Chapter 7 Evolution 1 
7
The Evolution of Living
Things
Biological evolution explains how populations change over time.
SECTION
1 Change over Time . . . . . . . . . . 166
2 How Does Evolution
Happen? . . . . . . . . . . . . . . . . . . 174
3 Natural Selection in Action. . . 180
PRE-READING
About the
Can you find two eyes and a mouth in this photo? The eyes and mouth belong to an adult flounder. Adult flounders swim on their sides and have both eyes on one side of their body.
These characteristics allow flounders to lie flat and still see all of their surroundings. Flounders also look like the sandy bottoms of coastal areas. These adaptations help flounders survive in their environment.
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Concept Map Before you read the chapter, create the graphic organizer entitled “Concept Map” described in the
Study Skills section of the Appendix. As you read the chapter, fill in the concept map with details about evolution and natural selection. START-UP
Out of Sight, Out of Mind
Analysis
In this activity, you will see how traits can affect the success of an organism in a particular environment.
1. How many white marshmallows did your partner pick? How many colored marshmallows did he or she pick?
Procedure
1. Count out 25 colored marshmallows and
25 white marshmallows.
2. What did the marshmallows and the cloth represent in your investigation? What effect did the color of the cloth have?
2. Ask your partner to look away while you spread the marshmallows out on a white cloth. Do not make a pattern with the marshmallows. Now, ask your partner to turn around and pick the first marshmallow that he or she sees.
3. When an organism blends into its environment, the organism is camouflaged. How does this activity model camouflaged organisms in the wild? What are some weaknesses of this model?
3. Repeat step 2 ten times.
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Change over Time
1
If someone asked you to describe a frog, you might say that a frog has long hind legs, has bulging eyes, and croaks. But what color skin would you say that a frog has?
What You Will Learn
Identify two kinds of evidence that show that organisms have evolved.
Describe one pathway through which a modern whale could have evolved from an ancient mammal.
Explain how comparing organisms can provide evidence that they have ancestors in common.
Vocabulary adaptation species evolution fossil fossil record
READING STRATEGY
Paired Summarizing Read this section silently. In pairs, take turns summarizing the material. Stop to discuss ideas that seem confusing.
Once you start to think about frogs, you realize that frogs differ in many ways. These differences set one kind of frog apart from another. The frogs in Figures 1, 2, and 3 look different from each other, yet they may live in the same areas.
Differences Among Organisms
As you can see, each frog has a different characteristic that might help the frog survive. A characteristic that helps an organism survive and reproduce in its environment is called an adaptation. Adaptations may be physical, such as a long neck or striped fur. Or adaptations may be behaviors that help an organism find food, protect itself, or reproduce.
Living things that have the same characteristics may be members of the same species. A species is a group of organisms that can mate with one another to produce fertile offspring. For example, all strawberry poison arrow frogs are members of the same species and can mate with each other to produce more strawberry poison arrow frogs. Groups of individuals of the same species living in the same place make up a population.
✓
Reading Check How can you tell that organisms are members of the same species? (See the Appendix for answers to Reading Checks.)
Figure 2 The bright coloring of the strawberry poison arrow frog warns predators that the frog is poisonous.
Figure 1 The red-eyed tree frog hides among a tree’s leaves during the day and comes out at night.
Figure 3 The smokey jungle frog blends into the forest floor.
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Do Species Change over Time?
In a single square mile of rain forest, there may be dozens of species of frogs. Across the Earth, there are millions of different species of organisms. The species that live on Earth today range from single-celled bacteria, which lack cell nuclei, to multicellular fungi, plants, and animals. Have these species always existed on Earth?
Scientists think that Earth has changed a great deal during its history, and that living things have changed, too. Scientists estimate that the planet is 4.6 billion years old. Since life first appeared on Earth, many species have died out, and many new species have appeared. Figure 4 shows some of the species that have existed during Earth’s history.
Scientists observe that species have changed over time. They also observe that the inherited characteristics in populations change over time. Scientists think that as populations change over time, new species form. Thus, newer species descend from older species. The process in which populations gradually change over time is called evolution. Scientists continue to develop theories to explain exactly how evolution happens.
adaptation a characteristic that improves an individual’s ability to survive and reproduce in a particular environment species a group of organisms that are closely related and can mate to produce fertile offspring evolution the process in which inherited characteristics within a population change over generations such that new species sometimes arise Figure 4 This diagram shows some of the many kinds of organisms that have lived on Earth since the planet formed 4.6 billion years ago.
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Figure 5 The fossil on the left is of a trilobite, an ancient aquatic animal. The fossils on the right are of seed ferns.
Evidence of Changes over Time
Evidence that organisms have changed over time is buried within Earth’s crust. The layers are made up of different kinds of rock and soil stacked on top of each other. These layers form when sediments, particles of sand, dust, or soil, are carried by wind and water and are deposited in an orderly fashion.
Older layers are deposited before newer layers and are buried deeper within Earth.
Fossils
fossil the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock fossil record a historical sequence of life indicated by fossils found in layers of the Earth’s crust
The remains or imprints of once-living organisms found in layers of rock called fossils. Examples of fossils are shown in
Figure 5. Fossils can be complete organisms, parts of organisms, or just a set of footprints. Fossils usually form when a dead organism is covered by a layer of sediment. Over time, more sediment settles on top of the organism. Minerals in the sediment may seep into the organism and gradually replace the organism with stone. If the organism rots away completely after being covered, it may leave an imprint of itself in the rock.
The Fossil Record
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. Fossils found in newer layers of Earth’s crust tend to be similar to present-day organisms. This similarity indicates that the fossilized organisms were close relatives of present-day organisms.
Fossils from older layers are less similar to present-day organisms than fossils from newer layers are. The older fossils are of earlier life-forms, which may not exist anymore.
✓Reading Check
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How does the fossil record organize fossils?
Evidence of Ancestry
The fossil record provides evidence about the order in which species have existed. Scientists observe that all living organisms have characteristics in common and inherit characteristics in similar ways. So, scientists think that all living species descended from common ancestors. Evidence of common ancestors can be found in fossils and in living organisms.
Drawing Connections
Scientists examine the fossil record to figure out the relationships between extinct and living organisms. Scientists draw models, such as the one shown in Figure 6, that illustrate their hypotheses. The short horizontal line at the top left in the diagram represents a species that lived in the past. Each branch in the diagram represents a group of organisms that descended from that species.
Scientists think that whales and some types of hoofed mammals have a common ancestor, as Figure 6 shows. This ancestor was probably a mammal that lived on land between
50 million and 70 million years ago. During this time period, the dinosaurs died out and a variety of mammals appeared in the fossil record. The first ocean-dwelling mammals appeared about 50 million years ago. Scientists think that all mammal species alive today evolved from common ancestors.
Scientists have named and described hundreds of thousands of living and ancient species. Scientists use information about these species to sketch out a “tree of life” that includes all known organisms. But scientists know that their information is incomplete. For example, parts of Earth’s history lack a fossil record. In fact, fossils are rare because specific conditions are necessary for fossils to form.
Sedimentary Rock Fossils are most often found in sedimentary rock. Sedimentary rock usually forms when rock is broken into sediment by wind, water, and other means.
The wind and water move the sediment around and deposit it. Over time, layers of sediment pile up. Lower layers are compressed and changed into rock. Find out if your area has any sedimentary rocks that contain fossils. Mark the location of such rocks on a copy of a local map.
Figure 6 This diagram is a model of the proposed relationships between ancient and modern mammals that have characteristics similar to whales.
camels and IIamas pigs and peccaries hoofed grazing mammals hippopotamuses toothed whales baleen whales
70
Approximate time (millions of years ago)
0
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Examining Organisms
Examining an organism carefully can give scientists clues about its ancestors. For example, whales seem similar to fish. But unlike fish, whales breathe air, give birth to live young, and produce milk. These traits show that whales are mammals. Thus, scientists think that whales evolved from ancient mammals.
Case Study: Evolution of the Whale
Scientists think that the ancient ancestor of whales was probably a mammal that lived on land and that could run on four legs. A more recent ancestor was probably a mammal that spent time both on land and in water. Comparisons of modern whales and a large number of fossils have supported this hypothesis. Figure 7 illustrates some of this evidence.
✓
Reading Check What kind of organism do scientists think was an ancient ancestor of whales?
Figure 7
Evidence of Whale Evolution
a Pakicetus (PAK uh SEE tuhs)
Scientists think that whales evolved from land-dwelling mammals that could run on four legs. One of these ancestors may have been Pakicetus, which lived about
50 million years ago. The fossil skeleton and an artist’s illustration of Pakicetus are shown here. Pakicetus was about the size of a wolf.
b Ambulocetus (AM byoo loh SEE tuhs)
This mammal lived in coastal waters about 49 million years ago. It could swim by kicking its legs and using its tail for balance. It could also waddle on land by using its short legs. Ambulocetus was about the size of a dolphin.
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Walking Whales
The organisms in Figure 7 form a sequence between ancient four-legged mammals and modern whales. Several pieces of evidence indicate that these species are related by ancestry.
Each species shared some traits with an earlier species. However, some species had new traits that were shared with later species. Yet, each species had traits that allowed it to survive in a particular time and place in Earth’s history.
Further evidence can be found inside the bodies of living whales. For example, although modern whales do not have hind limbs, inside their bodies are tiny hip bones, as shown in Figure 7. Scientists think that these hip bones were inherited from the whales’ four-legged ancestors. Scientists often look at this kind of evidence when they want to determine the relationships between organisms.
The Weight of Whales
Whales are the largest animals ever known on Earth.
One reason whales can grow so large is that they live in water, which supports their weight in a way that their bones could not. The blue whale—the largest type of whale in existence—is about
24 m long and has a mass of about 99,800 kg. Convert these measurements into feet and pounds, and round to whole numbers.
c Dorudon (DOH roo DON)
This mammal lived in the oceans about 40 million years ago. It resembled a giant dolphin and propelled itself with its massive tail. Dorudon had tiny hind limbs that it could not use for walking or swimming.
d Modern toothed whale
Modern whales’ forelimbs are flippers.
