1 FOCUSObjectives16.2.1 Explain how natural selec-

tion affects single-gene andpolygenic traits.

16.2.2 Describe genetic drift.16.2.3 List the five conditions need-

ed to maintain geneticequilibrium.

Vocabulary PreviewChallenge students to predict whatthe Vocabulary terms directional selec-tion, stabilizing selection, anddisruptive selection refer to. Theyshould check to see if their predic-tions were correct after they read thesection.

Reading StrategyWhen completing their outlines, stu-dents should pay special attention tothe highlighted, boldface terms andthe boldface sentences.

2 INSTRUCT

Natural Selection onSingle-Gene TraitsUse VisualsFigure 16–5 Ask: How does coloraffect the fitness of the lizards?(Both red and brown lizards are less fitthan black lizards.) What do you pre-dict the lizard population will looklike by generation 50? Explain.(Students are likely to say that thelizard population will have more blacklizards, fewer brown lizards, and no redlizards by generation 50. They shoulddescribe the environmental conditionsthat would support their prediction.)

Evolution of Populations 397

16–2 Evolution as Genetic Change

Agenetic view of evolution offers a new way to look at keyevolutionary concepts. Each time an organism reproduces,

it passes copies of its genes to its offspring. We can thereforeview evolutionary fitness as an organism’s success in passinggenes to the next generation. In the same way, we can view anevolutionary adaptation as any genetically controlled physiolog-ical, anatomical, or behavioral trait that increases an individ-ual’s ability to pass along its genes.

Natural selection never acts directly on genes. Why?Because it is an entire organism—not a single gene—that eithersurvives and reproduces or dies without reproducing. Naturalselection, therefore, can only affect which individuals surviveand reproduce and which do not. If an individual dies withoutreproducing, the individual does not contribute its alleles to thepopulation’s gene pool. If an individual produces many offspring,its alleles stay in the gene pool and may increase in frequency.

Now recall that evolution is any change over time in therelative frequencies of alleles in a population. This reminds usthat it is populations, not individual organisms, that can evolveover time. Let us see how this can happen in different situations.

Natural Selection on Single-Gene TraitsNatural selection on single-gene traits can lead to

changes in allele frequencies and thus to evolution.Imagine that a hypothetical population of lizards, shown inFigure 16-5, is normally brown, but experiences mutations thatproduce red and black forms. What happens to those new alleles? If red lizards are more visible to predators, they mightbe less likely to survive and reproduce, and the allele for redcoloring might not become common.

Key Concepts• How does natural selection

affect single-gene and polygenic traits?

• What is genetic drift?• What five conditions are

needed to maintain geneticequilibrium?

Vocabularydirectional selectionstabilizing selectiondisruptive selectiongenetic driftfounder effectHardy-Weinberg principlegenetic equilibrium

Reading Strategy:Outlining Before you read,use the headings to make anoutline. As you read, add asentence after each heading toprovide key information.

� Figure 16–5 Natural selectionon single-gene traits can lead tochanges in allele frequencies and thusto evolution. Organisms of one color,for example, may produce feweroffspring than organisms of other colors.

SECTION RESOURCES

Print:

• Laboratory Manual B, Chapter 16 Lab• Teaching Resources, Section Review 16–2• Reading and Study Workbook A, Section 16–2• Adapted Reading and Study Workbook B,

Section 16–2• Lesson Plans, Section 16–2

Technology:

• iText, Section 16–2• Transparencies Plus, Section 16–2• Virtual Labs, Lab 14, Lab 15

Tim

eSaver

Section 16–2

398 Chapter 16

Natural Selection onPolygenic TraitsBuild Science SkillsApplying Concepts Point out howpolygenic traits show continuousvariation rather than falling into dis-crete, all-or-nothing categories likesingle-gene traits. Ask: Besidesheight and birth weight, what aresome other polygenic traits thatshow continuous variation? (Otherhuman polygenic traits that show continuous variation include body proportions such as limb length, bio-chemical traits such as skin pigmentation, and behavioral traitssuch as intelligence.)

