378 Chapter 15

1 FOCUSObjectives15.3.1 List events leading to

Darwin’s publication of Onthe Origin of Species.

15.3.2 Describe how natural varia-tion is used in artificialselection.

15.3.3 Explain how natural selec-tion is related to species’fitness.

15.3.4 Identify evidence Darwinused to present his case forevolution.

15.3.5 State Darwin’s theory of evo-lution by natural selection.

Vocabulary PreviewHave students think of examples ofeach of the concepts in theVocabulary list. As they read the sec-tion, they should check to see if theirexamples are suitable.

Reading StrategySuggest that students write theheadings and subheadings in outlineform and fill in details under eachtopic as they read the section.

2 INSTRUCT

Publication of Onthe Origin of SpeciesBuild Science SkillsMaking Judgments WhetherDarwin delayed publishing his workbecause of possible criticism by fel-low scientists or the general public,his reluctance to publish is a goodexample of how science is influencedby its social context. Help studentsappreciate this by challenging themto think of similar examples fromtoday. Ask: What current areas ofscientific research are controver-sial, much as evolution wascontroversial in Darwin’s time?(Possible answers include cloning,genetic engineering, and researchinvolving animals or human fetal tissue.)

When Darwin returned to England in 1836, he brought back

specimens from around the world. Subsequent findings

about these specimens soon had the scientific community abuzz.

Darwin learned that his Galápagos mockingbirds actually

belonged to three separate species found nowhere else in the

world! Even more surprising, the brown birds that Darwin had

thought to be wrens, warblers, and blackbirds were all finches.

They, too, were found nowhere else. The same was true of the

Galápagos tortoises, the marine iguanas, and many plants that

Darwin had collected on the islands. Each island species looked

a great deal like a similar species on the South American main-

land. Yet, the island species were clearly different from the

mainland species and from one another.

Publication of On the Origin of SpeciesDarwin began filling notebooks with his ideas about species

diversity and the process that would later be called evolution.

However, he did not rush out to publish his thoughts. Recall

that Darwin’s ideas challenged fundamental scientific beliefs of

his day. Darwin was not only stunned by his discoveries, he was

disturbed by them. Years later, he wrote, “It was evident that

such facts as these . . . could be explained on the supposition

that species gradually became modified, and the subject

haunted me.” Although he discussed his work with friends, he

shelved his manuscript for years and told his wife to publish it

in case he died.

In 1858, Darwin received a short essay

from Alfred Russel Wallace, a fellow

naturalist who had been doing field work

in Malaysia. That essay summarized the

thoughts on evolutionary change that

Darwin had been mulling over for almost

25 years! Suddenly, Darwin had an incen-

tive to publish his own work. At a scientific

meeting later that year, Wallace’s essay

was presented together with some of

Darwin’s work.

15–3 Darwin Presents His Case

Key Concepts• How is natural variation used

in artificial selection?• How is natural selection

related to a species’ fitness?• What evidence of evolution

did Darwin present?

Vocabularyartificial selectionstruggle for existencefitnessadaptationsurvival of the fittestnatural selectiondescent with modificationcommon descenthomologous structurevestigial organ

Reading Strategy:Building Vocabulary Asyou read, write a phrase orsentence in your own words todefine each highlighted,boldface term.

� Figure 15–9 Each zebra inherits genesthat give it a distinctive pattern of stripes.Those visibly different patterns are anexample of natural variation in a species.Formulating Hypotheses What might besome genetic variations that are not visible?

Section 15–3

SECTION RESOURCES

Print:

• Laboratory Manual A, Chapter 15 Lab• Laboratory Manual B, Chapter 15 Lab• Teaching Resources, Lesson Plan 15–3,

Adapted Section Summary 15–3, AdaptedWorksheets 15–3, Section Summary 15–3,Worksheets 15–3, Section Review 15–3

• Reading and Study Workbook A,Section 15–3

• Adapted Reading and Study Workbook B,Section 15–3

• Lab Worksheets, Chapter 15 Exploration

Technology:

• iText, Section 15–3• Transparencies Plus, Section 15–3

Tim

eSaver

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Inherited Variationand ArtificialSelection

Darwin’s Theory of Evolution 379

New vegetables from old? Materials various Brassica(cabbage family) vegetables

ProcedureExamine each of the vegetablesand compare them. Determinewhich organ of the ancestralplant breeders may have chosento produce each vegetable.

Analyze and ConcludeFormulating HypothesesChoose one of the vegetables.Explain how breeders might haveproduced that variety from theancestral plant, shown below.

