the environment and change over time - grade 7...

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Charles DarwinHow many species of birds can you name? Robins, penguins,

and chickens are a few. There are about 10,000 species of birds on Earth today. Each species has wings, feathers, and beaks. Scientists hypothesize that all birds evolved from an earlier, or ancestral, group of birdlike organisms. As this group evolved into different species, birds developed different sizes, colors, songs, and ways of eating. Yet, they kept their key bird traits.

How do species evolve? Charles Darwin, a scientist, worked to answer this question. Darwin was an English naturalist who, in the mid-1800s, developed a theory of how evolution works. A naturalist is a person who studies plants and animals by observing them. Darwin spent years studying plants and animals in nature before developing his theory. Recall that a theory is an explanation of the natural world that is well supported by evidence. Darwin’s theory of evolution was not the first, but his theory is the one best supported by evidence today.

Voyage of the BeagleDarwin worked as a naturalist on the HMS Beagle, a ship

of the British navy. During his trip around the world, Darwin observed and collected many plants and animals.

What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you’ve read this lesson, reread the statements to see if you have changed your mind.

Before Statement After

3. Environmental change causes variations in populations.

4. Variations can lead to adaptations.

Key Concepts • Who was Charles Darwin?• How does Darwin’s theory

of evolution by natural selection explain how species change over time?

• How are adaptations evidence of natural selection?

Make Flash Cards Think of a quiz question for each paragraph. Write the question on one side of a flash card. Write the answer on the other side. Work with a partner to quiz each other using your flash cards.

The Environment and Change Over TimeTheory of Evolution by Natural Selection

Key Concept Check1. Describe Who was Charles Darwin?

Reading Essentials The Environment and Change Over Time 91

C214_009_014_RE_L2_889406.indd 9C214_009_014_RE_L2_889406.indd 9 3/4/10 12:23:06 PM3/4/10 12:23:06 PM

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The Galápagos IslandsDarwin was interested in the organisms he saw on the

Galápagos (guh LAH puh gus) Islands. These islands are 1,000 km off the South American coast in the Pacific Ocean. Darwin saw that each island had a slightly different environment. Some were dry. Some were more humid. Others had mixed environments.

Tortoises Darwin saw that the giant tortoises on each island looked different. On one island, tortoises had shells that came close to their necks. They could eat only short plants. On other islands, tortoises had more space between the shell and neck. They could eat taller plants.

Mockingbirds and Finches Darwin was also curious about the different mockingbirds and finches he saw. Like the tortoises, different types of mockingbirds and finches lived in different island environments. Later, he was surprised to find that many were different enough to be separate species.

Darwin’s TheoryDarwin discovered a relationship between each species

and the food found on the island where it lived. Tortoises with long necks lived on islands that had tall cacti. Their long necks made it possible for them to reach high to eat the cacti. The tortoises with short necks lived on islands that had plenty of short grass.

Common AncestorsDarwin became convinced that all the tortoise species

were related. He thought they all shared a common ancestor.He suggested that millions of years before, a storm had carried a group of tortoises to one of the islands from South America. In time, the tortoises spread to the other islands. Their neck lengths and shell shapes changed to match their islands’ food sources. How did this happen?

VariationsDarwin knew that individual members of a species have

slight differences, or variations. A variation is a slight difference in the appearance of individual members of a species. Variations arise naturally in populations. They occur in the offspring as a result of sexual reproduction. You might recall that variations are caused by random mutations, or changes, in genes. Mutations can lead to changes in phenotype. Recall that an organism’s phenotype is all of the observable traits and characteristics of the organism. Genetic changes to phenotype can be passed on to future generations.

Reading Check2. Explain What made Darwin become curious about the organisms that lived on the Galápagos Islands?

ACADEMIC VOCABULARYconvince(verb) to overcome by argument

Make a small four-door shutterfold book. Use it to investigate the who, what, when, and where of Charles Darwin, the Galápagos Islands, and the theory of evolution by natural selection.

Who?