Build Science SkillsInferring Explain that polygenictraits are often susceptible to envi-ronmental influences. In fact, a shiftin the environment can lead to acorresponding shift in the pheno-types of a polygenic trait, which canmimic directional selection. Give stu-dents an example. Explain that,during the 1900s, average height inthe United States increased becauseof environmental factors. Ask: Whatenvironmental factors do you thinkled to this shift in phenotype? (Theincrease in average height has beenattributed largely to changes in dietand health care that maximizedgrowth potential.)

Comprehension: Ask QuestionsBeginning To help students understand therelationship between genetics and naturalselection, distribute a rewritten, modified ver-sion of the first three paragraphs on page 397that includes the most important information.Ask students questions that can be answereddirectly from the rewritten text. For example:When an organism reproduces, what does itpass to its offspring? How do genes affect anorganism’s adaptations?

Intermediate Have students read the modi-fied text that you prepared for beginningstudents and then read the actual text in thebook. Students can work in groups to writequestions about the text. Provide answers bothorally and in writing.

SUPPORT FOR ENGLISH LANGUAGE LEARNERS

Directional Selection

Beak Size

Num

ber

of

Bir

ds

in P

opul

atio

n

Black lizards, on the other hand, might absorb more sun-light and warm up faster on cold days. If high body temperatureallows them to move faster to feed and to avoid predators, theymight produce more offspring than brown forms. The allele forblack color might then increase in relative frequency. If a colorchange has no effect on fitness, the allele that produces it wouldnot be under pressure from natural selection.

Natural Selection on Polygenic TraitsWhen traits are controlled by more than one gene, the effects ofnatural selection are more complex. As you learned earlier, theaction of multiple alleles on traits such as height produces arange of phenotypes that often fit a bell curve. The fitness ofindividuals close to one another on the curve will not be verydifferent. But fitness can vary a great deal from one end of sucha curve to the other. And where fitness varies, natural selectioncan act. Natural selection can affect the distributionsof phenotypes in any of three ways: directional selection,stabilizing selection, or disruptive selection.

Directional Selection When individuals at one end of thecurve have higher fitness than individuals in the middle or atthe other end, takes place. The range ofphenotypes shifts as some individuals fail to survive and repro-duce while others succeed. To understand this, consider howlimited resources, such as food, can affect the long-term survivalof individuals and the evolution of populations.

Among seed-eating birds such as Darwin’s finches, for exam-ple, birds with bigger, thicker beaks can feed more easily on larger,harder, thicker-shelled seeds. Suppose a food shortage causes thesupply of small and medium-sized seeds to run low, leaving onlylarger seeds. Birds whose beaks enable them to open those largerseeds will have better access to food. Birds with the big-beakadaptation would therefore have higher fitness than small-beakedbirds. The average beak size of the population would probablyincrease, as shown in Figure 16–6.

directional selection

� Figure 16–6 Directionalselection occurs when individuals at one end of the curve have higherfitness than individuals in themiddle or at the other end. In thisexample, a population of seed-eatingbirds experiences directional selectionwhen a food shortage causes thesupply of small seeds to run low. Thedotted line shows the original dis-tribution of beak sizes. The solid lineshows how the distribution of beaksizes would change as a result ofselection.

16–2 (continued)

DemonstrationShow students illustrations ofmonarch and viceroy butterflies.Challenge them to detect any visibledifferences between the two species.Explain that monarch butterflies areavoided by bird predators becausethey taste bitter and that viceroy but-terflies are avoided by bird predatorsbecause they resemble the bitter-tasting monarch butterflies, asituation called mimicry that hasevolved through natural selection.Ask: If monarch butterflies evolvedwhite spots instead of orangespots, what do you think wouldhappen to viceroy butterflies? (If amutation for white spots occurred inviceroy butterflies, natural selectionwould cause the trait to increase in fre-quency.)

Use VisualsFigure 16–7 Ask: If the fitness ofphenotypes at both ends of thecurve were to decrease even more,how would it affect the shape ofthe curve? (The curve would becomenarrower.) If medical advances couldprevent problems for high birthweight babies but not for low birthweight babies, how might thecurve change then? (There might bea shift in the curve toward higher birthweights or at least a broadening of thecurve at the high end.)

Evolution of Populations 399

Birth Mass

Per

cen

tag

e o

f P

op

ula

tio

n

Stabilizing Selection

Selection againstboth extremes keeps curve narrow and in same place.