Modern whales do not have hind limbs, but they do have tiny hip bones.
Modern whales range in size from
1.4 m porpoises to 33 m blue whales.
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Human arm
Dolphin flipper
Cat leg
Bat wing
Figure 8 The bones in the front limbs of these animals are similar. Similar bones are shown in the same color. These limbs are different sizes in life.
Comparing Organisms
Evidence that groups of organisms have common ancestry can be found by comparing the groups’ DNA. Because every organism inherits DNA, every organism inherits the traits determined by DNA. Organisms contain evidence that populations and species undergo changes in traits and DNA over time.
Comparing Skeletal Structures
What do your arm, the front leg of a cat, the front flipper of a dolphin, and the wing of a bat have in common? You might notice that these structures do not look alike and are not used in the same way. But under the surface, they have similarities.
Look at Figure 8. The structure and order of bones of a human arm are similar to those of the front limbs of a cat, a dolphin, and a bat. These similarities suggest that cats, dolphins, bats, and humans had a common ancestor. Over millions of years, changes occurred in the limb bones. Eventually, the bones performed different functions in each type of animal.
Comparing DNA
For another activity related to this chapter, go to go.hrw.com and type in the keyword HL5EVOW.
When scientists compare organism’s traits, such as skeletal structures, much of the information that they get supports the theory that organisms share a common ancestor. To further support this theory, scientists compare organisms’ DNA at a molecular level. Scientists analyze many organisms’ DNA, RNA, proteins, and other molecules. Then, scientists compare the data for each species. The greater the number of similarities between the data sets for any two species, the more closely the two species are related through a common ancestor. Scientists use molecular data, the comparison of traits, and fossils to support the theory that because all existing species have DNA, all species share a common ancestor.
✓
Reading Check If two species have similar DNA, what hypothesis is supported?
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Review
Summary
is the process in which inherited
• Evolution characteristics within a population change
think that modern whales
• Scientists evolved from an ancient, land-dwelling
over generations, sometimes giving rise to new species. Scientists continue to develop theories to explain how evolution happens.
•
mammal ancestor. Fossil organisms that support this hypothesis have been found. of common ancestry among liv• Evidence ing organisms is provided by comparing
Evidence that organisms evolve can be found by comparing living organisms to each other and to the fossil record. Such comparisons provide evidence of common ancestry. DNA and inherited traits. Species that have a common ancestor will have traits and
DNA that are more similar to each other than to those of distantly related species.
Using Key Terms
Complete each of the following sentences by choosing the correct term from the word bank. adaptation species fossil evolution
1. Members of the same can mate with one another to produce offspring.
2. A(n)
helps an organism survive.
3. When populations change over time, occurred. has
Understanding Key Ideas
8. Forming Hypotheses Is a person’s DNA likely to be more similar to the DNA of his or her biological parents or to the DNA of one of his or her cousins? Explain your answer.
Interpreting Graphics
9. The photograph below shows the layers of sedimentary rock exposed during the construction of a road. Imagine that a species that lived 200 million years ago is found in layer b.
Would the species’ ancestor, which lived
250 million years ago, most likely be found in layer a or in layer c? Explain your answer.
a
4. A human’s arm, a cat’s front leg, a dolphin’s front flipper, and a bat’s wing
a. have similar kinds of bones.
b. are used in similar ways.
c. are very similar to insect wings and jellyfish tentacles. d. have nothing in common.
b c 5. How does the fossil record show that species have changed over time?
6. What evidence do fossils provide about the ancestors of whales?
Critical Thinking
7. Making Comparisons Other than the examples provided in the text, how are whales different from fishes?
For a variety of links related to this chapter, go to www.scilinks.org
Topic: Species and Adaptation;
Fossil Record
SciLinks code: HSM1433; HSM0615
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How Does Evolution
Happen?
2
What You Will Learn
List four sources of Charles Darwin’s ideas about evolution.
Describe the four parts of Darwin’s theory of evolution by natural selection. Relate genetics to evolution.
Vocabulary trait selective breeding natural selection
Imagine that you are a scientist in the 1800s. Fossils of some very strange animals have been found. And some familiar fossils have been found where you would least expect them.
How did seashells end up on the tops of mountains?
In the 1800s, geologists began to realize that the Earth is much older than anyone had previously thought. Evidence showed that gradual processes had changed the Earth’s surface over millions of years. Some scientists saw evidence of evolution in the fossil record. However, no one had been able to explain how evolution happens—until Charles Darwin.
Charles Darwin
READING STRATEGY
Brainstorming The key idea of this section is natural selection. Brainstorm words and phrases related to natural selection. In 1831, 21-year-old Charles Darwin, shown in Figure 1, graduated from college. Like many young people just out of college, Darwin didn’t know what he wanted to do with his life.
His father wanted him to become a doctor, but seeing blood made Darwin sick. Although he eventually earned a degree in theology, Darwin was most interested in the study of plants and animals.
So, Darwin signed on for a five-year voyage around the world. He served as the naturalist—a scientist who studies nature—on the British ship the HMS Beagle, similar to the ship in Figure 2. During the trip, Darwin made observations that helped him form a theory about how evolution happens.
Figure 1 Charles Darwin wanted to understand the natural world.
Figure 2 Darwin sailed around the world on a ship similar to this one.
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Figure 3 The course of the
HMS Beagle is shown by the red line. The journey began and ended in England.
Darwin’s Excellent Adventure
The Beagle’s journey is charted in Figure 3. Along the way,
Darwin collected thousands of plant and animal samples. He kept careful notes of his observations. One interesting place that the ship visited was the Galápagos Islands. These islands are found 965 km (600 mi) west of Ecuador, a country in
South America.
✓
Reading Check Where are the Galápagos Islands? (See the Appendix for answers to Reading Checks.)
Darwin’s Finches
Darwin noticed that the animals and plants on the Galápagos
Islands were a lot like those in Ecuador. However, they were not exactly the same. The finches of the Galápagos Islands, for example, were a little different from the finches in Ecuador.
And the finches on each island differed from the finches on the other islands. As Figure 4 shows, the beak of each finch is adapted to the way the bird usually gets food.
Figure 4
Some Finches of the Galápagos Islands
The large ground finch has a wide, strong beak that it uses to crack open big, hard seeds. This beak works like a nutcracker.
The cactus finch has a tough beak that it uses for eating cactus parts and insects. This beak works like a pair of needle-nose pliers.
Section 2
The warbler finch has a small, narrow beak that it uses to catch small insects. This beak works like a pair of tweezers.
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Darwin’s Thinking
After returning to England, Darwin puzzled over the animals of the Galápagos Islands. He tried to explain why the animals seemed so similar to each other yet had so many different adaptations. For example, Darwin hypothesized that the island finches were descended from South American finches. The first finches on the islands may have been blown from South America by a storm. Over many generations, the finches may have evolved adaptations for the various island environments.
During the course of his travels, Darwin came up with many new ideas. Before sharing these ideas, he spent several years analyzing his evidence and gathering ideas from other people. Ideas About Breeding trait a genetically determined characteristic selective breeding the human practice of breeding animals or plants that have certain desired traits Figure 5 Over the past 12,000 years, dogs have been selectively bred to produce more than 150 breeds.
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In Darwin’s time, farmers and breeders had produced many kinds of farm animals and plants. These plants and animals had traits that were desired by the farmers and breeders. A trait is a characteristic that can be passed from parent to offspring through genes. The process in which humans select which plants or animals to reproduce based on certain desired traits is called selective breeding. Most pets, such as the dogs in Figure 5, have been bred for various desired traits.
You can see the results of selective breeding in many kinds of organisms. For example, people have bred horses that are particularly fast or strong. And farmers have bred crops that produce large fruit or that grow in specific climates.
Population Growth Versus Food Supply
1. Get an egg carton and a bag of rice. Use a marker to label one row of the carton “Food supply.” Then, label the second row “Human population.” 2. In the row labeled “Food supply,” place one grain of rice in the first cup. Place two grains of rice in the second cup, and place three grains of rice in the third cup. In each subsequent cup, place one more grain than you placed in the previous cup. Imagine that each grain represents enough food for one person’s lifetime.
3. In the row labeled “Human population,” place one grain of rice in the first cup. Place two grains in the second cup, and place four grains in the third cup. In each subsequent cup, place twice as many grains as you placed in the previous cup. This rice represents people.
4. How many units of food are in the sixth cup?
How many “people” are in the sixth cup? If this pattern continued, what would happen?
5. Describe how the patterns in the food supply and in the human population differ. Explain how the patterns relate to Malthus’s hypothesis.
Ideas About Population
Malthus’s Description of Unlimited Population Growth
Human
population
Quantity
During Darwin’s time, Thomas Malthus wrote a famous book entitled An Essay on the Principle of
Population. Malthus noted that humans have the potential to reproduce rapidly. He warned that food supplies could not support unlimited population growth. Figure 6 illustrates this relationship. However, Malthus pointed out that human populations are limited by choices that humans make or by problems such as starvation and disease.
After reading Malthus’s work, Darwin realized that any species can produce many offspring. He also knew that the populations of all species are limited by starvation, disease, competition, and predation. Only a limited number of individuals survive to reproduce. Thus, there is something special about the survivors. Darwin reasoned that the offspring of the survivors inherit traits that help the offspring survive in their environment.
Food supply Time
Figure 6 Malthus thought that the human population could increase more quickly than the food supply, with the result that there would not be enough food for everyone.
Ideas About Earth’s History
Darwin had begun to think that species could evolve over time.
But most geologists at the time did not think that Earth was old enough to allow for slow changes. Darwin learned new ideas from Principles of Geology, a book by Charles Lyell. This book presented evidence that Earth had formed by natural processes over a long period of time. It became clear to Darwin that Earth was much older than anyone had imagined.
✓Reading Check
What did Darwin learn from Charles Lyell?
Section 2
How Does Evolution Happen?