When? Where?

What?

92 The Environment and Change Over Time Reading Essentials


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Natural SelectionDarwin did not know about genes. But he saw that

variations were the key to how evolution worked. He knew that there was not enough food on each island to feed every tortoise that was born. Tortoises had to compete for food. As the tortoises spread to the different islands, some were born with random variations in neck length. If a variation helped a tortoise compete for food, the tortoise lived longer than other tortoises without the variation. Because it lived longer, it reproduced more. It passed on the helpful variation to its offspring.

This is Darwin’s theory of evolution by natural selection. Natural selection is the process by which populations of organisms with variations that help them survive in their environments live longer, compete better, and reproduce more than those that do not have the variations. Natural selection explains how Galápagos tortoises became matched to their food sources, as shown below. It also explains why there were so many different kinds of Galápagos finches and mockingbirds. Birds with beak variations that helped them compete for food lived longer and reproduced more.

Visual Check4. Illustrate Mark all the tortoises in the figure that have short necks with the letter S. Mark those that have long necks with the letter L. What trend do you see over time?

Key Concept Check3. Analyze What role do variations have in the theory of evolution by natural selection?

Natural Selection1 Reproduction

A population of

tortoises produces

many offspring that

inherit its

characteristics.

2 VariationA tortoise is born

with a variation

that makes its neck

slightly longer.

3 CompetitionDue to limited

resources, not all

offspring will

survive. An offspring

with a longer neck

can eat more cacti

than other tortoises.

It lives longer and

produces more

offspring.

4 SelectionOver time, the

variation is

inherited by

more and more

offspring.

Eventually, all

tortoises have

longer necks.



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AdaptationsNatural selection explains how all species change over

time as their environments change. Through natural selection, a helpful variation in one individual can eventually pass to future members of a population.

As time passes, more variations come about. The buildup of many similar variations can lead to an adaptation (a dap TAY shun). An adaptation is a characteristic of a species that enables the species to survive in its environment. The long neck of certain species of tortoises is an adaptation to an environment with tall cacti.

Types of AdaptationsEvery species has many adaptations. Scientists classify

adaptations into three categories: structural, behavioral, and functional.

Structural Adaptations These adaptations involve color, shape, and other physical characteristics. The shape of a tortoise’s neck is a structural adaptation.

Behavioral Adaptations The way an organism behaves or acts is a behavioral adaptation. Hunting at night and moving in herds are behavioral adaptations.

Functional Adaptations The last category is functional adaptations. These adaptations involve chemical changes in body systems. A drop in body temperature during hibernation is a functional adaptation.

Environmental InteractionsMany species have evolved adaptations that make them

nearly invisible. For example, a seahorse may be the same color as and similar in texture to the coral it rests on. This is a structural adaptation called camouflage (KAM uh flahj). Camouflage is an adaptation that enables species to blend in with their environments.

Some species have adaptations that draw attention to them or make them more visible. A caterpillar may resemble a snake. Predators see it and are scared away. The resemblance of one species to another species is mimicry (MIH mih kree). Camouflage and mimicry are adaptations that help species avoid being eaten. Many other adaptations help species eat. For example, the pelican has a beak and mouth uniquely adapted to its food source—fish.

Key Concept Check5. Explain How do variations lead to adaptations?

6. Apply An opossum will play dead when a predator frightens it. That way the predator might think it is not good food and will leave it alone. What kind of adaptation is this? (Circle the correct answer.)a. structuralb. behavioralc. functional

Reading Check7. Contrast How do camouflage and mimicry differ?



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8. Synthesize How do you think some fur-bearing species might adapt to a gradual change in climate in which global temperature increased?

Role of Environment Environments are complex. Species must adapt to an environment’s living parts as well as to its nonliving parts. Some nonliving things are temperature, water, nutrients in soil, and climate. Deciduous trees shed their leaves due to changes in climate. Camouflage, mimicry, and mouth shape are adaptations mostly to an environment’s living parts.