Stabilizing Selection When individualsnear the center of the curve have higher fitnessthan individuals at either end of the curve,

takes place. This situationkeeps the center of the curve at its currentposition, but it narrows the overall graph.

As shown in Figure 16–7, the mass of humaninfants at birth is under the influence of stabiliz-ing selection. Human babies born much smallerthan average are likely to be less healthy andthus less likely to survive. Babies that are muchlarger than average are likely to have difficultybeing born. The fitness of these larger or smallerindividuals is, therefore, lower than that of moreaverage-sized individuals.

Disruptive Selection When individuals atthe upper and lower ends of the curve havehigher fitness than individuals near the middle,

takes place. In suchsituations, selection acts most strongly againstindividuals of an intermediate type. If the pres-sure of natural selection is strong enough andlasts long enough, this situation can cause thesingle curve to split into two. In other words,selection creates two distinct phenotypes.

For example, suppose a population of birdslives in an area where medium-sized seedsbecome less common and large and small seedsbecome more common. Birds with unusuallysmall or large beaks would have higher fitness.As shown in Figure 16– 8, the population mightsplit into two subgroups: one that eats smallseeds and one that eats large seeds.

How do stabilizing selection and disruptiveselection differ?

disruptive selection

stabilizing selection

� Figure 16–7 Stabilizing selection takesplace when individuals near the center of a curvehave higher fitness than individuals at either end.This example shows that human babies born at anaverage mass are more likely to survive than babiesborn either much smaller or much larger than average.

Disruptive Selection

Beak Size

Nu

mb

er o

f B

ird

sin

Po

pu

lati

on � Figure 16– 8 When individuals at the

upper and lower ends of the curve have higherfitness than individuals near the middle, disruptiveselection takes place. In this example, average-sizedseeds become less common, and larger and smallerseeds become more common. As a result, the birdpopulation splits into two subgroups specializing ineating different-sized seeds.

Answer to . . . Stabilizing selection is

selection for individuals at the centerof the curve. It keeps the center of thecurve at its current position. Disruptiveselection is selection for individuals atthe upper and lower ends of the curve.It splits the curve in two.

400 Chapter 16

Genetic DriftBuild Science SkillsUsing Models Divide the class intogroups, and provide each group witha bowl containing 10 beans each of 5different types, such as pinto, kidney,navy, white, and lima beans.Challenge groups to brainstorm away to use the beans to modelgenetic drift. (One way is by randomlyselecting only some of the beans fromthe bowl to represent alleles in the nextgeneration.) Ask: How would youshow with your model that geneticchange had occurred? (By calculat-ing the relative frequencies of thedifferent types of beans in the nextgeneration to show that their frequen-cies had changed)

Make ConnectionsEnvironmental Science State thatthe founder effect may be especiallylikely to occur when natural disasterstake place. Challenge students tothink of ways in which natural disas-ters might result in a small number ofindividuals from a population becom-ing and remaining isolated from therest of the group. (As one example,students might describe how a forestfire isolates a few rodents in a smallremnant of forest.)

Genetic DriftNatural selection is not the only source of evolutionary change.In small populations, an allele can become more or less commonsimply by chance. Recall that genetics is controlled by the lawsof probability. These laws can be used to predict the overallresults of genetic crosses in large populations. However, thesmaller a population is, the farther the results may be fromwhat the laws of probability predict. This kind of randomchange in allele frequency is called How doesgenetic drift.genetic drift take place? In small populations, individu-als that carry a particular allele may leave more descen-dants than other individuals, just by chance. Over time, aseries of chance occurrences of this type can cause anallele to become common in a population.

Genetic drift may occur when a small group of individualscolonizes a new habitat. These individuals may carry alleles indifferent relative frequencies than did the larger populationfrom which they came. If so, the population that they found willbe genetically different from the parent population. Here,however, the cause is not natural selection but simply chance—specifically, the chance that particular alleles were in one ormore of the founding individuals, as shown in Figure 16–9. Asituation in which allele frequencies change as a result of themigration of a small subgroup of a population is known as the

One example of the founder effect is the evolu-tion of several hundred species of fruit flies found on differentHawaiian Islands. All of those species descended from the sameoriginal mainland population. Those species in different habi-tats on different islands now have allele frequencies that aredifferent from those of the original species.