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Darwin’s Theory of Natural Selection
natural selection the process by which individuals that are better adapted to their environment survive and reproduce more successfully than less well adapted individuals do; a theory to explain the mechanism of evolution
After his voyage on the HMS Beagle, Darwin privately struggled with his ideas for about 20 years. Then, in 1858, Darwin received a letter from a fellow naturalist named Alfred Russel
Wallace. Wallace had arrived at the same ideas about evolution that Darwin had. In 1859, Darwin published a famous book called On the Origin of Species by Means of Natural Selection. In his book, Darwin proposed the theory that evolution happens through natural selection. Natural selection is the process by which organisms that are better adapted to their environment survive and reproduce more successfully than less well adapted organisms do. The process has four parts and is explained in
Figure 7.
✓Reading Check
Figure 7
What is natural selection?
Four Parts of Natural Selection
1 Overproduction A tarantula’s egg sac may hold
500–1,000 eggs. Some of the eggs will survive and develop into adult spiders. Some will not.
2 Inherited Variation Every individual has its own combination of traits. Each tarantula is similar to, but not identical to, its parents.
3 Struggle to Survive Some tarantulas may be caught by predators, such as this wasp. Other tarantulas may starve or get a disease. Only some of the tarantulas will survive to adulthood.
4 Successful Reproduction The tarantulas that are best adapted to their environment are likely to have many offspring that survive.
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Genetics and Evolution
Darwin lacked evidence for parts of his theory. For example, he knew that organisms inherit traits, but not how they inherit traits. He knew that there is great variation among organisms, but not how that variation occurs. Today, scientists have found most of the evidence that Darwin lacked. They know that variation happens as a result of differences in genes. Changes in genes may happen whenever organisms produce offspring.
Some genes make an organism more likely to survive to reproduce. The process called selection happens when only organisms that carry these genes can survive to reproduce. New fossil discoveries and new information about genes add to scientists’ understanding of natural selection and evolution.
Review
Summary
explained that
• Darwin evolution occurs through natural selection. His theory has four parts:
1. Each species produces more offspring than will survive to reproduce. 2. Individuals within a population have slightly different traits.
3. Individuals within a population compete with each other for limited resources.
4. Individuals that are better equipped to live in an environment are more likely to survive to reproduce. genetics helps
• Modern explain the theory of natural selection.
Using Key Terms
1. In your own words, write a definition for the term trait.
2. Use the following terms in the same sentence: selective breeding and natural selection.
Understanding Key Ideas
3. Modern scientific explanations of evolution
a. have replaced Darwin’s theory.
b. rely on genetics instead of natural selection.
c. fail to explain how traits are inherited. d. combine the principles of natural selection and genetic inheritance. Math Skills
8. In a sample of 80 beetles,
50 beetles had 4 spots each, and the rest had 6 spots each.
What was the average number of spots per beetle?
Critical Thinking
9. Making Comparisons In selective breeding, humans influence the course of evolution. What determines the course of evolution in natural selection?
10. Predicting Consequences
Suppose that an island in the
Pacific Ocean was just formed by a volcano. Over the next million years, how might species evolve on this island?
4. Describe the observations that
Darwin made about the species on the Galápagos Islands.
5. Summarize the ideas that
Darwin developed from books by Malthus and Lyell.
6. Describe the four parts of
Darwin’s theory of evolution by natural selection.
7. What knowledge did Darwin lack that modern scientists now use to explain evolution?
For a variety of links related to this chapter, go to www.scilinks.org
Topic: Galápagos Islands;
Darwin and Natural Selection
SciLinks code: HSM0631; HSM0378
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3
What You Will Learn
Give three examples of natural selection in action.
Outline the process of speciation.
Vocabulary generation time speciation READING STRATEGY
Prediction Guide Before reading this section, write the title of each heading in this section. Next, under each heading, write what you think you will learn.
Natural Selection in Action
Have you ever had to take an antibiotic? Antibiotics are supposed to kill bacteria. But sometimes, bacteria are not killed by the medicine. Do you know why?
A population of bacteria might develop an adaptation through natural selection. Most bacteria are killed by the chemicals in antibiotics. But a few of the bacteria have an adaptation that makes them naturally resistant to, or not killed by, the antibiotic. These few bacteria survive antibiotic treatment, continue to reproduce, and pass the adaptation to their offspring. After several generations, almost all the bacteria in the population carry the adaptation of antibiotic resistance.
Changes in Populations
The theory of natural selection explains how a population changes in response to its environment. Through ongoing natural selection, a population adapts to its environment.
Well-adapted individuals will likely survive and reproduce.
Adaptation to Hunting
Changes in populations are sometimes observed when a new force affects the survival of individuals. Scientists think that hunting in Uganda is affecting Uganda’s elephant population.
In 1930, about 99% of the male elephants in one area had tusks. Only 1% of the elephants were born without tusks.
Today, as many as 15% of the male elephants in that area lack tusks. What happened?
A male African elephant that has tusks is shown in Figure 1. The ivory of an elephant’s tusks is very valuable. People hunt the elephants for their tusks. As a result, fewer of the elephants that have tusks survive to reproduce, and more of the tuskless elephants survive. When the tuskless elephants reproduce, they pass the tuskless trait to their offspring.
Figure 1 The ivory tusks of African elephants are very valuable. Some elephants are born without tusks.
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Figure 2
Natural Selection of Insecticide Resistance
1 An insecticide will kill most insects, but a few may survive. These survivors have genes that make them resistant to the insecticide.
2 The survivors then reproduce, passing the insecticideresistance genes to their offspring.
3 In time, the replacement population of insects is made up mostly of individuals that have the insecticideresistance genes.
4 When the same kind of insecticide is used on the insects, only a few are killed because most of them are resistant to that insecticide. Insecticide Resistance
To control insect pests, many people use insecticides, chemicals that kill insects. Sometimes, an insecticide that used to work well no longer affects an insect population. The reason is that a few insects in the population are resistant to the chemical.
These insects survive insecticide treatment and pass the resistance trait to their offspring. Figure 2 shows how an insect population becomes resistant to some insecticides.
Insect populations can evolve quickly because insects produce many offspring and have a short generation time.
Generation time is the average time between one generation and the next.
✓
Reading Check Why do insects quickly develop resistance to insecticides? (See the Appendix for answers to Reading Checks.)
generation time the period between the birth of one generation and the birth of the next generation
Competition for Mates
For organisms that reproduce sexually, competition for mates can select for adaptations. For example, in many bird species, females prefer to mate with colorful males. So, colorful males have more offspring than noncolorful males do. Because colorful males are more likely to pass on their genes to the next generation, the proportion of colorful males is likely to increase from generation to generation.
Section 3
Natural Selection in Action
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Forming a New Species
speciation the formation of new species as a result of evolution
Sometimes, drastic changes that can form a new species take place. In the animal kingdom, a species is a group of organisms that can mate with each other to produce fertile offspring. A new species may form after a group becomes separated from the original population. This group forms a new population.
Over time, the new population adapts to its new environment.
Eventually, the new population and the original population differ so greatly that they can no longer mate successfully. The new population may then be considered a new species. The formation of a new species as a result of evolution is called speciation (SPEE shee AY shuhn). Figure 3 shows how new species of Galápagos finches may have formed. Speciation may happen in other ways as well.
Separation
Speciation often begins when a part of a population becomes separated from the rest. The process of separation can happen in several ways. For example, a newly formed canyon, mountain range, or lake can divide the members of a population.
✓Reading Check
How can parts of a population become
separated?
Figure 3
The Evolution of Galápagos Finch Species
1 Some finches left the mainland and reached one of the islands (separation).
2 The finches reproduced and adapted to the environment
(adaptation).
3 Some finches flew to a second island (separation).
4 The finches reproduced and adapted to the different environment (adaptation).
5 Some finches flew back to the first island but could no longer interbreed with the finches there (division).
6 This process may have occurred over and over again as the finches flew to the other islands.
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Adaptation
Division
Over many generations, two separated groups of a population may become very different. Even if a geographical barrier is removed, the groups may not be able to interbreed anymore. At this point, the two groups are no longer the same species.
Figure 4 shows another way that populations may stop interbreeding. Leopard frogs and pickerel frogs probably had the same ancestor species.
Then, at some point, some of these frogs began to mate at different times during the year.
Review
Mating Times
March 1
selection
• Natural explains how popula-
•
tions adapt to changes in their environment. A variety of examples of such adaptations can be found.
Natural selection also explains how one species may evolve into another. Speciation occurs as populations undergo separation, adaptation, and division.
April 1
May 1
June 1
July 1
Month
Figure 4 The leopard frog and the pickerel frog are similar species. However, leopard frogs do not search for mates at the same time of year that pickerel frogs do.
Using Key Terms
1. In your own words, write a definition for the term speciation.
Understanding Key Ideas
Summary
Pickerel frog
Bullfrog
Leopard frog
Tree frog
Mating behavior
Populations constantly undergo natural selection.
After two groups have separated, natural selection may act on each group in different ways.
Over many generations, the separated groups may evolve different sets of traits. If the environmental conditions for each group differ, the adaptations in the groups will also differ.
2. Two populations have evolved into two species when
a. the populations are separated.
b. the populations look different.
c. the populations can no longer interbreed. d. the populations adapt.
Critical Thinking
5. Forming Hypotheses Most kinds of cactus have leaves that grow in the form of spines. The stems or trunks become thick, juicy pads or barrels. Explain how these cactus parts might have evolved.
6. Making Comparisons Suggest an organism other than an insect that might evolve an adaptation to human activities.
3. Explain why the number of tuskless elephants in Uganda may be increasing.
Math Skills
4. A female cockroach can produce
80 offspring at a time. If half of the offspring produced by a certain female are female and each female produces 80 offspring, how many cockroaches are there in the third generation?
Developed and maintained by the
National Science Teachers Association
For a variety of links related to this chapter, go to www.scilinks.org
Topic: Species and Adaptation
SciLinks code: HSM1433
183
Using Scientific Methods
Lab
Survival of the Chocolates
OBJ ECTIVES
Form a hypothesis about the fate of the candy-coated chocolates. Predict what will happen to the candy-coated chocolates.
Design and conduct an experiment to test your hypothesis. MATE RIALS
•
•
chocolates, candy-coated, small, in a variety of colors
(about 100) items to be determined by the students and approved by the teacher
SAFETY
Imagine a world populated with candy, and hold that delicious thought in your head for just a moment. Try to apply the idea of natural selection to a population of candy-coated chocolates.