Eighteen months later, in 1859, Darwin published the

results of his work, On the Origin of Species. In his book, he

proposed a mechanism for evolution that he called natural

selection. He then presented evidence that evolution has been

taking place for millions of years—and continues in all living

things. Darwin’s work caused a sensation. Many people consid-

ered his arguments to be brilliant, while others strongly

opposed his message. But what did Darwin actually say?

What event motivated Darwin to publish his ideas?

Inherited Variation and Artificial SelectionOne of Darwin’s most important insights was that members of

each species vary from one another in important ways.

Observations during his travels and conversations with plant

and animal breeders convinced him that variation existed both

in nature and on farms. For example, some plants in a species

bear larger fruit than others. Some cows give more milk than

others. From breeders, Darwin learned that some of this was

heritable variation—differences that are passed from parents to

offspring. Darwin had no idea of how heredity worked. Today, we

know that heritable variation in organisms is

caused by variations in their genes. We also

know that genetic variation is found in wild

species as well as in domesticated plants and

animals.

Darwin argued that this variation mattered.

This was a revolutionary idea, because in Darwin’s

day, variations were thought to be unimportant,

minor defects. But Darwin noted that plant and

animal breeders used heritable variation—what

we now call genetic variation—to improve crops

and livestock. They would select for breeding only

the largest hogs, the fastest horses, or the cows

that produced the most milk. Darwin termed this

process In artificialselection, nature provided the variation, andhumans selected those variations that theyfound useful. Artificial selection has produced

many diverse domestic animals and crop plants,

including the plants shown in Figure 15–10, by

selectively breeding for different traits.

artificial selection.

Cauliflower

Broccoli

Kale

Ancestralspecies

Kohlrabi

Brusselssprouts

� Figure 15–10 In artificial selection, humansselect from among the naturally occurring geneticvariations in a species. From a single ancestral plant,breeders selecting for enlarged flower buds, leaf buds,leaves, or stems have produced all these plants.

Answers to . . . Darwin received

Wallace’s essay.

Figure 15–9 Answers may include:differences in speed of the animal,physical strength, resistance to disease,vision, hearing, and so on.

Comprehension: Key ConceptsBeginning Write the Key Concept sentenceabout artificial selection on the board, andread it aloud. Draw boxes around the follow-ing words: artificial selection, nature, variation,organisms, humans, and selected. Form groupsthat include beginners and—if possible—moreadvanced students who speak the beginners’native languages. Students in the groupsshould work together to ensure that all com-

prehend the meaning of the boxed words andthe Key Concept. Ask each group questions tocheck comprehension.

Intermediate To help students comprehendthe Key Concept, give them English words andphrases that mean the same as some of themore difficult words in the statement, forexample, selected/chose, organisms/livingthings, variations/differences.

SUPPORT FOR ENGLISH LANGUAGE LEARNERS

Objective Students will be able toformulate hypotheses to explainhow agricultural scientists useselective breeding to develop newvegetables. Skill Focus FormulatingHypothesesMaterials various BrassicavegetablesTime 20 minutesAdvance Prep Obtain a variety offresh Brassica vegetables, such asbroccoli, Brussels sprouts, cauli-flower, cabbage, and rutabaga.Strategies• Urge students to recall the smell

and taste of cooked Brassica veg-etables, noting their similarcabbagelike characteristics, inaddition to making observationsof the uncooked specimens.

• After students have completedthe procedure, challenge them torecall what selective breeding is.(If necessary, prompt them toreview Section 13–1.) Then,encourage them to observe theancestral species in Figure 15–10.

Expected Outcome Studentsshould identify the organ that wasdeveloped in each plant, such asflowers in cauliflower and leaves inkale.Analyze and Conclude Studentsshould hypothesize that plants inthe ancestral species must haveshown great variation, for example,some probably had larger leaves orflowers. Breeders would haveselected plants with similar traitsand crossed them to produce newvegetables. Students may also sug-gest that chemicals were used forgenerating polyploids, therebydeveloping new variants to workwith.

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380 Chapter 15

Use Community ResourcesArrange for students to visit a localzoo, wildlife preserve, or museum ofnatural history. Challenge smallgroups of students to identify adap-tive traits in 10 different animalspecies. Tell students to considerbehavioral as well as biological traits.After the visit, urge groups to sharetheir lists of adaptive traits. In thecase of traits for which the adaptivesignificance is not obvious, ask: Whydo you think the trait is adaptive?(Answers will vary depending on thespecies and trait identified. For example, students might say that anorangutan’s long arms are adaptivebecause they help the animal swingthrough trees in its arboreal habitat.)

