Heredity and How Traits Change - Christ the King School

Heredity and How Traits Change

Adaptation and Evolution

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. A population that lacks variation among its individuals might not be able to adapt to a changing environment.

6. Extinction occurs when the last individual of a species dies.

Key Concepts

? How does natural selection occur?

? What is an adaptation?

? Why do traits change over time?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Mutations, Variation, and Natural Selection

Recall that mutations can lead to changes in traits. Therefore, mutations can produce differences among individuals. Slight differences in inherited traits among individuals in a population are called variations.

In 1976, scientists measured several traits in a population of medium ground finches on one of the Gal?pagos Islands. They discovered that the birds had variations in beak size. Most had smaller beaks, but all of the birds of this species preferred to eat small, soft seeds. The next year, it did not rain on the island. None of the plants reproduced, so no seeds formed. After all of the small, soft seeds had been eaten, many of the finches died. The few seeds left were fairly large and hard. The birds that survived were those that could crack and eat these larger seeds. These finches tended to be the birds with fairly large beaks.

In 1978, the scientists measured the beaks of the surviving birds' offspring. They compared the average beak size of birds hatched in 1978 to that of birds hatched in 1976. In just two years, the average beak size of birds in the population had increased. How did this happen?

Building Vocabulary As you read, underline the words and phrases that you do not understand. When you finish reading, discuss these words and phrases with another student or your teacher.

Make a vertical three-tab book and use it to organize notes on the different types of adaptations.

Structural

" E

B

Q

Functional

U B

U

J

P

Behavioral

O T

Reading Essentials

Heredity and How Traits Change 411

Key Concept Check 1. Explain How does natural selection occur?

Visual Check 2. Analyze What happened when individual sunflowers competed for limited resources?

Natural Selection

The process by which individuals with variations that help them survive in their environment live longer, compete better, and reproduce more than those individuals without these variations is called natural selection. For the finches, there was variation among individual birds for the trait of beak size. Some of the birds had small beaks, and some of them had larger beaks. When a change in the environment--a drought--occurred, the birds with larger beaks were better able to survive than birds with smaller beaks. The surviving individuals then passed on the favorable trait to their offspring. Over the two-year span of natural selection and reproduction, the average beak size of the birds in the population increased. The birds with larger beaks were naturally selected by environmental conditions and survived. Another example of natural selection in plants is shown in the figure below.

Natural Selection: Sunflowers

1 Variation Individuals in a population differ from one another. In this population, some sunflowers are taller than others.

2 Inheritance Traits are inherited from parents. Tall sunflowers produce tall sunflowers. Short sunflowers produce short sunflowers.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

3 Competition Due to limited resources, not all offspring will survive. Individuals with a trait that better suits the environment are more likely to survive and reproduce. In this environment, short sunflowers are more successful.

412 Heredity and How Traits Change

4 Natural Selection Over time, the average height of the sunflower population is short if the short sunflowers continue to reproduce successfully.

Reading Essentials

Adaptations

The traits of surviving individuals, such as larger beaks or shorter sunflowers, become more common as the survivors reproduce and pass the genes for their traits to their offspring. An adaptation is an inherited trait that increases an organism's chance of surviving and reproducing in a particular environment. Adaptations can be structural, functional, or behavioral.

Structural Adaptations

A flying squirrel has a flap of skin between its forelegs and hind legs. This flap of skin enables the squirrel to glide distances of up to 45 m. The behavior possibly is used to escape from predators. This flap of skin is an example of a structural adaptation. Structural adaptations involve physical characteristics, such as color or shape.

Another example of a structural adaptation occurs in many desert plants. Some types of cactus plants have spines. Spines are leaves that are reduced in size. The adaptation of smaller leaves helps reduce water loss in a dry environment.

Functional Adaptations

Functional adaptations involve internal systems that affect an organism's physiology or biochemistry. For example, the alpine snowbell, a flower species, has adapted to survive in an environment with a short growing season.

