chapter 16 evolution of populations summary · chapter 16 evolution of populations 16–1 genes and...

Name______________________________ Class __________________ Date ______________

© Pearson Education, Inc., publishing as Pearson Prentice Hall.54

Chapter 16 Evolution of Populations

16–1 Genes and VariationDarwin’s original ideas can now be under-stood in genetic terms. Beginning withvariation, we now know that traits are con-trolled by genes and that many genes haveat least two forms, or alleles. We also knowthat individuals of all species are heterozy-gous for many genes.

To understand evolution, genetic varia-tion is studied in populations. A populationis defined as a group of individuals of thesame species that interbreed. Members of apopulation share a common group of genes,called a gene pool. A gene pool consists ofall the genes, including all the differentalleles, that are present in the population. Ingenetic terms, evolution is any change inthe relative frequency of alleles in a popula-tion. The relative frequency of an allele isthe number of times the allele occurs in agene pool, compared with the number oftimes other alleles for the same gene occur.

The two main sources of genetic varia-tion are mutations and gene shuffling. Amutation is any change in a sequence ofDNA. Gene shuffling occurs during theproduction of gametes in sexual reproduc-tion. It can result in millions of differentcombinations of genes. Mutation and geneshuffling do not change relative allele fre-quencies. However, they increase geneticvariation by increasing the number of dif-ferent genotypes.

The number of different phenotypes fora given trait depends on how many genescontrol the trait. A single-gene trait is con-trolled by one gene. If there are two allelesfor the gene, two or three different geno-types are possible. An example in humansis the presence or absence of widow’s peak.A polygenic trait is controlled by two ormore genes, and each gene may have morethan one allele. An example of a humanpolygenic trait is height.

Polygenic traits such as height producemany different phenotypes. Variation in apolygenic trait in a population often pro-duces a bell-shaped curve, with most peo-ple falling near the middle of the curve.

16–2 Evolution as GeneticChangeNatural selection acts on individuals. Evo-lution acts on populations. Natural selec-tion acting on individuals leads to theevolution of populations.

Natural selection on a trait controlled bya single gene with two alleles can cause oneallele to increase and the other allele todecrease. Natural selection on polygenictraits is more complicated. Natural selectionon polygenic traits can occur as directionalselection, stabilizing selection, or disruptiveselection.

Directional selection takes place whenindividuals at one end of the bell-shapedcurve have higher fitness than individualsnear the middle or at the other end of thecurve. The result of directional selection is ashift in the curve toward the higher fitnessend.

Stabilizing selection takes place whenindividuals near the middle of the curvehave higher fitness than individuals at eitherend. The result of stabilizing selection is anarrowing of the curve around the middle.

Disruptive selection takes place whenindividuals at the upper and lower ends ofthe curve have higher fitness than individu-als near the middle. As a result of disrup-tive selection, the curve develops a peak ateach end and a low point in the middle.

Summary

Bio07_TR_U05_CH16.QXD 5/5/06 1:44 PM Page 54

Name______________________________ Class __________________ Date ______________


Natural selection is not the only sourceof evolutionary change. In small popula-tions, alleles can become more or less com-mon simply by chance. This kind of changein allele frequency is called genetic drift. Itoccurs when individuals with a particularallele leave more descendants than otherindividuals, just by chance. Over time, thiscan cause an allele to become more or lesscommon in a population.

Genetic drift also may occur when asmall group of individuals colonizes a newhabitat. By chance, the small group mayhave different relative allele frequenciesthan the original population. When thishappens, it is called the founder effect.

To understand how evolution occurs,scientists first had to answer the question:Under what conditions does evolution notoccur? The answer to this question is calledthe Hardy-Weinberg principle. The princi-ple states that allele frequencies in a popula-tion will remain constant unless one ormore factors cause those frequencies tochange. The situation in which allele fre-quencies remain constant is called geneticequilibrium. For a population to be ingenetic equilibrium, five conditions arerequired: random mating, very large popu-lation size, no migrations, no mutations,and no natural selection. Random matingassures that each individual has an equalchance of reproducing. Very large popula-tion size prevents genetic drift from occur-ring. If all five conditions are met, relativeallele frequencies will not change and evo-lution will not occur.