Address MisconceptionsThe phrase “struggle for existence”may lead to the misconception thatthe struggle is a physical contest inwhich bigger or stronger individualsalways win. Challenge students tothink of situations in which smaller,weaker individuals might be winners,for example, because small size helpsthem hide from predators, locatefood, or find safe resting places.Stress that fitness involves passinginherited traits to the next generation.

Evolution by Natural SelectionDarwin’s next insight was to compare processes in nature to

artificial selection. By doing so, he developed a scientific hypothe-

sis to explain how evolution occurs. This is where Darwin made

his greatest contribution—and his strongest break with the past.

The Struggle for Existence Darwin was convinced that a

process like artificial selection worked in nature. But how? He

recalled Malthus’s work on population growth. Darwin realized

that high birth rates and a shortage of life’s basic needs would

eventually force organisms into a competition for resources. The

means that members of each species

compete regularly to obtain food, living space, and other necessi-

ties of life. In this struggle, the predators that are faster or have

a particular way of ensnaring other organisms can catch more

prey. Those prey that are faster, better camouflaged, or better

protected, such as the porcupine shown in Figure 15–11, can

avoid being caught. This struggle for existence was central to

Darwin’s theory of evolution.

Survival of the Fittest A key factor in the struggle for

existence, Darwin observed, was how well suited an organism

is to its environment. Darwin called the ability of an individual

to survive and reproduce in its specific environment

Darwin proposed that fitness is the result of adaptations. An

is any inherited characteristic that increases an

organism’s chance of survival. Successful adaptations, Darwin

concluded, enable organisms to become better suited to their

environment and thus better able to survive and reproduce.

Adaptations can be anatomical, or structural, characteristics,

such as a porcupine’s sharp quills. Adaptations also include an

organism’s physiological processes, or functions, such as the way

in which a plant performs photosynthesis. More complex features,

such as behavior in which some animals live and hunt in groups,

can also be adaptations.

adaptation

fitness.

struggle for existence

� Figure 15–11 Survival of thefittest can take many differentforms. For one species, it may be anability to run fast, whereas foranother species, it may be behav-ioral tactics that it uses to outsmartpredators. For the porcupine, sharpquills make a powerful, hungrypredator back away from an attack.Inferring What other types ofcharacteristics might increase chancesof survival?

For: Links on natural selection

Visit: www.SciLinks.orgWeb Code: cbn-5153

NSTA

I like to take my class to the gym and play “sur-vival of the fittest.” Several exercises are set upto test for different abilities, such as jumping,crawling, hearing soft sounds, and seeing colordistinctions. The “winners” are likely to haveadaptations for their specialty areas. For exam-ple, students who excel in crawling throughtight openings are likely to be smaller than

average. This activity highlights individualuniqueness and is a great springboard for dis-cussing adaptations that contribute to fitness.

—Dick JordanBiology TeacherTimberline High SchoolBoise, ID

TEACHER TO TEACHER

Evolution byNatural Selection

NSTA

Download a worksheet on naturalselection for students to complete,and find additional teacher supportfrom NSTA SciLinks.

15–3 (continued)

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Build Science SkillsApplying Concepts Divide theclass into small groups. Instruct eachgroup to brainstorm ways Earthmight change over the next 1000years. Have each group select anorganism living today and explainhow populations descended fromthat organism might evolve to adaptto the changes. The groups shoulddescribe or sketch specific adapta-tions in their organism. Invite eachgroup to share its ideas with theclass.

Address MisconceptionsStudents may hold the misconcep-tion that if a trait is favored bynatural selection, it must be adaptivein every respect. Provide an exampleto make the point that some adap-tive traits have drawbacks as well asbenefits. Show students a picture of(or have them imagine) a deer withlarge antlers. Tell students that theantlers help the deer to compete fora mate and, therefore, to reproduce.However, a deer uses energy to pro-duce the antlers and they may get inthe way when the deer tries to runthrough thick woods or feed indense brush. The reproductiveadvantage of antlers outweighs theserisks to survival.

Darwin’s Theory of Evolution 381

The concept of fitness, Darwin argued, was central to the

process of evolution by natural selection. Generation after

generation, individuals compete to survive and produce off-

spring. The baby birds in Figure 15–12, for example, compete for

food and space while in the nest. Because each individual differs

from other members of its species, each has unique advantages

and disadvantages. Individuals with characteristics that are not

well suited to their environment—that is, with low levels of

fitness—either die or leave few offspring. Individuals that are

better suited to their environment—that is, with adaptations

that enable fitness—survive and reproduce most successfully.