Extinct Species Living and nonliving factors are always changing. Even slight environmental changes affect how species adapt. If a species is unable to adapt, it becomes extinct. The fossil record contains many fossils of species unable to adapt to change.

Artificial SelectionAdaptations show how closely Earth’s species match their

environments. This is exactly what Darwin’s theory of evolution by natural selection predicted. Darwin gave many examples of adaptation in On the Origin of Species, the book he wrote to explain his theory. Darwin wrote his book 20 years after he developed his theory. He spent those years collecting more evidence for his theory.

Darwin also had a hobby of breeding pigeons. He bred pigeons of different colors and shapes. In this way, he produced new, fancy varieties. The breeding of organisms for desired characteristics is called selective breeding. Like many plants and animals produced from selective breeding, pigeons look different from their ancestors.

Darwin saw that changes caused by selective breeding were much like changes caused by natural selection. Instead of nature selecting variations, humans selected them. Darwin called this process artificial selection.

Artificial selection explains and supports Darwin’s theory. In Lesson 3, you will read about other evidence that supports the idea that species evolve from other species.

Reading Check9. Compare How are artificial selection and natural selection alike?



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Mini Glossary

Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind?

What do you think

END OF LESSON

Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.

ConnectED

adaptation (a dap TAY shun): a characteristic of a species

that enables the species to survive in its environment

camouflage (KAM uh flahj): an adaptation that enables

species to blend in with their environments

mimicry (MIH mih kree): the resemblance of one species to

another species

naturalist: a person who studies plants and animals by

observing them

natural selection: the process by which populations of

organisms with variations that help them survive in their

environments live longer, compete better, and reproduce

more than those that do not have the variations

selective breeding: the breeding of organisms for desired

characteristics

variation: a slight difference in the appearance of individual

members of a species

1. Review the terms and their definitions in the Mini Glossary. Describe a living organism that depends on camouflage or mimicry to survive.

2. Write a letter in each box to show the correct sequence that demonstrates the process of natural selection.

a. Birds eat more light green beetles. Dark green beetles live longer and reproduce more.

b. A beetle is born with a variation in its color: It is dark green.

c. Over time, all beetles in the environment are dark green.

d. A population of beetles is light green. They stand out against dark green leaves.

3. Compare selective breeding and evolution.



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Evidence for EvolutionThe pictures of horse fossils in Lesson 1 seem to show

that horses evolved in a straight line. That is, one species replaced another in a series of orderly steps. Evolution does not occur this way. Different horse species were sometimes alive at the same time. They are related to one another because each descended from a common ancestor.

Living species that are closely related share a close common ancestor. How closely they are related depends on how closely in time they diverged, or split, from that ancestor. Evidence of common ancestors can be found in the fossil record and in living organisms.

Comparative AnatomyIt is easy to see that some species evolved from a common

ancestor. For example, robins, finches, and hawks have similar body parts. They all have feathers, wings, and beaks. The same is true for tigers, leopards, and house cats. But how are hawks related to cats?

Studying the structural and functional similarities and differences in species that do not look alike can show the relationships. The study of similarities and differences among structures of living species is called comparative anatomy.

What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you’ve read this lesson, reread the statements to see if you have changed your mind.

Before Statement After

5. Living species contain no evidence that they are related to each other.

6. Plants and animals share similar genes.

Key Concepts • What evidence from living

species supports the theory that species descended from other species over time?

• How are Earth’s organisms related?

Identify Main Ideas Highlight the main idea of each paragraph. Highlight two details that support each main idea with a different color. Use your highlighted copy to review what you studied in this lesson.

Make a table with five rows and three columns. Label the rows and columns of the table as shown below. Give your table a title.