In the high altitude of the mountains, where the alpine snowbell grows, light and temperature conditions for flowering are favorable for only a short period during the summer. The alpine snowbell produces flower buds at the end of the previous season. Over the winter, the buds remain dormant--alive, but not actively growing. In the spring, increased light triggers the plant to bloom even when it is still surrounded by snow. These adaptations enable the species to survive.

Behavioral Adaptations

Migration, and other behavioral adaptations, involve the ways an organism behaves or acts. For example, caribou migrate south for the winter. Other animals, such as birds, whales, and butterflies, also migrate. Animal species that migrate to find adequate food and suitable temperatures survive and reproduce more successfully.

Reading Check 3. Identify How can a species benefit from an adaptation?

4. Hypothesize How

might a monkey's long tail be an example of a structural adaptation?

Key Concept Check 5. Describe three types of adaptations.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

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Key Concept Check 6. State Why do traits change over time?

ACADEMIC VOCABULARY

survive (verb) to remain alive

Visual Check 7. Draw a circle around the dish with the largest number of resistant bacteria.

Evolution of Populations-- Why Traits Change

Once an inherited trait has become more frequent in a population, the population has adapted and evolved. Evolution is change over time. Evolution by natural selection is a way that populations change over time. When populations evolve, species can look and behave differently than their ancestors. This happens because the frequency of genetic traits changes over time. As the environment changes, different inherited traits might enable survival, and the population can evolve again.

A Modern Example of Change Over Time

Bacteria can cause infections in your body, such as strep throat or pneumonia. Sometimes a doctor might prescribe an antibiotic to help you fight an infection. Antibiotics are drugs that kill bacteria. Although antibiotics often kill bacteria effectively, variation exists within a population of bacteria. As shown in the figure below, some bacteria in a population might have a mutation that lets them survive when exposed to an antibiotic. When the surviving bacteria reproduce, that trait passes to their offspring. Soon, most individuals in the population survive when exposed to the antibiotic. Bacteria that survive when exposed to an antibiotic are called antibiotic-resistant. Antibiotic-resistant bacteria have caused deadly infections and are of great concern to scientists.

Antibiotic Resistance in Bacteria

An antibiotic is added to a population of bacteria. A few of the bacteria already have mutations that enable them to resist the antibiotic.

New resistant and nonresistant bacteria are treated with the same antibiotic.

Nonresistant bacteria Resistant bacteria

Antibiotic

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

The antibiotic kills most of the nonresistant bacteria. The surviving bacteria reproduce.

414 Heredity and How Traits Change

The antibiotic now affects only a small percentage of the bacteria. The surviving bacteria continue to reproduce. Most of the bacteria are resistant to the antibiotic.

Reading Essentials

Extinction and Conservation Biology

You already have read that for evolution by natural section to occur, variation within a population is needed. A population with diversity can survive changes in its environment and persist through time. But what happens when a population lacks variation among its individuals and the environment changes? The population might lose its ability to reproduce successfully and fail to survive. When the last individual of a species dies, the species has undergone extinction.

Today, many species are threatened with extinction. A species' habitat might have been altered or destroyed. Some species have been hunted to extinction. For others, new species introduced into many habitats make it difficult for some native species to survive and reproduce.

A relatively new field of science focuses on saving species. Conservation biology is a branch of biology that studies why many species are in trouble and what can be done to save them. Sometimes scientists' knowledge of genetics helps species that are in danger of extinction. For example, by 1995, the population of Florida panthers was between 20 and 30 individuals. The population had lost much of its natural variation and was struggling to survive.

Fortunately, scientists' understanding of genetics and heredity saved the population from extinction. Scientists introduced into the Florida population several female panthers from a population in Texas. This was done to increase genetic diversity in the Florida population. By 2003, the Florida panther population had increased to 80 individuals, and the effort was considered a success.

8. Name one animal

species that is in danger of becoming extinct.

SCIENCE USE V. COMMON USE

introduce Science Use to bring a substance or organism into a habitat or a population Common Use to make someone known to others

Reading Check 9. Explain Why did scientists introduce panthers from Texas into the Florida panther population?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

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