Darwin called this process

Because of its similarities to artificial selection, Darwin

referred to the survival of the fittest as In

both artificial selection and natural selection, only certain

individuals of a population produce new individuals. However,

in natural selection, the traits being selected—and therefore

increasing over time—contribute to an organism’s fitness in its

environment. Natural selection also takes place without human

control or direction. Over time, natural selection

results in changes in the inherited characteristics of a

population. These changes increase a species’ fitness in

its environment. Natural selection cannot be seen directly; it

can only be observed as changes in a population over many

successive generations.

What did Darwin mean when he described certainorganisms as “more fit” than others?

Descent With Modification Darwin proposed that over

long periods, natural selection produces organisms that have

different structures, establish different niches, or occupy differ-

ent habitats. As a result, species today look different from their

ancestors. Each living species has descended, with changes,

from other species over time. He referred to this principle as

descent with modification.

natural selection.

survival of the fittest.

� Figure 15–12 Each of thesebaby tanagers has its own set ofinherited traits that affect its survival.A stronger bird may take food from aweaker sibling. A faster bird mayescape predators more easily. Onlythose birds that survive and reproducehave the chance to pass their traits tothe next generation. Over time,natural selection results in changesin the inherited characteristics of apopulation.

I like to use a make-believe scenario to illustratenatural selection. I have students pretend that asmall group of humans is rocketed at “warpspeed” to Planet X, without any books,weapons, or technology. Planet X has no ozonelayer to protect its surface from cancer-causingultraviolet radiation, but native plants and ani-mals have evolved to live safely there. Below thesurface, the planet has a network of under-ground tunnels, just a meter high, that are

inhabited by carnivorous predators. I challengestudents to describe how descendants of thegroup of humans would look and act afterevolving for 100,000 years on Planet X.

—Dennis GlasgowBiology TeacherLittle Rock School DistrictLittle Rock, AR

TEACHER TO TEACHER Answers to . . . He meant organisms

that are better suited to survive in theirenvironment and pass their traits on tothe next generation.

Figure 15–11 Possible answersinclude characteristics that help theorganism evade predators, such as achameleon’s ability to change color, orhelp the organism obtain resources,such as a giraffe’s long neck.

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382 Chapter 15

Build Science SkillsApplying Concepts Find a tree oflife that includes major taxonomiccategories, including the kingdoms,phyla, and classes. Explain that theclassification is based on similaritiesin traits among living organisms. Theclassification also shows how differ-ent kinds of organisms are related toone another as shown by the “treeof life.” Have students look closely atone of the lineages in the chart, suchas the lineage that leads to thespecies Homo sapiens. Check theirinterpretation of the chart as arecord of evolution by asking: Towhich family do humans belong?(The hominid family) Which otherfamilies are most closely related tothe hominids? (The pongids, or greatapes, and the hylobatids, or lesserapes) What type of animal was thecommon ancestor of humans andapes? (A hominoid)

Evidence of EvolutionDemonstrationHalf fill some beakers with water.Place a small leaf, shell, or otherobject in the bottom of each beaker.As students observe, gradually add acouple of handfuls of a mixture ofsand and soil to each beaker. Do notstir or shake the beakers. Have stu-dents observe the beakers again atthe end of class and once a day forthe next two days. Then, ask: Whathappened to the leaves, shells, andother objects? (They became coveredwith deposits of sediment.) Point outthat this is also what happens toorganisms when they die and fallinto ponds or lakes. Explain how thepressure of water and additionaldeposits eventually turns the sedi-ments into rock. Ask: What mighteventually happen to organicremains that are covered over withsediments? (They turn into fossils.)

Descent with modification also implies that all living organ-

isms are related to one another. Look back in time, and you will

find common ancestors shared by tigers, panthers, and cheetahs.

Look farther back, and you will find ancestors that these felines

share with horses, dogs, and bats. Farther back still are the

common ancestors of mammals, birds, alligators, and fishes. If

we look far enough back, the logic concludes, we could find the

common ancestors of all living things. This is the principle

known as According to this principle, all

species—living and extinct—were derived from common ances-

tors. Therefore, a single “tree of life” links all living things.

Evidence of EvolutionWith this unified, dynamic theory of life, Darwin could finally

explain many of the observations he had made during his

travels aboard the Beagle. Darwin argued that livingthings have been evolving on Earth for millions of years.Evidence for this process could be found in the fossilrecord, the geographical distribution of living species,homologous structures of living organisms, and similari-ties in early development, or embryology.