16–3 The Process of SpeciationSpeciation means the formation of newspecies. For one species to evolve into twonew species, the gene pools of two popula-tions must become separated, or reproduc-tively isolated. Reproductive isolation occurswhen members of two populations cannotinterbreed and produce fertile offspring.Reproductive isolation can involve behav-ioral, geographic, or temporal isolation.

Behavioral isolation occurs when popu-lations have different courtship rituals orother behaviors involved in reproduction.Geographic isolation occurs when popula-tions are separated by geographic barriers,such as mountains or rivers. Temporal isola-tion occurs when populations reproduce atdifferent times.

Recently, Peter and Rosemary Grantproved that natural selection is still causingevolution of finches on the GalápagosIslands. The Grants showed that there wasenough heritable variation in finch beaks toprovide raw material for natural selection.They also showed that differences in beaksproduced differences in fitness. These dif-ferences in fitness caused directional selec-tion to occur.

Darwin thought that different finchspecies evolved on the Galápagos Islandsfrom a single species of founding birds. Wenow know how this could have happened.A few finches may have traveled frommainland South America to one of theislands to found a new population. There,they survived and reproduced. Some birdscrossed to a second island, and the twopopulations became geographically iso-lated. They no longer shared a gene pool.Seed sizes on the second island favoredbirds with larger beaks. The population onthe second island evolved into a populationwith larger beaks. Eventually, the large-beaked birds on the second island becamereproductively isolated and evolved into anew species.

Evolution continues today. For example,bacteria are evolving to have resistance todrugs. Evolutionary theory can help usunderstand these changes.


Name______________________________ Class __________________ Date ______________


Chapter 16 Evolution of Populations

Section 16–1 Genes and Variation (pages 393–396)

Key Concepts• What are the main sources of heritable variation in a population?

• How is evolution defined in genetic terms?

• What determines the numbers of phenotypes for a given trait?

Introduction (page 393)

1. Is the following sentence true or false? Mendel’s work on inheritance was published

after Darwin’s lifetime.

2. Which two important factors was Darwin unable to explain without an understanding

of heredity?

How Common Is Genetic Variation? (page 393)

3. All organisms have additional that is “invisible” because it involves small differences in biochemical processes.

Variation and Gene Pools (page 394)

4. A group of individuals of the same species that interbreed is a(an) .

5. All of the genes in a population are called a(an) .

6. Is the following sentence true or false? A gene pool typically contains just one allele for

each inheritable trait.

7. The number of times that an allele occurs in a gene pool compared with the number of

times other alleles for the same gene occur is called the of the allele.

Sources of Genetic Variation (pages 394–395)

8. What is a mutation?

9. Why do mutations occur?


Name______________________________ Class __________________ Date ______________


10. Circle the letter of each choice that is true about mutations.

a. They do not always change an amino acid.

b. They always affect lengthy segments of a chromosome.

c. They always affect an organism’s phenotype.

d. They always affect an organism’s fitness.

11. Is the following sentence true or false? Most heritable differences are due to gene

shuffling that occurs during the production of gametes.

12. Circle the letter of each choice that is true about sexual reproduction.

a. It is a major source of variation in many populations.

b. It can produce many different phenotypes.

c. It can produce many different genetic combinations.

d. It can change the relative frequency of alleles in a population.

Single-Gene and Polygenic Traits (pages 395–396)

13. Is the following sentence true or false? The number of phenotypes produced for a

given trait depends on how many genes control the trait.

14. Is the following sentence true or false? Most traits are controlled by a single gene.

15. Label the two graphs to show which one represents a single-gene trait and which one represents a polygenic trait.

100

80

60

40

20

Widow's peak0

No widow's peak

Phenotype

Freq

uenc

y of

Phe

noty

pe(%

)

Freq

uenc

y of

Phe

noty

pe

Phenotype (height)


Name______________________________ Class __________________ Date ______________


Section 16–2 Evolution as Genetic Change(pages 397–402)

Key Concepts• How does natural selection affect single-gene and polygenic traits?

• What is genetic drift?

• What is the Hardy-Weinberg principle?

Natural Selection on Single-Gene Traits (pages 397–398)

1. Is the following sentence true or false? Natural selection on single-gene traits cannot

lead to changes in allele frequencies.

2. If a trait made an organism less likely to survive and reproduce, what would happen to

the allele for that trait?