Explanation ExampleComparativeAnatomyVestigialStructures

DevelopmentalBiology

MolecularBiology

The Environment and Change Over TimeBiological Evidence of Evolution

LESSON 3

CHAPTER 6



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Homologous Structures Humans, cats, frogs, bats, and birds look different and move in different ways. Humans use their arms for balance and their hands to grasp objects. Cats use their forelimbs to walk, run, and jump. Frogs use their forelimbs to jump. The forelimbs of bats and birds are wings and are used for flying. However, the forelimb bones of all these species show similar patterns, as shown in the figure above. The forelimbs of the species in the figure are different sizes, but their placement and structure suggest common ancestry.

Homologous (huh MAH luh gus) structures are body parts of organisms that are similar in structure and position but different in function. Homologous structures, such as the forelimbs of humans, cats, frogs, bats, and birds, suggest that these species are related. The more alike two structures are, the more likely it is that the species have evolved from a recent common ancestor.

Analogous Structures Can you think of a body part in two species that does the same job but differs in structure? How about the wings of birds and flies? The wings in both species are used for flight. But bird wings are covered with feathers. Fly wings are covered with tiny hairs. Though used for the same function—flight—the wings of birds and insects are too different in structure to suggest close common ancestry.

Bird wings and fly wings are analogous (uh NAH luh gus) structures. Analogous structures are body parts that perform a similar function but differ in structure. The differences in wing structure show that birds and flies are not closely related.

Visual Check1. Infer What is the function of the bones in bats that are homologous to finger bones in humans?

Key Concept Check2. Explain How do homologous structures provide evidence for evolution?

Homologous StructuresHuman Cat Frog Bat Bird

Humerus

Ulna

Radius



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Vestigial Structures Ostriches have wings. Yet they cannot fly. An ostrich’s

wings are an example of vestigial structures. Vestigial (veh STIH jee ul) structures are body parts that have lost their original function through evolution. The best explanation for vestigial structures is that the species with a vestigial structure is related to an ancestral species that used the structure for a specific purpose.

The whale shown in the figure above has tiny pelvic bones inside its body. Pelvic bones are hip bones, which in many species attach the leg bones to the body. Modern whales do not have legs. The pelvic bones in whales suggest that whales came from ancestors that used legs for walking on land. The fossil evidence supports this conclusion. Many fossils of whale ancestors show a slow loss of legs over millions of years. They also show, at the same time, that whale ancestors became better adapted to their watery environments.

Developmental BiologyStudying the internal structures of living organisms is not

the only way that scientists learn about common ancestors. Studying how embryos develop can also show how species are related. The science of the development of embryos from fertilization to birth is called embryology (em bree AH luh jee).

Visual Check3. Infer Why does a vestigial pelvis show that the ancestors of the modern whale once had legs?

Key Concept Check4. Explain How are vestigial structures evidence of descent from ancestral species?

Vestigial Structures

Ambulocetus natans

Modern toothed whale

Pelvis

Vestigial pelvis

After 10–15 million more years of evolution, the

ancestors of modern whales could not walk on

land. They were adapted to an aquatic environment.

Modern whales have two small vestigial pelvic

bones that no longer support legs.

Between 50–40 million years ago, this mammal breathed air and walked clumsily on

land. It spent a lot of time in water, but swimming was difficult because of its rear

legs. Individuals born with variations that made their rear legs smaller lived longer

and reproduced more. This mammal is an ancestor of modern whales.



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Pharyngeal Pouches Embryos of different species often look like each other at different stages of their growth. For example, all vertebrate embryos have pharyngeal (fuh rihn JEE ul) pouches at one stage. These pouches become different body parts in each vertebrate. Yet, in all vertebrates, each part is in the face or neck.

In reptiles, birds, and humans, part of the pharyngeal pouch develops into a gland in the neck. This gland regulates, or balances, the body’s calcium levels.

In fish, the same part becomes the gills. One function of gills is to regulate calcium. The similar function and location of gills and glands suggest a close evolutionary relationship between fish and other vertebrates.

Molecular BiologyStudies of fossils, comparative anatomy, and embryology

provide support for Darwin’s theory of evolution by natural selection. Molecular biology is the study of gene structure and function.