What is genetic drift?

founder effect.

Sample ofOriginal Population

Founding Population A

Descendants

Founding Population B

Figure 16–9 In small popula-tions, individuals that carry aparticular allele may have moredescendants than other individu-als. Over time, a series of chanceoccurrences of this type can causean allele to become more com-mon in a population. This modeldemonstrates how two smallgroups from a large, diversepopulation could produce newpopulations that differ from theoriginal group.

Mutiny on the BountyA good example of founder effect in human pop-ulations is the population of Pitcairn Island in theSouth Pacific. The island’s population today haslimited genetic variability, because it was foundedby only a handful of people in the late 1700s. Thefounders consisted of nine mutineers from the HMS Bounty, all of whom were English, alongwith a small number of Tahitian men and Tahitianwomen. A few years after the population was

founded, the number of people declined evenmore because of a disagreement between theEnglish and Tahitian men. When Pitcairn Islandwas discovered by American whalers in 1808, thepopulation consisted of just one Englishman, sev-eral Tahitian women, and some children. Becausethe population was geographically isolated, fewnew genes entered the gene pool over subse-quent years, and genetic variation remainedlimited.

FACTS AND FIGURES

16–2 (continued)

Evolution VersusGenetic EquilibriumMake ConnectionsMathematics Explain that in addi-tion to allele frequencies remainingconstant when a population is ingenetic equilibrium, genotype propor-tions also remain constant and can becalculated from the allele frequencies.If p is the frequency of allele A for atrait and q is the frequency of allele afor the same trait, then genotype pro-portions are given by (p � q)2 � p2 (AA) � 2pq (Aa) �q2 (aa). Ask: If a population is ingenetic equilibrium and the value ofp is 0.3, what proportion of the pop-ulation has each genotype? (Theproportion of AA individuals is p2, or0.09; the proportion of Aa individuals is2pq, or 0.42; and the proportion of aaindividuals is q2, or 0.49.) Point outthat the genotype proportions mustadd up to 1.00.

Evolution of Populations 401

Evolution Versus Genetic EquilibriumTo clarify how evolutionary change operates, scientists often findit helpful to determine what happens when no change takesplace. So biologists ask: Are there any conditions under whichevolution will not occur? Is there any way to recognize when thatis the case? The answers to those questions are provided by theHardy-Weinberg principle, named after two researchers whoindependently proposed it in 1908.

The states that allele frequen-cies in a population will remain constant unless one or morefactors cause those frequencies to change. The situation inwhich allele frequencies remain constant is called

If the allele frequencies do not change, thepopulation will not evolve.

Under what conditions does the Hardy-Weinberg principlehold? Five conditions are required to maintain geneticequilibrium from generation to generation: (1) Theremust be random mating; (2) the population must be verylarge; and (3) there can be no movement into or out of thepopulation, (4) no mutations, and (5) no natural selection.

In some populations, these conditions may be met or nearlymet for long periods of time. If, however, the conditions are notmet, the genetic equilibrium will be disrupted, and the popula-tion will evolve.

equilibrium.genetic

Hardy-Weinberg principle

Can the environment affect survival?

Materials scissors, construction paper (severalcolors), transparent tape, 15-cm ruler, watch with a second hand

Procedure

1. Predicting Predict what would happen to apopulation of butterflies that includes someindividuals that are easy for predators to see andsome that blend in with the environment.

2. Choose three different-colored sheets of construc-tion paper. Cut out a butterfly shape from eachsheet, 5 � 10 cm in size, as shown. CAUTION:Be careful with scissors.

3. Tape your butterflies to different-colored surfaces.Then, return to your seat.

4. Record how many shapes of each color you cancount from your desk in 5 seconds.

5. Exchange your observations with your classmatesto determine the class total for each color.

Analyze and Conclude1. Analyzing Data According to your class data,

which colors of butterfly are easiest to see? Whichcolor of butterfly would be most easily caught by apredator?

2. Inferring What will happen to the butterflypopulation after many generations if predatorsconsume most of the easy-to-see butterflies?