According to the theory of natural selection, individuals who have favorable adaptations are more likely to survive. In the
“species” of candy-coated chocolates you will study in this experiment, the characteristics of individual chocolates may help them “survive.” For example, shell strength (the strength of the candy coating) could be an adaptive advantage. Plan an experiment to find out which characteristics of the chocolates are favorable “adaptations.”
Ask a Question
1
Form a Hypothesis
2
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What might “survival” mean for a candy-coated chocolate?
What are some ways you can test which chocolates are the
“strongest” or “most fit” for their environment? Also, write down any other questions that you could ask about the “survival” of the chocolates.
Form a hypothesis, and make a prediction. For example, if you chose to study candy color, your prediction might be similar to colored shell is the strongest, then fewer of the this: If the chocolates with this color of shell will when .
The Evolution of Living Things
Test the Hypothesis
Analyze the Results
3
Design a procedure to determine which type of candy-coated chocolate is most likely to survive. In your plan, be sure to include materials and tools you may need to complete this procedure.
1
4
Check your experimental design with your teacher before you begin. Your teacher will supply the candy and assist you in gathering materials and tools.
2
5
Record your results in a data table. Be sure to organize your data in a clear and understandable way.
Describing Events
Write a report that describes your experiment. Be sure to include tables and graphs of the data you collected.
Draw Conclusions
Evaluating Data In your report, explain how your data either support or do not support your hypothesis. Include possible errors and ways to improve your procedure.
Applying Your Data
Can you think of another characteristic of the chocolates that can be tested to determine which type is best adapted to survive? Explain your idea, and describe how you might test it.
Chapter Lab
185
USING KEY TERMS
Complete each of the following sentences by choosing the correct term from the word bank.
UNDERSTANDING KEY IDEAS
Multiple Choice
8 Fossils are commonly found in
a. sedimentary rock.
adaptation evolution generation time species speciation fossil record selective breeding natural selection
b. all kinds of rock.
c. granite.
d. loose sand.
9 The fact that all organisms have
DNA as their genetic material is evidence that
a. all organisms undergo natural
selection.
b. all organisms may have descended from a common ancestor.
c. selective breeding takes place every day.
d. genetic resistance rarely occurs.
1 When a single population evolves into two populations that cannot interbreed anymore, has occurred.
2 Darwin’s theory of
explained the process by which organisms become well-adapted to their environment.
3 A group of organisms that can mate with each other to produce offspring is known as a(n)
.
0 Charles Darwin puzzled over differ-
ences in the of the different species of Galápagos finches.
a. webbed feet
4 The
b. beaks
provides information about organisms that have lived in the past.
5 In
, humans select organisms with desirable traits that will be passed from one generation to another.
c. bone structure of the wings
d. eye color
q Darwin observed variations among
6 A(n)
helps an organism survive better in its environment.
7 Populations of insects and bacteria can evolve quickly because they usually have a short
.
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individuals within a population, but he did not realize that these variations were caused by
a. interbreeding.
b. differences in food.
c. differences in genes.
d. selective breeding.
p Analyzing Relationships Geologists
have evidence that the continents were once a single giant continent.
This giant landform eventually split apart, and the individual continents moved to their current positions.
What role might this drifting of continents have played in evolution?
w Identify two ways that organisms can be compared to provide evidence of evolution from a common ancestor.
e Describe evidence that supports the
hypothesis that whales evolved from land-dwelling mammals.
r Why are some animals more likely
to survive to adulthood than other animals are?
t Explain how genetics is related to evolution. y Outline an example of the process of speciation.
INTERPRETING GRAPHICS
The graphs below show information about the infants that are born and the infants that have died in a population.
The weight of each infant was measured at birth. Use the graphs to answer the questions that follow.
terms to create a concept map: struggle to survive, theory, genetic variation,
Darwin, overpopulation, natural selection, and successful reproduction.
i Making Inferences How could natural selection affect the songs that birds sing?
o Forming Hypotheses In Australia,
many animals look like mammals from other parts of the world. But most of the mammals in Australia are marsupials, which carry their young in pouches after birth. Few kinds of marsupials are found anywhere else in the world. What is a possible explanation for the presence of so many of these unique mammals in Australia?
Percentage of births
u Concept Mapping Use the following
Infant Deaths by Birth Weight
Infant Births by Birth Weight
CRITICAL THINKING
20
15
10
5
0
2 4 6 8 10
Weight ( lb)
Probability of death (%)
Short Answer
100
10
1
2
4 6
8
(
)
Weight lb
a What is the most common birth weight? s At which birth weight is an infant most likely to survive?
d How do the principles of natural selection help explain why there are more deaths among babies whose birth weights are low than among babies whose birth weights are average?
Chapter Review
187
READI NG
Read each of the passages below. Then, answer the questions that follow each passage.
Passage 1 When the Grand Canyon was forming, a single population of tassel-eared squirrels may have been separated into two groups. Today, descendants of the two groups live on opposite sides of the canyon. The two groups share many characteristics, but they do not look the same. For example, both groups have tasseled ears, but each group has a unique fur color pattern. An important difference between the groups is that the Abert squirrels live on the south rim of the canyon, and the Kaibab squirrels live on the north rim.
The environments on the two sides of the
Grand Canyon are different. The north rim is about 370 m higher than the south rim. Almost twice as much precipitation falls on the north rim than on the south rim every year. Over many generations, the two groups of squirrels have adapted to their new environments. Over time, the groups became very different. Many scientists think that the two types of squirrels are no longer the same species. The development of these two squirrel groups is an example of speciation in progress.
1. Which of the following statements best describes the main idea of this passage?
A Speciation is evident in two groups of squirrels in the Grand Canyon area.
B Two groups of squirrels in the Grand
Canyon area are closely related.
C Two species can form from one species.
This process is called speciation.
D There are two groups of squirrels because the Grand Canyon has two sides.
2. Which of the following statements about the two types of squirrels is true?
F They look the same.
G They live in similar environments.
H They have tasseled ears.
I They can interbreed with each other.
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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. 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.
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
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
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.
I NTE RPRETI NG G RAPH ICS
MATH
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.
Read each question below, and choose the best answer.
Average Beak Measurements of
Birds of the Colores Islands
Beak size (mm)
Average Beak Size in Galápagos Finches
10.0
Wet year
Dry year 9.0
1978
Dry year Dry year 1980
1982
Year
1984
1. In which of the years studied was average beak size the largest?
A 1977
B 1980
C 1982
D 1984
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
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.
Average beak width
(mm)
Number of unique species
Verde
9.7
6.5
5
Azul
8.9
8.7
15
Rosa
5.2
8.0
10
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
B 1:5
C 1:6
D 5:15
2. What percentage of all bird species on the
Colores Islands are on Rosa Island?
F approximately 15%
G approximately 30%
H approximately 50%
I approximately 80%
3. On which of the islands is the ratio of average beak length to average beak width closest to 1:1?
A Verde Island
B Azul Island
C Rosa Island
D There is not enough information to determine the answer.
4. On which island does the bird with the smallest beak length live?
F Verde Island
G Azul Island
H Rosa Island
I There is not enough information to determine the answer.
Standardized Test Preparation
189
Standardized Test Preparation
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.
Island
Average beak length
(mm)
in Action
Science, Technology, and Society
Seed Banks
All over the world, scientists are making deposits in a special kind of bank. These banks are not for money, but for seeds. Why should seeds be saved? Saving seeds saves plants that may someday save human lives.
These plants could provide food or medicine in the future. Throughout human history, many medicines have been developed from plants. And scientists keep searching for new chemicals among the incredible variety of plants in the world. But time is running out.
Many plant species are becoming extinct before they have even been studied.
“The Anatomy Lesson” by Scott Sanders
Do you know the feeling you get when you have an important test? A medical student faces a similar situation in this story. The student needs to learn the bones of the human body for an anatomy exam the next day. The student goes to the anatomy library to study. The librarian lets him check out a box of bones that are supposed to be from a human skeleton. But something is wrong.
There are too many bones. They are the wrong shape. They don’t fit together correctly. Somebody must be playing a joke!
Find out what’s going on and why the student and the librarian will never be the same after “The Anatomy Lesson.” You can read it in the Holt Anthology of Science Fiction.
Math
Many drugs were originally developed from plants. Suppose that 100 plants are used for medicines this year, but 5% of plant species become extinct each year. How many of the medicinal plants would be left after 1 year? after 10 years? Round your answers to whole numbers.
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Language Arts
Before you read this story, predict what you think will happen.
Write a paragraph that “gives away” the ending that you predict. After you have read the story, listen to some of the predictions made by your classmates. Discuss your opinions about the possible endings.
WRITING
SKILL
People in Science
Raymond Pierotti
Canine Evolution Raymond Pierotti thinks that it’s natural that he became an evolutionary biologist. He grew up exploring the desert around his home in New Mexico.
He was fascinated by the abundant wildlife surviving in the bleak landscape. “One of my earliest memories is getting coyotes to sing with me from my backyard,” he says.
Pierotti now studies the evolutionary relationships between wolves, coyotes, and domestic dogs. Some of his ideas come from the traditions of the Comanches. According to the Comanche creation story, humans came from wolves. Although Pierotti doesn’t believe that humans evolved from wolves, he sees the creation story as a suggestion that humans and wolves have evolved together. “Wolves are very similar to humans in many ways,” says Pierotti. “They live in family groups and hunt together. It is possible that wolves actually taught humans how to hunt in packs, and there are ancient stories of wolves and humans hunting together and sharing the food. I think it was this relationship that inspired the Comanche creation stories.”
Social Studies
Research a story of creation that comes from a Greek, Roman, or
Native American civilization. Write a paragraph summarizing the myth, and share it with a classmate.
WRITING
SKILL
To learn more about these
Science in Action topics, visit go.hrw.com and type in the keyword HL5EVOF.
Check out Current Science® articles related to this chapter by visiting go.hrw.com. Just type in the keyword HL5CS07.
Science in Action
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Things
Biological evolution explains how populations change over time.