The Fossil Record By Darwin’s time, scientists knew that

fossils were the remains of ancient life and that different layers

of rock had been formed at different times during Earth’s

history. Darwin saw fossils as a record of the history of life on

Earth. Darwin, like Lyell, proposed that Earth was many mil-

lions—rather than thousands—of years old. During this long

time, Darwin proposed, countless species had come into being,

lived for a time, and then vanished. By comparing fossils from

older rock layers with fossils from younger layers, scientists

could document the fact that life on Earth has changed over

time as shown in Figure 15–13.

common descent.

� Figure 15–13 Darwinargued that the fossil recordprovided evidence that livingthings have been evolving formillions of years. Often, the fossilrecord includes a variety of differentextinct organisms that are relatedto one another and to livingspecies. The four fossil organismsshown here are cephalopods, agroup that includes squid, octopi,and the chambered nautilus. Thefossil record contains more than7500 species of cephalopods,which vary, as these fossils show,from species with short, straightshells, to species with longer, coiledshells. Darwin and his colleaguesnoticed that the sizes, shapes, andvarieties of related organismspreserved in the fossil recordchanged over time.

Monkeys old and newUntil a few decades ago, scientists had longbelieved that Old and New World monkeysdiverged from a common prosimian ancestor atleast 50 million years ago. According to this view,the two primate groups became separated asAfrica and South America drifted apart, but theyevolved many of the same traits because of theirsimilar tropical arboreal habitats. This interpreta-tion was challenged in the 1970s by Richard

Hoffstetter, an anthropologist, who hypothesizedthat Old and New World monkeys evolved froma common monkey ancestor much more recent-ly. According to Hoffstetter, small numbers ofmonkeys accidentally rafted on fallen trees acrossthe Atlantic Ocean from Africa to South America,after the two continents had already driftedapart. Later, genetic evidence was found to sup-port Hoffstetter’s hypothesis, which is acceptedby most experts today.

HISTORY OF SCIENCE

15–3 (continued)

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Build Science SkillsUsing Analogies Explain to stu-dents that the evolution of sharedtraits in unrelated species because ofsimilar environments is called conver-gent evolution. Give students achance to see how convergent evo-lution works by using an analogysimilar to one that Darwin himselfsuggested. Have small groups of stu-dents brainstorm several ways tosolve the same problem, such asimproving traffic flow in the schoolor decreasing crowding in the class-rooms. Tell students to be creative asthey brainstorm ideas. After severalminutes, have the groups share theirideas and identify those that are sim-ilar. Ask: What do the differentgroups represent in terms of con-vergent evolution? (Unrelatedspecies) What do the similar ideasrepresent? (Similar adaptations)What does the common problemthat the groups worked on repre-sent? (Similarities in the environmentsof the unrelated species)

Darwin’s Theory of Evolution 383

N O R T HA M E R I C A

S O U T HA M E R I C A

BeaverMuskratBeaver andMuskrat

CoypuCapybaraCoypu andCapybara

Beaver

Muskrat

Coypu

Capybara

Since Darwin’s time, the number of known

fossil forms has grown enormously. Researchers

have discovered many hundreds of transitional

fossils that document various intermediate stages

in the evolution of modern species from organisms

that are now extinct. Gaps remain, of course, in

the fossil records of many species, although a lot of

them shrink each year as new fossils are discov-

ered. These gaps do not indicate weakness in the

theory of evolution itself. Rather, they point out

uncertainties in our understanding of exactly how

some species evolved.

Geographic Distribution of LivingSpecies Remember that many parts of the

biological puzzle that Darwin saw on his Beaglevoyage involved living organisms. After Darwin

discovered that those little brown birds he col-

lected in the Galápagos were all finches, he began

to wonder how they came to be similar, yet dis-

tinctly different from one another. Each species

was slightly different from every other species.

They were also slightly different from the most

similar species on the mainland of South America.

Could the island birds have changed over time, as

populations in different places adapted to different

local environments? Darwin struggled with this

question for a long time. He finally decided that all

these birds could have descended with modifica-

tion from a common mainland ancestor.

There were other parts to the living puzzle as

well. Recall that Darwin found entirely different

species of animals on the continents of South

America and Australia. Yet, when he looked at

similar environments on those continents, he

sometimes saw different animals that had similar

anatomies and behaviors. Darwin’s theory of

descent with modification made scientific sense of

this part of the puzzle as well. Species now living

on different continents, as shown in Figure 15–14,had each descended from different ancestors.