3. If a trait had no effect on an organism’s fitness, what would happen to the allele for that

trait?

Natural Selection on Polygenic Traits (pages 398–399)

4. List the three ways that natural selection can affect the distributions of phenotypes.

a.

b.

c.

Match the type of selection with the situation in which it occurs.Type of Selection5. Directional

6. Stabilizing

7. Disruptive

8. An increase in the average size of beaks in Galápagos finches is an example of

selection.

9. Is the following sentence true or false? The weight of human infants at birth is under the

influence of disruptive selection.

Situationa. Individuals at the upper and lower ends of the curve

have higher fitness than individuals near the middle.b. Individuals at one end of the curve have higher fitness

than individuals in the middle or at the other end.c. Individuals near the center of the curve have higher

fitness than individuals at either end.

Bio07_TR_U05_CH16.QXD 5/5/06 1:44 PM 8

Name______________________________ Class __________________ Date ______________


10. Draw the missing graph to show how disruptive selection affects beak size.

Genetic Drift (page 400)

11. Is the following sentence true or false? Natural selection is the only source of

evolutionary change.

12. Random change in allele frequencies in small populations is called

.

13. A situation in which allele frequencies change as a result of the migration of a small

subgroup of a population is known as the .

14. What is an example of the founder effect?

Evolution Versus Genetic Equilibrium (page 401–402)

15. What does the Hardy-Weinberg principle state?

16. The situation in which allele frequencies remain constant is called

.

17. List and describe the five conditions required to maintain genetic equilibrium.

a.

b.

Disruptive SelectionN

umbe

r of

Bir

dsin

Pop

ulat

ion

Beak Size

Largest and smallest seeds become more common.

Num

ber

of B

irds

in P

opul

atio

n

Beak Size

Population splitsinto two subgroupsspecializing indifferent seeds.


Name______________________________ Class __________________ Date ______________


Reading Skill PracticeWhen you read about related concepts, making a graphic organizer such as a Venndiagram can help you focus on their similarities and differences. Make a Venndiagram comparing and contrasting single-gene and polygenic traits. For moreinformation on Venn diagrams, see Appendix A of your textbook. Do your work ona separate sheet of paper.

c.

d.

e.

18. Is it common for a population to remain in genetic equilibrium? Explain your answer.


Name______________________________ Class __________________ Date ______________


Section 16–3 The Process of Speciation(pages 404–410)

Key Concepts• What factors are involved in the formation of new species?

• Describe the process of speciation in the Galápagos finches.

Introduction (page 404)

1. What is speciation?

Isolating Mechanisms (pages 404–405)

2. Is the following sentence true or false? Individuals in different species can have the

same gene pool.

3. What does it mean for two species to be reproductively isolated from each other?

4. What must happen in order for new species to evolve?

5. List three ways that reproductive isolation occurs.

a. c.

b.

6. When does behavioral isolation occur?

7. Is the following sentence true or false? Eastern and Western meadowlarks are an

example of behavioral isolation.

8. When does geographic isolation occur?

9. Abert and Kaibab squirrels in the Southwest are an example of isolation.

10. Is the following sentence true or false? Geographic barriers guarantee the formation of

new species.

11. What is an example of temporal isolation?

Testing Natural Selection in Nature (pages 406–407)

12. Is the following sentence true or false? The basic mechanisms of evolutionary change

cannot be observed in nature.


Name______________________________ Class __________________ Date ______________


13. Circle the letter of each hypothesis about the evolution of Galápagos finches that was tested by the Grants.

a. The finches’ beak size and shape has enough inheritable variation to provide rawmaterial for natural selection.

b. The different finch species are the descendants of a common mainland ancestor.

c. Differences in the finches’ beak size and shape produce differences in fitness thatcause natural selection to occur.

d. The evolution of the finches is proceeding slowly and gradually.

Speciation in Darwin’s Finches (pages 408–409)

14. Complete the flowchart to show how speciation probably occurred in the Galápagos finches.

15. How could differences in beak size lead to reproductive isolation?

Studying Evolution Since Darwin (page 410)

16. Why is the study of evolution important?

Founders arrive

Continued evolution


chapter 16 evolution of populations summary · chapter 16 evolution of populations 16–1 genes and...

Documents