SECTION
1 Change over Time . . . . . . . . . . 166
2 How Does Evolution
Happen? . . . . . . . . . . . . . . . . . . 174
3 Natural Selection in Action. . . 180
PRE-READING
About the
Can you find two eyes and a mouth in this photo? The eyes and mouth belong to an adult flounder. Adult flounders swim on their sides and have both eyes on one side of their body.
These characteristics allow flounders to lie flat and still see all of their surroundings. Flounders also look like the sandy bottoms of coastal areas. These adaptations help flounders survive in their environment.
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Chapter 7
Concept Map Before you read the chapter, create the graphic organizer entitled “Concept Map” described in the
Study Skills section of the Appendix. As you read the chapter, fill in the concept map with details about evolution and natural selection. START-UP
Out of Sight, Out of Mind
Analysis
In this activity, you will see how traits can affect the success of an organism in a particular environment.
1. How many white marshmallows did your partner pick? How many colored marshmallows did he or she pick?
Procedure
1. Count out 25 colored marshmallows and
25 white marshmallows.
2. What did the marshmallows and the cloth represent in your investigation? What effect did the color of the cloth have?
2. Ask your partner to look away while you spread the marshmallows out on a white cloth. Do not make a pattern with the marshmallows. Now, ask your partner to turn around and pick the first marshmallow that he or she sees.
3. When an organism blends into its environment, the organism is camouflaged. How does this activity model camouflaged organisms in the wild? What are some weaknesses of this model?
3. Repeat step 2 ten times.
The Evolution of Living Things
165
Change over Time
1
If someone asked you to describe a frog, you might say that a frog has long hind legs, has bulging eyes, and croaks. But what color skin would you say that a frog has?
What You Will Learn
Identify two kinds of evidence that show that organisms have evolved.
Describe one pathway through which a modern whale could have evolved from an ancient mammal.
Explain how comparing organisms can provide evidence that they have ancestors in common.
Vocabulary adaptation species evolution fossil fossil record
READING STRATEGY
Paired Summarizing Read this section silently. In pairs, take turns summarizing the material. Stop to discuss ideas that seem confusing.
Once you start to think about frogs, you realize that frogs differ in many ways. These differences set one kind of frog apart from another. The frogs in Figures 1, 2, and 3 look different from each other, yet they may live in the same areas.
Differences Among Organisms
As you can see, each frog has a different characteristic that might help the frog survive. A characteristic that helps an organism survive and reproduce in its environment is called an adaptation. Adaptations may be physical, such as a long neck or striped fur. Or adaptations may be behaviors that help an organism find food, protect itself, or reproduce.
Living things that have the same characteristics may be members of the same species. A species is a group of organisms that can mate with one another to produce fertile offspring. For example, all strawberry poison arrow frogs are members of the same species and can mate with each other to produce more strawberry poison arrow frogs. Groups of individuals of the same species living in the same place make up a population.
✓
Reading Check How can you tell that organisms are members of the same species? (See the Appendix for answers to Reading Checks.)
Figure 2 The bright coloring of the strawberry poison arrow frog warns predators that the frog is poisonous.
Figure 1 The red-eyed tree frog hides among a tree’s leaves during the day and comes out at night.
Figure 3 The smokey jungle frog blends into the forest floor.
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Do Species Change over Time?
In a single square mile of rain forest, there may be dozens of species of frogs. Across the Earth, there are millions of different species of organisms. The species that live on Earth today range from single-celled bacteria, which lack cell nuclei, to multicellular fungi, plants, and animals. Have these species always existed on Earth?
Scientists think that Earth has changed a great deal during its history, and that living things have changed, too. Scientists estimate that the planet is 4.6 billion years old. Since life first appeared on Earth, many species have died out, and many new species have appeared. Figure 4 shows some of the species that have existed during Earth’s history.
Scientists observe that species have changed over time. They also observe that the inherited characteristics in populations change over time. Scientists think that as populations change over time, new species form. Thus, newer species descend from older species. The process in which populations gradually change over time is called evolution. Scientists continue to develop theories to explain exactly how evolution happens.
adaptation a characteristic that improves an individual’s ability to survive and reproduce in a particular environment species a group of organisms that are closely related and can mate to produce fertile offspring evolution the process in which inherited characteristics within a population change over generations such that new species sometimes arise Figure 4 This diagram shows some of the many kinds of organisms that have lived on Earth since the planet formed 4.6 billion years ago.
Section 1
Change over Time
167
Figure 5 The fossil on the left is of a trilobite, an ancient aquatic animal. The fossils on the right are of seed ferns.
Evidence of Changes over Time
Evidence that organisms have changed over time is buried within Earth’s crust. The layers are made up of different kinds of rock and soil stacked on top of each other. These layers form when sediments, particles of sand, dust, or soil, are carried by wind and water and are deposited in an orderly fashion.
Older layers are deposited before newer layers and are buried deeper within Earth.
Fossils
fossil the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock fossil record a historical sequence of life indicated by fossils found in layers of the Earth’s crust
The remains or imprints of once-living organisms found in layers of rock called fossils. Examples of fossils are shown in
Figure 5. Fossils can be complete organisms, parts of organisms, or just a set of footprints. Fossils usually form when a dead organism is covered by a layer of sediment. Over time, more sediment settles on top of the organism. Minerals in the sediment may seep into the organism and gradually replace the organism with stone. If the organism rots away completely after being covered, it may leave an imprint of itself in the rock.
The Fossil Record
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. Fossils found in newer layers of Earth’s crust tend to be similar to present-day organisms. This similarity indicates that the fossilized organisms were close relatives of present-day organisms.
Fossils from older layers are less similar to present-day organisms than fossils from newer layers are. The older fossils are of earlier life-forms, which may not exist anymore.
✓Reading Check
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How does the fossil record organize fossils?
Evidence of Ancestry
The fossil record provides evidence about the order in which species have existed. Scientists observe that all living organisms have characteristics in common and inherit characteristics in similar ways. So, scientists think that all living species descended from common ancestors. Evidence of common ancestors can be found in fossils and in living organisms.
Drawing Connections
Scientists examine the fossil record to figure out the relationships between extinct and living organisms. Scientists draw models, such as the one shown in Figure 6, that illustrate their hypotheses. The short horizontal line at the top left in the diagram represents a species that lived in the past. Each branch in the diagram represents a group of organisms that descended from that species.
Scientists think that whales and some types of hoofed mammals have a common ancestor, as Figure 6 shows. This ancestor was probably a mammal that lived on land between
50 million and 70 million years ago. During this time period, the dinosaurs died out and a variety of mammals appeared in the fossil record. The first ocean-dwelling mammals appeared about 50 million years ago. Scientists think that all mammal species alive today evolved from common ancestors.
Scientists have named and described hundreds of thousands of living and ancient species. Scientists use information about these species to sketch out a “tree of life” that includes all known organisms. But scientists know that their information is incomplete. For example, parts of Earth’s history lack a fossil record. In fact, fossils are rare because specific conditions are necessary for fossils to form.
Sedimentary Rock Fossils are most often found in sedimentary rock. Sedimentary rock usually forms when rock is broken into sediment by wind, water, and other means.
The wind and water move the sediment around and deposit it. Over time, layers of sediment pile up. Lower layers are compressed and changed into rock. Find out if your area has any sedimentary rocks that contain fossils. Mark the location of such rocks on a copy of a local map.
Figure 6 This diagram is a model of the proposed relationships between ancient and modern mammals that have characteristics similar to whales.
camels and IIamas pigs and peccaries hoofed grazing mammals hippopotamuses toothed whales baleen whales
70
Approximate time (millions of years ago)
0
Section 1
Change over Time
169
Examining Organisms
Examining an organism carefully can give scientists clues about its ancestors. For example, whales seem similar to fish. But unlike fish, whales breathe air, give birth to live young, and produce milk. These traits show that whales are mammals. Thus, scientists think that whales evolved from ancient mammals.
Case Study: Evolution of the Whale
Scientists think that the ancient ancestor of whales was probably a mammal that lived on land and that could run on four legs. A more recent ancestor was probably a mammal that spent time both on land and in water. Comparisons of modern whales and a large number of fossils have supported this hypothesis. Figure 7 illustrates some of this evidence.
✓
Reading Check What kind of organism do scientists think was an ancient ancestor of whales?
Figure 7
Evidence of Whale Evolution
a Pakicetus (PAK uh SEE tuhs)
Scientists think that whales evolved from land-dwelling mammals that could run on four legs. One of these ancestors may have been Pakicetus, which lived about
50 million years ago. The fossil skeleton and an artist’s illustration of Pakicetus are shown here. Pakicetus was about the size of a wolf.
b Ambulocetus (AM byoo loh SEE tuhs)
This mammal lived in coastal waters about 49 million years ago. It could swim by kicking its legs and using its tail for balance. It could also waddle on land by using its short legs. Ambulocetus was about the size of a dolphin.
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Walking Whales
The organisms in Figure 7 form a sequence between ancient four-legged mammals and modern whales. Several pieces of evidence indicate that these species are related by ancestry.
Each species shared some traits with an earlier species. However, some species had new traits that were shared with later species. Yet, each species had traits that allowed it to survive in a particular time and place in Earth’s history.
Further evidence can be found inside the bodies of living whales. For example, although modern whales do not have hind limbs, inside their bodies are tiny hip bones, as shown in Figure 7. Scientists think that these hip bones were inherited from the whales’ four-legged ancestors. Scientists often look at this kind of evidence when they want to determine the relationships between organisms.
The Weight of Whales
Whales are the largest animals ever known on Earth.
One reason whales can grow so large is that they live in water, which supports their weight in a way that their bones could not. The blue whale—the largest type of whale in existence—is about
24 m long and has a mass of about 99,800 kg. Convert these measurements into feet and pounds, and round to whole numbers.
c Dorudon (DOH roo DON)
This mammal lived in the oceans about 40 million years ago. It resembled a giant dolphin and propelled itself with its massive tail. Dorudon had tiny hind limbs that it could not use for walking or swimming.
d Modern toothed whale
Modern whales’ forelimbs are flippers.
Modern whales do not have hind limbs, but they do have tiny hip bones.