However, because some animals on each continent

were living under similar ecological conditions,

they were exposed to similar pressures of natural

selection. Because of these similar selection

pressures, different animals ended up evolving

certain striking features in common.

How can two species that look verydifferent from each other be more closely related thantwo other species that look similar to each other?

� Figure 15–14 The existence of similar butunrelated species was a puzzle to Darwin. Later, herealized that similar animals in different locationswere the product of different lines of evolutionarydescent. Here, the beaver and the capybara aresimilar species that inhabit similar environments ofNorth America and South America. The SouthAmerican coypu also shares many characteristicswith the North American muskrat. InterpretingGraphics Which animal has a larger geographicalrange, the coypu or the muskrat?

Comparing DNAInstead of comparing homologous structures orembryos, a more direct way of determining evo-lutionary relationships among species is tocompare the sequence of the four nitrogenousbases—adenine, guanine, cytosine, andthymine—in the DNA of all living things onEarth. The exact sequence of nitrogenous bases isunique to each species, but it is more similar in

closely related species than in species that are notas closely related. Scientists can use this informa-tion, along with knowledge of mutation rates, toestimate how long it has been since two speciesshared a common ancestor. Since the 1990s, sci-entists have also been able to sequence the DNAof fossils to determine how extinct organismswere related to one another and to their livingdescendants.

FACTS AND FIGURESAnswers to . . .

Regardless of theirappearance, two species are closelyrelated when they share a commonancestry. If one of those species expe-rienced natural selection in a particularkind of environment, such as a desert,it may look similar to some unrelateddesert species from a different location.

Figure 15–14 Muskrat

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384 Chapter 15

Turtle Alligator Bird Mammal

Ancient, lobe-

finned fish

Homologous, from the Greekwords homos, meaning “same,”and legein, meaning “say,”describes similar body structuresthat come from a common ances-tor. If the word morphe means“shape,” what are homomorphicstructures?

� Figure 15–15 The limbs ofthese four modern vertebrates arehomologous structures. Theyprovide evidence of a commonancestor whose bones may haveresembled those of the ancient fishshown here. Notice that the samecolors are used to show relatedstructures. Homologousstructures are one type of evi-dence for the evolution of livingthings.

Homologous Body Structures Further evidence of evolu-

tion can be found in living animals. By Darwin’s time, researchers

had noticed striking anatomical similarities among the body parts

of animals with backbones. For example, the limbs of reptiles,

birds, and mammals—arms, wings, legs, and flippers—vary

greatly in form and function. Yet, they are all constructed from the

same basic bones, as shown in Figure 15–15.Each of these limbs has adapted in ways that enable organ-

isms to survive in different environments. Despite these different

functions, however, these limb bones all develop from the same

clumps of cells in embryos. Structures that have different mature

forms but develop from the same embryonic tissues are called

(hoh-MAHL-uh-guhs) Homologous

structures provide strong evidence that all four-limbed vertebrates

have descended, with modifications, from common ancestors.

There is still more information to be gathered from homologous

structures. If we compare the front limbs, we can see that all bird

wings are more similar to one another than any of them are to bat

wings. Other bones in bird skeletons most closely resemble the

homologous bones of certain reptiles—including crocodiles and

extinct reptiles such as dinosaurs. The bones that support the wings

of bats, by contrast, are more similar to the front limbs of humans,

whales, and other mammals than they are to those of birds. These

similarities and differences help biologists group animals according

to how recently they last shared a common ancestor.

Not all homologous structures serve important functions.

The organs of many animals are so reduced in size that they are

just vestiges, or traces, of homologous organs in other species.

These may resemble miniature legs, tails, or

other structures. The legs of the skinks shown in Figure 15–16are an example of vestigial organs. Why would an organism

possess organs with little or no function? One possibility is that

the presence of a vestigial organ may not affect an organism’s

ability to survive and reproduce. In that case, natural selection

would not cause the elimination of that organ.

vestigial organs

structures.homologous

Clues to evolutionMany species of animals have vestigial organs. Anexample is the human appendix, an extension ofthe cecum at the beginning of the large intestine.Some plant-eating mammals such as koalas alsohave cecal extensions, which they require fordigestion. Humans also have a set of miniaturetailbones at the base of the spine, even thoughhumans do not have tails. In addition, in humans

the muscles that move the ears are vestigial.Vestigial organs often persist in a species indefi-nitely because they are unaffected by natural se-lection. Natural selection increases the frequencyof useful traits and decreases the frequency ofharmful traits, but it does not change the fre-quency of traits that have little or no effect on anorganism’s ability to survive and reproduce.