Discoveries in molecular biology have confirmed and extended much of the data already collected about the theory of evolution. Darwin did not know about genes, but scientists today know that mutations in genes are the source of variations upon which natural selection acts. Genes provide powerful support for evolution.

Comparing Sequences All living organisms have genes. All genes are made of DNA, and all genes work in similar ways. This supports the idea that all living organisms are related.

Scientists can study how living organisms are related by comparing their genes. For example, nearly all organisms have a gene for cytochrome c, a protein required for cellular respiration. Some species, such as humans and rhesus monkeys, have nearly identical cytochrome c. The more closely related two species are, the more similar their genes and proteins are.

Divergence Scientists have found that some stretches of shared DNA mutate at regular, predictable rates. Scientists use this “molecular clock” to estimate when in the past living species split from common ancestors. This is how scientists have shown that whales and porpoises are more closely related to hippopotamuses than they are to other living things. Whales and hippopotamuses share an ancestor that lived 50–60 million years ago.

Key Concept Check5. Analyze How do pharyngeal pouches provide evidence of relationships among species?

Reading Check6. Describe What is molecular biology?

Key Concept Check7. Explain How is molecular biology used to determine relationships among species?



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The Study of Evolution TodayThe theory of evolution by natural selection is the

cornerstone of modern biology. Since Darwin published his theory, scientists have confirmed, refined, and extended his work. They have observed natural selection in hundreds of living species. Their studies of fossils, anatomy, embryology, and molecular biology have shown relationships among living and extinct species.

How New Species FormNew evidence supporting the theory of evolution by

natural selection is discovered nearly every day. But scientists debate some of the details. The figure below shows how scientists have different ideas about the rate at which natural selection produces new species. Some say it works slowly and gradually. Others say it works quickly, in bursts. How different species first came about is difficult to study on human time scales. It is also difficult to study with the incomplete fossil record. Yet, new fossils that fill in the holes are discovered all the time. Further fossil discoveries will help scientists study more details about the origin of new species.

DiversityEvolution has produced Earth’s wide diversity of living

things using the same basic building blocks called genes. This is an active area of study in evolutionary biology. Scientists are finding that genes can be reorganized in simple ways and give rise to dramatic changes in organisms. Scientists now study evolution by looking at molecules. Yet, they still use the same basic ideas that Darwin came up with over 150 years ago.

Reading Check9. Describe a difference of opinion in regard to how scientists interpret the theory of evolution by natural selection.

Reading Check8. Connect What is the connection between modern biology and the theory of evolution by natural selection?

Visual Check10. Analyze What does a flat (horizontal) line mean in the figure? (Circle the correct answer.)a. gradual changeb. no variationc. rapid change

Rates of Evolution

Gradual change

Novariation

Novariation

Novariation

Rapid change

Rapid change

Bursts of change

Tim

e

Changes occur

slowly as small

variations are

gradually selected

in a population.

Change occurs

quickly. Long

periods of time

pass with no

variations.



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Mini Glossary

Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind?

What do you think

END OF LESSON

Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.

ConnectED

analogous (uh NAH luh gus) structure: a body part that

performs a similar function to the body part of another

organism, though it differs in structure

comparative anatomy: the study of similarities and

differences among structures of living species

embryology (em bree AH luh jee): the science of the

development of embryos from fertilization to birth

homologous (huh MAH luh gus) structure: a body part

that is similar in structure and position to the body part of

another organism, though it has a different function

vestigial (veh STIH jee ul) structure: a body part that has

lost its original function through evolution

1. Review the terms and their definitions in the Mini Glossary. Use one of the terms to write your own sentence.

2. Use what you have learned about analogous, homologous, and vestigial structures to complete the table. The last row has been completed for you.

3. How did highlighting the main idea in each paragraph help you study this lesson?

Structures Example Pair of Structures Similar Structure or Function (circle one)

Analogoussimilar

structure or function

Homologoussimilar


Vestigialpelvic bone in modern whales

pelvic bone in whale ancestors

similar




the environment and change over time - grade 7...

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