Modern whales range in size from
1.4 m porpoises to 33 m blue whales.
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Change over Time
171
Human arm
Dolphin flipper
Cat leg
Bat wing
Figure 8 The bones in the front limbs of these animals are similar. Similar bones are shown in the same color. These limbs are different sizes in life.
Comparing Organisms
Evidence that groups of organisms have common ancestry can be found by comparing the groups’ DNA. Because every organism inherits DNA, every organism inherits the traits determined by DNA. Organisms contain evidence that populations and species undergo changes in traits and DNA over time.
Comparing Skeletal Structures
What do your arm, the front leg of a cat, the front flipper of a dolphin, and the wing of a bat have in common? You might notice that these structures do not look alike and are not used in the same way. But under the surface, they have similarities.
Look at Figure 8. The structure and order of bones of a human arm are similar to those of the front limbs of a cat, a dolphin, and a bat. These similarities suggest that cats, dolphins, bats, and humans had a common ancestor. Over millions of years, changes occurred in the limb bones. Eventually, the bones performed different functions in each type of animal.
Comparing DNA
For another activity related to this chapter, go to go.hrw.com and type in the keyword HL5EVOW.
When scientists compare organism’s traits, such as skeletal structures, much of the information that they get supports the theory that organisms share a common ancestor. To further support this theory, scientists compare organisms’ DNA at a molecular level. Scientists analyze many organisms’ DNA, RNA, proteins, and other molecules. Then, scientists compare the data for each species. The greater the number of similarities between the data sets for any two species, the more closely the two species are related through a common ancestor. Scientists use molecular data, the comparison of traits, and fossils to support the theory that because all existing species have DNA, all species share a common ancestor.
✓
Reading Check If two species have similar DNA, what hypothesis is supported?
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Review
Summary
is the process in which inherited
• Evolution characteristics within a population change
think that modern whales
• Scientists evolved from an ancient, land-dwelling
over generations, sometimes giving rise to new species. Scientists continue to develop theories to explain how evolution happens.
•
mammal ancestor. Fossil organisms that support this hypothesis have been found. of common ancestry among liv• Evidence ing organisms is provided by comparing
Evidence that organisms evolve can be found by comparing living organisms to each other and to the fossil record. Such comparisons provide evidence of common ancestry. DNA and inherited traits. Species that have a common ancestor will have traits and
DNA that are more similar to each other than to those of distantly related species.
Using Key Terms
Complete each of the following sentences by choosing the correct term from the word bank. adaptation species fossil evolution
1. Members of the same can mate with one another to produce offspring.
2. A(n)
helps an organism survive.
3. When populations change over time, occurred. has
Understanding Key Ideas
8. Forming Hypotheses Is a person’s DNA likely to be more similar to the DNA of his or her biological parents or to the DNA of one of his or her cousins? Explain your answer.
Interpreting Graphics
9. The photograph below shows the layers of sedimentary rock exposed during the construction of a road. Imagine that a species that lived 200 million years ago is found in layer b.
Would the species’ ancestor, which lived
250 million years ago, most likely be found in layer a or in layer c? Explain your answer.
a
4. A human’s arm, a cat’s front leg, a dolphin’s front flipper, and a bat’s wing
a. have similar kinds of bones.
b. are used in similar ways.
c. are very similar to insect wings and jellyfish tentacles. d. have nothing in common.
b c 5. How does the fossil record show that species have changed over time?
6. What evidence do fossils provide about the ancestors of whales?
Critical Thinking
7. Making Comparisons Other than the examples provided in the text, how are whales different from fishes?
For a variety of links related to this chapter, go to www.scilinks.org
Topic: Species and Adaptation;
Fossil Record
SciLinks code: HSM1433; HSM0615
173
How Does Evolution
Happen?
2
What You Will Learn
List four sources of Charles Darwin’s ideas about evolution.
Describe the four parts of Darwin’s theory of evolution by natural selection. Relate genetics to evolution.
Vocabulary trait selective breeding natural selection
Imagine that you are a scientist in the 1800s. Fossils of some very strange animals have been found. And some familiar fossils have been found where you would least expect them.
How did seashells end up on the tops of mountains?
In the 1800s, geologists began to realize that the Earth is much older than anyone had previously thought. Evidence showed that gradual processes had changed the Earth’s surface over millions of years. Some scientists saw evidence of evolution in the fossil record. However, no one had been able to explain how evolution happens—until Charles Darwin.
Charles Darwin
READING STRATEGY
Brainstorming The key idea of this section is natural selection. Brainstorm words and phrases related to natural selection. In 1831, 21-year-old Charles Darwin, shown in Figure 1, graduated from college. Like many young people just out of college, Darwin didn’t know what he wanted to do with his life.
His father wanted him to become a doctor, but seeing blood made Darwin sick. Although he eventually earned a degree in theology, Darwin was most interested in the study of plants and animals.
So, Darwin signed on for a five-year voyage around the world. He served as the naturalist—a scientist who studies nature—on the British ship the HMS Beagle, similar to the ship in Figure 2. During the trip, Darwin made observations that helped him form a theory about how evolution happens.
Figure 1 Charles Darwin wanted to understand the natural world.
Figure 2 Darwin sailed around the world on a ship similar to this one.
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Figure 3 The course of the
HMS Beagle is shown by the red line. The journey began and ended in England.
Darwin’s Excellent Adventure
The Beagle’s journey is charted in Figure 3. Along the way,
Darwin collected thousands of plant and animal samples. He kept careful notes of his observations. One interesting place that the ship visited was the Galápagos Islands. These islands are found 965 km (600 mi) west of Ecuador, a country in
South America.
✓
Reading Check Where are the Galápagos Islands? (See the Appendix for answers to Reading Checks.)
Darwin’s Finches
Darwin noticed that the animals and plants on the Galápagos
Islands were a lot like those in Ecuador. However, they were not exactly the same. The finches of the Galápagos Islands, for example, were a little different from the finches in Ecuador.
And the finches on each island differed from the finches on the other islands. As Figure 4 shows, the beak of each finch is adapted to the way the bird usually gets food.
Figure 4
Some Finches of the Galápagos Islands
The large ground finch has a wide, strong beak that it uses to crack open big, hard seeds. This beak works like a nutcracker.
The cactus finch has a tough beak that it uses for eating cactus parts and insects. This beak works like a pair of needle-nose pliers.
Section 2
The warbler finch has a small, narrow beak that it uses to catch small insects. This beak works like a pair of tweezers.
How Does Evolution Happen?
175
Darwin’s Thinking
After returning to England, Darwin puzzled over the animals of the Galápagos Islands. He tried to explain why the animals seemed so similar to each other yet had so many different adaptations. For example, Darwin hypothesized that the island finches were descended from South American finches. The first finches on the islands may have been blown from South America by a storm. Over many generations, the finches may have evolved adaptations for the various island environments.
During the course of his travels, Darwin came up with many new ideas. Before sharing these ideas, he spent several years analyzing his evidence and gathering ideas from other people. Ideas About Breeding trait a genetically determined characteristic selective breeding the human practice of breeding animals or plants that have certain desired traits Figure 5 Over the past 12,000 years, dogs have been selectively bred to produce more than 150 breeds.
176
In Darwin’s time, farmers and breeders had produced many kinds of farm animals and plants. These plants and animals had traits that were desired by the farmers and breeders. A trait is a characteristic that can be passed from parent to offspring through genes. The process in which humans select which plants or animals to reproduce based on certain desired traits is called selective breeding. Most pets, such as the dogs in Figure 5, have been bred for various desired traits.
You can see the results of selective breeding in many kinds of organisms. For example, people have bred horses that are particularly fast or strong. And farmers have bred crops that produce large fruit or that grow in specific climates.
Population Growth Versus Food Supply
1. Get an egg carton and a bag of rice. Use a marker to label one row of the carton “Food supply.” Then, label the second row “Human population.” 2. In the row labeled “Food supply,” place one grain of rice in the first cup. Place two grains of rice in the second cup, and place three grains of rice in the third cup. In each subsequent cup, place one more grain than you placed in the previous cup. Imagine that each grain represents enough food for one person’s lifetime.
3. In the row labeled “Human population,” place one grain of rice in the first cup. Place two grains in the second cup, and place four grains in the third cup. In each subsequent cup, place twice as many grains as you placed in the previous cup. This rice represents people.
4. How many units of food are in the sixth cup?
How many “people” are in the sixth cup? If this pattern continued, what would happen?
5. Describe how the patterns in the food supply and in the human population differ. Explain how the patterns relate to Malthus’s hypothesis.
Ideas About Population
Malthus’s Description of Unlimited Population Growth
Human
population
Quantity
During Darwin’s time, Thomas Malthus wrote a famous book entitled An Essay on the Principle of
Population. Malthus noted that humans have the potential to reproduce rapidly. He warned that food supplies could not support unlimited population growth. Figure 6 illustrates this relationship. However, Malthus pointed out that human populations are limited by choices that humans make or by problems such as starvation and disease.
After reading Malthus’s work, Darwin realized that any species can produce many offspring. He also knew that the populations of all species are limited by starvation, disease, competition, and predation. Only a limited number of individuals survive to reproduce. Thus, there is something special about the survivors. Darwin reasoned that the offspring of the survivors inherit traits that help the offspring survive in their environment.
Food supply Time
Figure 6 Malthus thought that the human population could increase more quickly than the food supply, with the result that there would not be enough food for everyone.
Ideas About Earth’s History
Darwin had begun to think that species could evolve over time.
But most geologists at the time did not think that Earth was old enough to allow for slow changes. Darwin learned new ideas from Principles of Geology, a book by Charles Lyell. This book presented evidence that Earth had formed by natural processes over a long period of time. It became clear to Darwin that Earth was much older than anyone had imagined.
✓Reading Check
What did Darwin learn from Charles Lyell?
Section 2
How Does Evolution Happen?