Using a series of overhead transparencies, Iexplain the Hardy-Weinberg principle to theentire class. Then, I distribute a problem sheetto the students and have them work in pairs.One student is the tutor, and the other is thelearner. While students are teaching one anoth-er, I circulate through the classroom, providinghelp where needed. After each pair of studentshas solved three problems, I have the studentsreverse roles with their partners. I find that stu-dents understand the Hardy-Weinberg principle

much more quickly and thoroughly when theyare able to explain it to one another. At the endof this activity, I have each pair write a newproblem on an overhead transparency. I thenuse these new problems as a warm-up activityfor genetics.

—Marion LaFemina Biology TeacherRidgewood High SchoolRidgewood, NJ

TEACHER TO TEACHER

Objective Students will be able toanalyze data and infer that the envi-ronment affects survival. Skills Focus Analyzing Data,InferringMaterials scissors, constructionpaper (several colors), transparenttape, 15-cm ruler, watch with a second handTime 20 minutesAdvance Prep Select surfaces thatwill not be harmed by tape. Providestudents with construction paper insome colors that blend with andother colors that contrast with theselected surfaces.Strategy Relate the lab to naturalselection.Expected Outcome Students willfind that butterflies in contrastingcolors are easier to see.Analyze and Conclude1. Butterflies in colors that contrastwith the surfaces are the easiest tosee. These butterflies would be themost easily caught by a predator.2. After many generations, the popula-tion will be made up only of butterfliesin colors that are difficult to see.

Answer to . . . Random change in allele

frequencies

402 Chapter 16

Build Science SkillsInferring Explain that selection forheterozygotes can also lead to equi-librium in allele frequencies, and givethe following example. State that, insome African populations wheremalaria is prevalent, heterozygotes for sickle cell hemoglobin have the highest fitness, because they aresomewhat resistant to malaria andlargely unaffected by sickle cell ane-mia. Homozygotes for sickle cellhemoglobin have the lowest fitness,because they have sickle cell anemia.Normal homozygotes have somewhatreduced fitness, because they have noresistance to malaria. As a result, theallele for sickle cell hemoglobin per-sists in these populations. Ask: Whatdo you think would happen to thesickle cell allele in these popula-tions if malaria were eradicated?(The allele would be selected againstand become less common.)

3 ASSESSEvaluate UnderstandingAsk students to write a paragraphsummarizing the different types ofnatural selection on polygenic traits.

ReteachUsing the board or a transparency,work with students to make a con-cept map showing the conditionsrequired for genetic equilibrium.

Random Mating All members of the popula-tion must have an equal opportunity to produceoffspring. Random mating ensures that eachindividual has an equal chance of passing on itsalleles to offspring.

In natural populations, however, mating israrely completely random. Many species, includ-ing lions and wolves, select mates based onparticular heritable traits, such as size orstrength. Such nonrandom mating means thatthe genes for those traits are not in equilibriumbut are under strong selection pressure.

Large Population A large population size is also important inmaintaining genetic equilibrium. That is because genetic drift hasless effect on large populations, such as the population of birdsshown in Figure 16–10, than on small ones.

No Movement Into or Out of the PopulationBecause individuals may bring new alleles into a population,there must be no movement of individuals into or out of apopulation. In genetic terms, the population’s gene pool must bekept together and kept separate from the gene pools of otherpopulations.

No Mutations If genes mutate from one form into another,new alleles may be introduced into the population, and allelefrequencies will change.

No Natural Selection All genotypes in the populationmust have equal probabilities of survival and reproduction.No phenotype can have a selective advantage over another.In other words, there can be no natural selection operating onthe population.

� Figure 16–10 One of thefive conditions that are needed tomaintain genetic equilibriumfrom one generation to the nextis large population size. The allelefrequencies of large populations,such as this group of birds, are lesslikely to be changed through theprocess of genetic drift.

1. Key Concept Describehow natural selection can affecttraits controlled by single genes.

2. Key Concept Describethree patterns of natural selectionon polygenic traits. Which oneleads to two distinct phenotypes?

3. Key Concept How doesgenetic drift lead to a change in apopulation’s gene pool?

4. Key Concept What is theHardy-Weinberg principle?

5. Critical Thinking Comparingand Contrasting How aredirectional selection and disrup-tive selection similar? How arethey different?

Using Models Demonstrate natural selectionon polygenic traits by cuttinga sheet of paper into squaresof five different sizes torepresent sizes in apopulation. Use the squaresto model directional,stabilizing, and disruptiveselection. Then, think of analternative way to model onetype of selection. Decidewhich model works best, andgive your reasons.