FACTS AND FIGURES

Build Science SkillsApplying Concepts Help studentsunderstand homologous structuresby contrasting them with analogousstructures. Tell students that homolo-gous structures are similar becausethey were inherited from a commonancestor, whereas analogous struc-tures are similar because theyevolved to fulfill the same function inunrelated species. Challenge stu-dents to apply the concepts byasking: Are bat wings and butterflywings homologous or analogousstructures? (Analogous)

Use VisualsFigure 15–15 Call students’ atten-tion to the figure. Ask: Whatsimilarities in the limbs suggestthat they developed from thesame basic structure? (The numberand placement of the bones) How isthe form of each limb adapted fora specific type of movement?(Students might say, for example, thatthe thin, light bones of a bird’s winghelp it to fly and the thick, strongbones of an alligator’s leg help itmaneuver in the water.)

Build Science SkillsClassifying Challenge students touse the information in the text tocreate a graphic organizer to classifythe following animals according totheir similarities and differences:snakes, lizards, robins, hawks, bats,and whales. (Students’ graphicsshould show that snakes are mostclosely related to lizards, robins tohawks, and bats to whales. Graphicorganizers also should show thatrobins and hawks are more closelyrelated to snakes and lizards than theyare to bats and whales.) Urge stu-dents to share their work with theclass. Then, ask: Would you expectthe vestigial limb bones of a snaketo be more like the bones of abird’s wing or a rabbit’s leg? (Abird’s wing)

Homomorphic structures are differ-ent structures that have the same, orsimilar, shapes.

15–3 (continued)

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Build Science SkillsInferring Tell students that similari-ties and differences in the DNA ofdifferent species are used to recon-struct evolutionary relationships. Ask:Why do DNA comparisons providethe most direct evidence of evolu-tionary relationships? (Because DNAis passed directly from common ances-tors to their descendants and controlsthe development of other traits)

Use VisualsFigure 15–17 Find a picture of ahuman embryo, and show it to stu-dents. Call their attention to theembryos in the figure. Ask: In whatways is the human embryo similarand in what ways is it different?(There will be slight differences, butoverall the human embryo should bevery similar to the embryos in the fig-ure.) What do the similaritiessuggest? (That humans are related tothese other animals with backbones)

Darwin’s Theory of Evolution 385

Homologies also appear in other aspects of plant and animal

anatomy and physiology. Certain groups of plants and algae, for

example, share homologous variations in stem, leaf, root, and

flower structures, and in the way they carry out photosynthesis.

Mammals share many homologies that distinguish them from

other vertebrates. Dolphins may look something like fishes, but

homologies show that they are mammals. For example, like

other mammals, they have lungs rather than gills and obtain

oxygen from air rather than water.

Similarities in Embryology The early stages, or embryos,

of many animals with backbones are very similar. This does not

mean that a human embryo is ever identical to a fish or a bird

embryo. However, as you can see in Figure 15–17, many embryos

look especially similar during early stages of development. What

do these similarities mean?

There have, in the past, been incorrect explanations for

these similarities. Also, the biologist Ernst Haeckel fudged some

of his drawings to make the earliest stages of some embryos

seem more similar than they actually are! Errors aside, how-

ever, it is clear that the same groups of embryonic cells develop

in the same order and in similar patterns to produce the tissues

and organs of all vertebrates. These common cells and tissues,

growing in similar ways, produce the homologous structures

discussed earlier.

What are homologous structures?

Figure 15–17 In their early stagesof development, chickens, turtles,and rats look similar, providingevidence that they shared a commonancestry. Inferring How could astudy of these embryos help show therelationships among animals withbackbones?

Figure 15–16 These three animals are skinks, a type of lizard. In somespecies of skinks, legs havebecome vestigial. They areso reduced that they nolonger function in walking.In humans, the appendix isan example of a vestigialorgan because it carries outno function in digestion.Inferring How mightvestigial organs provide cluesto an animal’s evolutionaryhistory?

Chicken RatTurtle

Comparative embryology explainedThe explanation for embryonic similarities inorganisms that are very different as adults lieswith the timing of gene action. All of an organ-ism’s genes are not active at the same time. Inaddition, those that are active during early devel-opment are less subject to change than thosethat are active later. This is because mutationsoccurring early in development have such far-reaching effects that they tend to be lethal.

Therefore, they are not passed on to successivegenerations. Mutations occurring later in devel-opment, on the other hand, tend to have morelimited effects, so they are less likely to be lethaland more likely to be passed on. Thus, genescontrolling later development are more subject toevolutionary change, explaining why humansand chickens are so different at later stagesdespite their similarity as embryos.