177
Darwin’s Theory of Natural Selection
natural selection the process by which individuals that are better adapted to their environment survive and reproduce more successfully than less well adapted individuals do; a theory to explain the mechanism of evolution
After his voyage on the HMS Beagle, Darwin privately struggled with his ideas for about 20 years. Then, in 1858, Darwin received a letter from a fellow naturalist named Alfred Russel
Wallace. Wallace had arrived at the same ideas about evolution that Darwin had. In 1859, Darwin published a famous book called On the Origin of Species by Means of Natural Selection. In his book, Darwin proposed the theory that evolution happens through natural selection. Natural selection is the process by which organisms that are better adapted to their environment survive and reproduce more successfully than less well adapted organisms do. The process has four parts and is explained in
Figure 7.
✓Reading Check
Figure 7
What is natural selection?
Four Parts of Natural Selection
1 Overproduction A tarantula’s egg sac may hold
500–1,000 eggs. Some of the eggs will survive and develop into adult spiders. Some will not.
2 Inherited Variation Every individual has its own combination of traits. Each tarantula is similar to, but not identical to, its parents.
3 Struggle to Survive Some tarantulas may be caught by predators, such as this wasp. Other tarantulas may starve or get a disease. Only some of the tarantulas will survive to adulthood.
4 Successful Reproduction The tarantulas that are best adapted to their environment are likely to have many offspring that survive.
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Genetics and Evolution
Darwin lacked evidence for parts of his theory. For example, he knew that organisms inherit traits, but not how they inherit traits. He knew that there is great variation among organisms, but not how that variation occurs. Today, scientists have found most of the evidence that Darwin lacked. They know that variation happens as a result of differences in genes. Changes in genes may happen whenever organisms produce offspring.
Some genes make an organism more likely to survive to reproduce. The process called selection happens when only organisms that carry these genes can survive to reproduce. New fossil discoveries and new information about genes add to scientists’ understanding of natural selection and evolution.
Review
Summary
explained that
• Darwin evolution occurs through natural selection. His theory has four parts:
1. Each species produces more offspring than will survive to reproduce. 2. Individuals within a population have slightly different traits.
3. Individuals within a population compete with each other for limited resources.
4. Individuals that are better equipped to live in an environment are more likely to survive to reproduce. genetics helps
• Modern explain the theory of natural selection.
Using Key Terms
1. In your own words, write a definition for the term trait.
2. Use the following terms in the same sentence: selective breeding and natural selection.
Understanding Key Ideas
3. Modern scientific explanations of evolution
a. have replaced Darwin’s theory.
b. rely on genetics instead of natural selection.
c. fail to explain how traits are inherited. d. combine the principles of natural selection and genetic inheritance. Math Skills
8. In a sample of 80 beetles,
50 beetles had 4 spots each, and the rest had 6 spots each.
What was the average number of spots per beetle?
Critical Thinking
9. Making Comparisons In selective breeding, humans influence the course of evolution. What determines the course of evolution in natural selection?
10. Predicting Consequences
Suppose that an island in the
Pacific Ocean was just formed by a volcano. Over the next million years, how might species evolve on this island?
4. Describe the observations that
Darwin made about the species on the Galápagos Islands.
5. Summarize the ideas that
Darwin developed from books by Malthus and Lyell.
6. Describe the four parts of
Darwin’s theory of evolution by natural selection.
7. What knowledge did Darwin lack that modern scientists now use to explain evolution?
For a variety of links related to this chapter, go to www.scilinks.org
Topic: Galápagos Islands;
Darwin and Natural Selection
SciLinks code: HSM0631; HSM0378
179
3
What You Will Learn
Give three examples of natural selection in action.
Outline the process of speciation.
Vocabulary generation time speciation READING STRATEGY
Prediction Guide Before reading this section, write the title of each heading in this section. Next, under each heading, write what you think you will learn.
Natural Selection in Action
Have you ever had to take an antibiotic? Antibiotics are supposed to kill bacteria. But sometimes, bacteria are not killed by the medicine. Do you know why?
A population of bacteria might develop an adaptation through natural selection. Most bacteria are killed by the chemicals in antibiotics. But a few of the bacteria have an adaptation that makes them naturally resistant to, or not killed by, the antibiotic. These few bacteria survive antibiotic treatment, continue to reproduce, and pass the adaptation to their offspring. After several generations, almost all the bacteria in the population carry the adaptation of antibiotic resistance.
Changes in Populations
The theory of natural selection explains how a population changes in response to its environment. Through ongoing natural selection, a population adapts to its environment.
Well-adapted individuals will likely survive and reproduce.
Adaptation to Hunting
Changes in populations are sometimes observed when a new force affects the survival of individuals. Scientists think that hunting in Uganda is affecting Uganda’s elephant population.
In 1930, about 99% of the male elephants in one area had tusks. Only 1% of the elephants were born without tusks.
Today, as many as 15% of the male elephants in that area lack tusks. What happened?
A male African elephant that has tusks is shown in Figure 1. The ivory of an elephant’s tusks is very valuable. People hunt the elephants for their tusks. As a result, fewer of the elephants that have tusks survive to reproduce, and more of the tuskless elephants survive. When the tuskless elephants reproduce, they pass the tuskless trait to their offspring.
Figure 1 The ivory tusks of African elephants are very valuable. Some elephants are born without tusks.
180
Figure 2
Natural Selection of Insecticide Resistance
1 An insecticide will kill most insects, but a few may survive. These survivors have genes that make them resistant to the insecticide.
2 The survivors then reproduce, passing the insecticideresistance genes to their offspring.
3 In time, the replacement population of insects is made up mostly of individuals that have the insecticideresistance genes.
4 When the same kind of insecticide is used on the insects, only a few are killed because most of them are resistant to that insecticide. Insecticide Resistance
To control insect pests, many people use insecticides, chemicals that kill insects. Sometimes, an insecticide that used to work well no longer affects an insect population. The reason is that a few insects in the population are resistant to the chemical.
These insects survive insecticide treatment and pass the resistance trait to their offspring. Figure 2 shows how an insect population becomes resistant to some insecticides.
Insect populations can evolve quickly because insects produce many offspring and have a short generation time.
Generation time is the average time between one generation and the next.
✓
Reading Check Why do insects quickly develop resistance to insecticides? (See the Appendix for answers to Reading Checks.)
generation time the period between the birth of one generation and the birth of the next generation
Competition for Mates
For organisms that reproduce sexually, competition for mates can select for adaptations. For example, in many bird species, females prefer to mate with colorful males. So, colorful males have more offspring than noncolorful males do. Because colorful males are more likely to pass on their genes to the next generation, the proportion of colorful males is likely to increase from generation to generation.
Section 3
Natural Selection in Action
181
Forming a New Species
speciation the formation of new species as a result of evolution
Sometimes, drastic changes that can form a new species take place. In the animal kingdom, a species is a group of organisms that can mate with each other to produce fertile offspring. A new species may form after a group becomes separated from the original population. This group forms a new population.
Over time, the new population adapts to its new environment.
Eventually, the new population and the original population differ so greatly that they can no longer mate successfully. The new population may then be considered a new species. The formation of a new species as a result of evolution is called speciation (SPEE shee AY shuhn). Figure 3 shows how new species of Galápagos finches may have formed. Speciation may happen in other ways as well.
Separation
Speciation often begins when a part of a population becomes separated from the rest. The process of separation can happen in several ways. For example, a newly formed canyon, mountain range, or lake can divide the members of a population.
✓Reading Check
How can parts of a population become
separated?
Figure 3
The Evolution of Galápagos Finch Species
1 Some finches left the mainland and reached one of the islands (separation).
2 The finches reproduced and adapted to the environment
(adaptation).
3 Some finches flew to a second island (separation).
4 The finches reproduced and adapted to the different environment (adaptation).
5 Some finches flew back to the first island but could no longer interbreed with the finches there (division).
6 This process may have occurred over and over again as the finches flew to the other islands.
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Adaptation
Division
Over many generations, two separated groups of a population may become very different. Even if a geographical barrier is removed, the groups may not be able to interbreed anymore. At this point, the two groups are no longer the same species.
Figure 4 shows another way that populations may stop interbreeding. Leopard frogs and pickerel frogs probably had the same ancestor species.
Then, at some point, some of these frogs began to mate at different times during the year.
Review
Mating Times
March 1
selection
• Natural explains how popula-
•
tions adapt to changes in their environment. A variety of examples of such adaptations can be found.
Natural selection also explains how one species may evolve into another. Speciation occurs as populations undergo separation, adaptation, and division.
April 1
May 1
June 1
July 1
Month
Figure 4 The leopard frog and the pickerel frog are similar species. However, leopard frogs do not search for mates at the same time of year that pickerel frogs do.
Using Key Terms
1. In your own words, write a definition for the term speciation.
Understanding Key Ideas
Summary
Pickerel frog
Bullfrog
Leopard frog
Tree frog
Mating behavior
Populations constantly undergo natural selection.
After two groups have separated, natural selection may act on each group in different ways.
Over many generations, the separated groups may evolve different sets of traits. If the environmental conditions for each group differ, the adaptations in the groups will also differ.
2. Two populations have evolved into two species when
a. the populations are separated.
b. the populations look different.
c. the populations can no longer interbreed. d. the populations adapt.
Critical Thinking
5. Forming Hypotheses Most kinds of cactus have leaves that grow in the form of spines. The stems or trunks become thick, juicy pads or barrels. Explain how these cactus parts might have evolved.
6. Making Comparisons Suggest an organism other than an insect that might evolve an adaptation to human activities.
3. Explain why the number of tuskless elephants in Uganda may be increasing.
Math Skills
4. A female cockroach can produce
80 offspring at a time. If half of the offspring produced by a certain female are female and each female produces 80 offspring, how many cockroaches are there in the third generation?
Developed and maintained by the
National Science Teachers Association
For a variety of links related to this chapter, go to www.scilinks.org
Topic: Species and Adaptation
SciLinks code: HSM1433
183
Using Scientific Methods
Lab
Survival of the Chocolates
OBJ ECTIVES
Form a hypothesis about the fate of the candy-coated chocolates. Predict what will happen to the candy-coated chocolates.
Design and conduct an experiment to test your hypothesis. MATE RIALS
•
•
chocolates, candy-coated, small, in a variety of colors
(about 100) items to be determined by the students and approved by the teacher
SAFETY
Imagine a world populated with candy, and hold that delicious thought in your head for just a moment. Try to apply the idea of natural selection to a population of candy-coated chocolates.
