research.

2. Following your teachers instructions, each pair in the group
should search for at least one reference about your topic.

3. Use Student Sheet 16.1, “Manipulating Genes Research,” to
record the information you obtain from your reference.249250251252

4. When both pairs have completed Steps 2 and 3, share the results
of your research with each other, being sure to compare and
contrast your findings.

5. Each group should prepare a summary of your research as
directed by your teacher to share with the rest of the class.

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82 EVOLUTION

MANIPULATING GENES  ACTIVITY 16

ANALYSIS

1. How has understanding the cause-and-effect relationship
between genes and traits led to advanced methods of changing
traits in organisms?253254

2. Compare and contrast the use of selective breeding and genetic
modification for manipulating the traits of other organisms
researched by the class. Be sure to include255256257

a. how they are similar.
b. how they are different.
c. the advantages of each method.

3. Any method that changes the heritable traits in a population
has an effect on the evolution of that population. Do you think
people should intentionally manipulate genes and evolution in
this way?258 259

EXTENSION

Medical researchers are exploring ways to manipulate human genes
to treat certain health conditions, such as some cancers and genetic
conditions. This is called gene therapy. Visit the SEPUP Third Edition
Evolution website at www.sepuplhs.org/middle/third-edition, and go to
the gene therapy links to learn more about recent developments in
this field. Develop a brief presentation to share what you learn.

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EVOLUTION  83



17 Evolution and Us
project

As you have learned, evolution of new species and extinction
of older species continue today as they have in the past. You
have also learned that people can cause intentional or unintentional
changes that affect evolution, and these changes can, in turn, affect
people. In this activity, you will synthesize and communicate some of
the ideas you have learned in this unit.260261262263264

GUIDING QUESTION

How are humans affecting and affected by evolution?

222226666640321 NNENNLGGGGWCLCLHSSCC446CCBC8EE12112 Mosquitoes and the diseases they carry have evolved
in response to our attempts to eliminate them.
This tropical rain forest is home to many species
that are yet to be discovered by people.
The passenger pigeon went extinct in 1901 due to
hunting by people.

EVOLUTION  85

ACTIVITY 17  EVOLUTION AND US

MATERIALS

For each group of four students
supplies for creating presentations/visual displays

PROCEDURE

1. You are working with a scientist who wants to communicate ideas
about evolution to the public. Design and produce a presentation
to communicate to a nonscientific audience how
• evolution by natural selection has a direct impact on their lives.
• understanding evolution can help people make choices about
whether and how they try to change other organisms.

2. Follow your teacher’s instructions about the formats you may
use for your presentation.Your presentation should do the
following:265266267
• Communicate your ideas clearly and convincingly.
• Include both illustrations and any text needed to convey your
key points.
• Describe at least one way that evolution affects people’s
day-to-day lives.
• Describe at least one choice you recommend based on your
understanding of evolution.

3. Be prepared to share your presentation with the class according to
your teacher’s instructions.268

ANALYSIS

1. Reflection: How have your ideas about evolution and its effects
on people changed during this unit?

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86 EVOLUTION

Evolution

unit summary

Evolution by Natural Selection

Within a population, individual organisms typically exhibit a variety
of traits. Many of these variations are caused by variations in genes.
Gene variations arise through the process of mutation, which causes
changes to the structure and function of the organism. Under certain
conditions, some variations may enhance the survival of individuals.
For example, a short, strong beak in a Galapagos finch is a physical
trait that enhances the bird’s survival when nuts are the primary
available food. When there is a change in the environment, such as
a change in food source or a new predator, some organisms in a
population may be better able to survive because of their traits. These
organisms are better adapted to the new environment, and the traits
that help them survive are called adaptations. Over time, individuals
with traits that allow them to survive and produce more offspring will
transmit their traits to their offspring and generations to come. The
trait will become relatively more common in the population. This
process of selection of traits through natural environmental change is
called natural selection.

Speciation and Extinction

Natural selection leads to changes in populations, and in some cases,
to the evolution of new species. For example, in the Galapagos
Islands, 13 species of finch have evolved from one original finch
population that came to the islands many generations ago. The many
species of finch have varying beak shapes and sizes adapted to eating
different foods. The millions of species of plants, animals, and micro-
organisms that live on Earth today evolved over many generations
and are related by descent from common ancestors. Evolutionary
trees, diagrams that represent hypotheses for evolutionary ancestry,
can show these relationships. While there are millions of species on
Earth today, billions of species have existed in the past but are now
extinct. Sometimes species go extinct because no individuals have
traits that allow them to survive and successfully reproduce, whereas
other times extinction is caused by major geological events. Some
extinctions are likely to have multiple or complex causes.

EVOLUTION  87

UNIT SUMMARY

Evidence for Evolution

The geologic timeline reflects the vast time scale since Earth orig-
inated approximately 4.5 billion years ago. This timeline reveals
patterns of appearances and disappearances in the fossil record.
The oldest fossil of a single-celled organism dates back to 3.7 billion
years ago. Multicellular life originated relatively recently in geologic
time, approximately 640 million years ago. Most key events in the
evolution of life occurred in very recent geologic time with the start
of the Cambrian explosion 550 million years ago. Based on evidence
from fossils, scientists have developed and tested hypotheses for how
major groups of organisms, such as vertebrates, evolved. Scientists
use stratigraphy and radiometric dating to determine the age of rock
layers and the fossils within them. Stratigraphy is based on the idea
that older rock layers are found beneath layers that were formed more
recently. Fossils in lower layers are generally older than fossils in upper
layers, allowing scientists to reconstruct the evolutionary history of the
organisms. Another source of evidence for evolutionary relationships
comes from embryos, some of which exhibit traits that are lost during
development and are not present in the adults. Scientists use patterns
in the data from these lines of evidence to develop cause-and-effect
explanations about evolution.

Humans and Evolution

Humans have mutual cause-and-effect evolutionary relationships with
other organisms. Most of the extinctions that have occurred in the
past 500 years have been caused by human activities, such as hunting,
habit loss, or climate change. The current rate of extinction due to
human activity may represent a sixth extinction event. With the loss
of so many species, humans may be losing some of the benefits that
could be derived from these species. Humans also affect how species
evolve, especially those species that cause problems for us, because we
have used chemicals like pesticides that lead to selection for resistance
to these chemicals. These species may become even more difficult to
control in the long run.

People have intentionally affected species, including many domestic
animals and crops, through selective breeding and genetic engineering.
In selective breeding (artificial selection), humans select individuals
with desirable traits and breed them in the hopes of producing
offspring with combinations of the desired traits. Artificial selection

88 EVOLUTION

UNIT SUMMARY

produces organisms with traits that are desired by the breeder. For
example, dogs have been bred to produce a wide variety of appear-
ances and behaviors desired by pet owner. Crops and livestock have
been bred or genetically modified for such traits as pest and disease
resistance and to increase the amount of food they produce. There
is debate among people about whether we should genetically modify
organisms, which requires us to evaluate the evidence and trade-offs
of this type of modification.

Essential Scientific Terms mutation
adaptation natural selection
cause and effect patterns
embryos speciation
evidence and trade-offs structure and function
evolution traits
extinction variation
fossils

EVOLUTION  89