16–2 Section Assessment

16–2 (continued)

One way students can modelselection is to use the different-sized squares to representindividual phenotypes in a popula-tion. They can increase thenumber of either small or largesquares to model directional selec-tion, of medium-sized squares tomodel stabilizing selection, and ofboth small and large squares tomodel disruptive selection.

If your class subscribes to the iText,use it to review the Key Concepts inSection 16–2.

16–2 Section Assessment1. It can lead to changes in allele frequencies

and the evolution of traits.2. Directional selection favors one extreme; sta-

bilizing selection favors the middle of therange; disruptive selection favors bothextremes and leads to two phenotypes.

3. Genetic drift causes random changes in allelefrequencies in small populations.

4. Allele frequencies in a population remainconstant unless one or more factors causethe frequencies to change.

5. Both are types of selection on polygenic traitsin which the curve shifts away from the mid-dle. In directional selection, the curve shiftstoward one end, and in disruptive selection,toward both ends.

Suggest that students investigate therole patients play in the developmentof resistance by demanding antibi-otics and then failing to take themcorrectly. Also, have students investi-gate bacterial resistance to otheragents because of the widespreaduse of antibacterial products, rangingfrom disinfectant sprays to hand gels,soaps, and lotions.

Research and Decide1. Advantages of restricting the useof antibiotics include a reduced riskof bacteria becoming resistant toantibiotics and less danger of anincurable bacterial epidemic.Disadvantages include the likelihoodof more deaths and suffering frominfectious diseases and a possiblereduction in the food supply becauseof more infections in farm animals.2. Some students might say thatantibiotics should be restricted tohuman use or to people who haveserious infectious diseases.

Evolution of Populations 403

Should the Use of AntibioticsBe Restricted?

Antibiotic Use Should Not Be RestrictedResearchers are coming up with new drugs all thetime. These drugs can be reserved for human useonly. Doctors need to be able to prescribe antibi-otics as they choose, and our food supply dependson the use of antibiotics in agriculture. The med-ical profession and the livestock industry need thefreedom to find solutions that work best for them.

Research and Decide1. Analyzing the Viewpoints To make an

informed decision, learn more about this issueby consulting library and Internet resources.Then, list the advantages and disadvantages ofrestricting the use of antibiotics.

2. Forming Your Opinion Should antibiotics berestricted? Are there some situations in whichsuch regulations would be more appropriatethan others?

Natural selection is everywhere. One dramaticexample of evolution in action poses a serious

threat to public health. Many kinds of disease-causing bacteria are evolving resistance to antibi-otics—drugs intended to kill them or interferewith their growth.

Antibiotics are one of medicine’s greatestweapons against bacterial diseases. When antibi-otics were discovered, they were called “magic bul-lets” and “wonder drugs” because they were soeffective. They have made diseases like pneumoniamuch less of a threat than they were about sixtyyears ago. However, people may be overusingantibiotics. Doctors sometimes prescribe themfor diseases for which they are not effective.Commercial feed for chickens and other farm ani-mals is laced with antibiotics to prevent infection.

This wide use has caused many bacteria—including Mycobacterium tuberculosis, whichcauses tuberculosis—to evolve resistance to antibi-otics. This resistance is a prime example of the evo-lution of a genetically controlled physiologicaltrait. Resistance evolved because bacterial popu-lations contained a few individuals with genes thatenabled them to destroy, inactivate, or eliminateantibiotics. Descendants of those physiologicallysimilar individuals survived and reproduced, andbecame today’s resistant strains. Once-powerfulantibiotics are now useless against resistant bac-teria. Given this risk, should government agenciesrestrict the use of antibiotics?

The Viewpoints

Antibiotic Use Should Be RestrictedThe danger of an incurable bacterial epidemic is sohigh that action must be taken on a national levelas soon as possible. Doctors overuse antibiotics inhumans because patients demand them. The live-stock industry likes using antibiotics in animalfeeds and will not change their practice unlessforced to do so. For: Links from the authors

Visit: PHSchool.comWeb Code: cbe-5162

Students can research antibioticresistance on the site developed byauthors Ken Miller and Joe Levine.