According to the theory of natural selection, individuals who have favorable adaptations are more likely to survive. In the
“species” of candy-coated chocolates you will study in this experiment, the characteristics of individual chocolates may help them “survive.” For example, shell strength (the strength of the candy coating) could be an adaptive advantage. Plan an experiment to find out which characteristics of the chocolates are favorable “adaptations.”
Ask a Question
1
Form a Hypothesis
2
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Chapter 7
What might “survival” mean for a candy-coated chocolate?
What are some ways you can test which chocolates are the
“strongest” or “most fit” for their environment? Also, write down any other questions that you could ask about the “survival” of the chocolates.
Form a hypothesis, and make a prediction. For example, if you chose to study candy color, your prediction might be similar to colored shell is the strongest, then fewer of the this: If the chocolates with this color of shell will when .
The Evolution of Living Things
Test the Hypothesis
Analyze the Results
3
Design a procedure to determine which type of candy-coated chocolate is most likely to survive. In your plan, be sure to include materials and tools you may need to complete this procedure.
1
4
Check your experimental design with your teacher before you begin. Your teacher will supply the candy and assist you in gathering materials and tools.
2
5
Record your results in a data table. Be sure to organize your data in a clear and understandable way.
Describing Events
Write a report that describes your experiment. Be sure to include tables and graphs of the data you collected.
Draw Conclusions
Evaluating Data In your report, explain how your data either support or do not support your hypothesis. Include possible errors and ways to improve your procedure.
Applying Your Data
Can you think of another characteristic of the chocolates that can be tested to determine which type is best adapted to survive? Explain your idea, and describe how you might test it.
Chapter Lab
185
USING KEY TERMS
Complete each of the following sentences by choosing the correct term from the word bank.
UNDERSTANDING KEY IDEAS
Multiple Choice
8 Fossils are commonly found in
a. sedimentary rock.
adaptation evolution generation time species speciation fossil record selective breeding natural selection
b. all kinds of rock.
c. granite.
d. loose sand.
9 The fact that all organisms have
DNA as their genetic material is evidence that
a. all organisms undergo natural
selection.
b. all organisms may have descended from a common ancestor.
c. selective breeding takes place every day.
d. genetic resistance rarely occurs.
1 When a single population evolves into two populations that cannot interbreed anymore, has occurred.
2 Darwin’s theory of
explained the process by which organisms become well-adapted to their environment.
3 A group of organisms that can mate with each other to produce offspring is known as a(n)
.
0 Charles Darwin puzzled over differ-
ences in the of the different species of Galápagos finches.
a. webbed feet
4 The
b. beaks
provides information about organisms that have lived in the past.
5 In
, humans select organisms with desirable traits that will be passed from one generation to another.
c. bone structure of the wings
d. eye color
q Darwin observed variations among
6 A(n)
helps an organism survive better in its environment.
7 Populations of insects and bacteria can evolve quickly because they usually have a short
.
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The Evolution of Living Things
individuals within a population, but he did not realize that these variations were caused by
a. interbreeding.
b. differences in food.
c. differences in genes.
d. selective breeding.
p Analyzing Relationships Geologists
have evidence that the continents were once a single giant continent.
This giant landform eventually split apart, and the individual continents moved to their current positions.
What role might this drifting of continents have played in evolution?
w Identify two ways that organisms can be compared to provide evidence of evolution from a common ancestor.
e Describe evidence that supports the
hypothesis that whales evolved from land-dwelling mammals.
r Why are some animals more likely
to survive to adulthood than other animals are?
t Explain how genetics is related to evolution. y Outline an example of the process of speciation.
INTERPRETING GRAPHICS
The graphs below show information about the infants that are born and the infants that have died in a population.
The weight of each infant was measured at birth. Use the graphs to answer the questions that follow.
terms to create a concept map: struggle to survive, theory, genetic variation,
Darwin, overpopulation, natural selection, and successful reproduction.
i Making Inferences How could natural selection affect the songs that birds sing?
o Forming Hypotheses In Australia,
many animals look like mammals from other parts of the world. But most of the mammals in Australia are marsupials, which carry their young in pouches after birth. Few kinds of marsupials are found anywhere else in the world. What is a possible explanation for the presence of so many of these unique mammals in Australia?
Percentage of births
u Concept Mapping Use the following
Infant Deaths by Birth Weight
Infant Births by Birth Weight
CRITICAL THINKING
20
15
10
5
0
2 4 6 8 10
Weight ( lb)
Probability of death (%)
Short Answer
100
10
1
2
4 6
8
(
)
Weight lb
a What is the most common birth weight? s At which birth weight is an infant most likely to survive?
d How do the principles of natural selection help explain why there are more deaths among babies whose birth weights are low than among babies whose birth weights are average?
Chapter Review
187
READI NG
Read each of the passages below. Then, answer the questions that follow each passage.
Passage 1 When the Grand Canyon was forming, a single population of tassel-eared squirrels may have been separated into two groups. Today, descendants of the two groups live on opposite sides of the canyon. The two groups share many characteristics, but they do not look the same. For example, both groups have tasseled ears, but each group has a unique fur color pattern. An important difference between the groups is that the Abert squirrels live on the south rim of the canyon, and the Kaibab squirrels live on the north rim.
The environments on the two sides of the
Grand Canyon are different. The north rim is about 370 m higher than the south rim. Almost twice as much precipitation falls on the north rim than on the south rim every year. Over many generations, the two groups of squirrels have adapted to their new environments. Over time, the groups became very different. Many scientists think that the two types of squirrels are no longer the same species. The development of these two squirrel groups is an example of speciation in progress.
1. Which of the following statements best describes the main idea of this passage?
A Speciation is evident in two groups of squirrels in the Grand Canyon area.
B Two groups of squirrels in the Grand
Canyon area are closely related.
C Two species can form from one species.
This process is called speciation.
D There are two groups of squirrels because the Grand Canyon has two sides.
2. Which of the following statements about the two types of squirrels is true?
F They look the same.
G They live in similar environments.
H They have tasseled ears.
I They can interbreed with each other.
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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. 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.
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
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
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.
I NTE RPRETI NG G RAPH ICS
MATH
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.
Read each question below, and choose the best answer.
Average Beak Measurements of
Birds of the Colores Islands
Beak size (mm)
Average Beak Size in Galápagos Finches
10.0
Wet year
Dry year 9.0
1978
Dry year Dry year 1980
1982
Year
1984
1. In which of the years studied was average beak size the largest?
A 1977
B 1980
C 1982
D 1984
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
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.
Average beak width
(mm)
Number of unique species
Verde
9.7
6.5
5
Azul
8.9
8.7
15
Rosa
5.2
8.0
10
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
B 1:5
C 1:6
D 5:15
2. What percentage of all bird species on the
Colores Islands are on Rosa Island?
F approximately 15%
G approximately 30%
H approximately 50%
I approximately 80%
3. On which of the islands is the ratio of average beak length to average beak width closest to 1:1?
A Verde Island
B Azul Island
C Rosa Island
D There is not enough information to determine the answer.
4. On which island does the bird with the smallest beak length live?
F Verde Island
G Azul Island
H Rosa Island
I There is not enough information to determine the answer.
Standardized Test Preparation
189
Standardized Test Preparation
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.
Island
Average beak length
(mm)
in Action
Science, Technology, and Society
Seed Banks
All over the world, scientists are making deposits in a special kind of bank. These banks are not for money, but for seeds. Why should seeds be saved? Saving seeds saves plants that may someday save human lives.
These plants could provide food or medicine in the future. Throughout human history, many medicines have been developed from plants. And scientists keep searching for new chemicals among the incredible variety of plants in the world. But time is running out.
Many plant species are becoming extinct before they have even been studied.
“The Anatomy Lesson” by Scott Sanders
Do you know the feeling you get when you have an important test? A medical student faces a similar situation in this story. The student needs to learn the bones of the human body for an anatomy exam the next day. The student goes to the anatomy library to study. The librarian lets him check out a box of bones that are supposed to be from a human skeleton. But something is wrong.
There are too many bones. They are the wrong shape. They don’t fit together correctly. Somebody must be playing a joke!
Find out what’s going on and why the student and the librarian will never be the same after “The Anatomy Lesson.” You can read it in the Holt Anthology of Science Fiction.
Math
Many drugs were originally developed from plants. Suppose that 100 plants are used for medicines this year, but 5% of plant species become extinct each year. How many of the medicinal plants would be left after 1 year? after 10 years? Round your answers to whole numbers.
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Language Arts
Before you read this story, predict what you think will happen.
Write a paragraph that “gives away” the ending that you predict. After you have read the story, listen to some of the predictions made by your classmates. Discuss your opinions about the possible endings.
WRITING
SKILL
People in Science
Raymond Pierotti
Canine Evolution Raymond Pierotti thinks that it’s natural that he became an evolutionary biologist. He grew up exploring the desert around his home in New Mexico.
He was fascinated by the abundant wildlife surviving in the bleak landscape. “One of my earliest memories is getting coyotes to sing with me from my backyard,” he says.
Pierotti now studies the evolutionary relationships between wolves, coyotes, and domestic dogs. Some of his ideas come from the traditions of the Comanches. According to the Comanche creation story, humans came from wolves. Although Pierotti doesn’t believe that humans evolved from wolves, he sees the creation story as a suggestion that humans and wolves have evolved together. “Wolves are very similar to humans in many ways,” says Pierotti. “They live in family groups and hunt together. It is possible that wolves actually taught humans how to hunt in packs, and there are ancient stories of wolves and humans hunting together and sharing the food. I think it was this relationship that inspired the Comanche creation stories.”
Social Studies
Research a story of creation that comes from a Greek, Roman, or
Native American civilization. Write a paragraph summarizing the myth, and share it with a classmate.
WRITING
SKILL
To learn more about these
Science in Action topics, visit go.hrw.com and type in the keyword HL5EVOF.
Check out Current Science® articles related to this chapter by visiting go.hrw.com. Just type in the keyword HL5CS07.
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