FACTS AND FIGURES

Answers to . . . Homologous structures

are structures that have differentmature forms but develop from thesame embryonic tissue.

Figure 15–16 A specific vestigialorgan suggests that an organism’sancestor once used that organ.

Figure 15–17 Embryos of less closelyrelated species would likely look differ-ent earlier in their development, whileembryos of more closely related speciesprobably look similar for longer periods.

If your class subscribes to the iText,use it to review the Key Conceptsin Section 15–3.

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386 Chapter 15

Summary ofDarwin’s TheoryBuild Science SkillsApplying Concepts Darwin’s“struggle for existence” refers to com-petition but may be confused withpredation. Explain the differencebetween the two. Describe severalcases of each, and have studentsidentify which are examples ofDarwin’s concept.

Strengths andWeaknesses ofEvolutionary TheoryMake ConnectionsHealth Science Describe antibioticresistance in bacteria. Ask students touse Darwin’s theory of natural selec-tion to explain how it comes about.

3 ASSESSEvaluate UnderstandingAsk students to make a table listingsimilarities and differences betweenartificial and natural selection and togive examples of each.

ReteachWrite the following concepts on theboard, and review their meaning: fit-ness, adaptation, natural selection,struggle for existence, survival of thefittest, and descent with modification.Then, have students write a para-graph in which they correctly useeach concept.

Answer to . . .

Figure 15–18 Based on Darwin’stheory, you could observe how thesespecies are adapted to survive andreproduce in their environments.

� Figure 15–18 Darwin’s On theOrigin of Species presented arevolutionary view of the livingworld. Many scientists agree withDarwin’s statement that “There is agrandeur in this view of life, . . .that . . . from so simple a begin-ning, endless forms so beautiful andwonderful have been and are beingevolved.” Applying ConceptsNew species are continually beingdiscovered. How could you useDarwin’s theory to learn more aboutthese new species?

Summary of Darwin’s TheoryDarwin’s theory of evolution can be summarized as follows:

• Individual organisms differ, and some of this variation is

heritable.

• Organisms produce more offspring than can survive, and

many that do survive do not reproduce.

• Because more organisms are produced than can survive, they

compete for limited resources.

• Each unique organism has different advantages and disad-

vantages in the struggle for existence. Individuals best suited

to their environment survive and reproduce most successfully.

These organisms pass their heritable traits to their offspring.

Other individuals die or leave fewer offspring. This process of

natural selection causes species to change over time.

• Species alive today are descended with modification from

ancestral species that lived in the distant past. This process,

by which diverse species evolved from common ancestors,

unites all organisms on Earth into a single tree of life.

Strengths and Weaknesses of Evolutionary TheoryScientific advances in many fields of biology, along with geology

and physics, have confirmed and expanded most of Darwin’s

hypotheses. Today, evolutionary theory offers vital insights to all

biological and biomedical sciences—from infectious-disease

research to ecology. In fact, evolution is often called the “grand

unifying theory of the life sciences.”

Like any scientific theory, evolutionary theory continues to

change as new data are gathered and new ways of thinking arise.

As you will see shortly, researchers still debate such important

questions as precisely how new species arise and why species

become extinct. There is also uncertainty about how life began.

1. Key Concept How isartificial selection dependent onvariation in nature?

2. Key Concept The theoryof evolution by natural selectionexplains, in scientific terms, howliving things evolve over time.What is being selected in thisprocess?

3. Key Concept What typesof evidence did Darwin use tosupport his theory of changeover time?

4. Critical Thinking EvaluatingUse scientific evidence to evalu-ate Darwin’s theory of evolutionby natural selection.

Newspaper ArticleWrite a newspaper articleabout the meeting in whichDarwin’s and Wallace’shypotheses of evolution werefirst presented. Explain thetheory of evolution by naturalselection for an audience whoknows nothing about the subject.

15–3 Section Assessment

15–3 (continued)

15–3 Section Assessment1. Nature provides the variation, and humans

select the variations that are useful.2. The traits that help an organism survive in a

particular environment3. The fossil record, geographic distribution of

species, homologous structures, and similari-ties in embryology

4. Scientific advances in many fields of biology,along with geology and physics, have con-firmed most of Darwin’s hypotheses. Specificexamples of evidence supporting Darwin’stheory include similarities in embryology andhomologous structures.

Students’ newspaper articles shouldexplain how heritable traits thathelp organisms survive and repro-duce tend to be passed on tosuccessive generations and increasein frequency in populations.

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