Unique IslandsOver 150 years ago, Charles Darwin visited a small cluster of islands—the GalápagosIslands—isolated in the Pacific Ocean. Darwin found no large mammals but someunusual reptiles. Some of the islands were barren, while others were mostly sparsecactus forest. Yet, what Darwin learned from these islands changed the way scientiststhought about all living things. What is so special about the Galápagos Islands?

Due to the climatic variation, the islands have four distinct zones of vegetation: drylowlands, transition zone between lowlands and highlands, moist forest region of thehighlands, and high upland zone. Each vegetation zone supports a different group ofanimal species.

The question of how these plants and animals came to the Galápagos Islands has longpuzzled scientists. At one time, scientists thought that a land bridge connected SouthAmerica and the Galápagos Islands. However, no evidence has been found to supportthat idea, and scientists question why some species would have crossed the land bridge,whereas others did not.

Today, scientists think that the plants and animals were carried to the islands onnatural rafts of plant material and logs. Such rafts regularly float down South Americanrivers during heavy storms. Carried by the Humboldt Current, the rafts could havefloated across the ocean and washed up on the shores of the Galápagos. These rafts,possibly as large as football fields, might have carried plants, seeds, insects, and animals.If the organisms found a way to survive and reproduce, they could have establishedpopulations on the islands. Of course, many of the organisms that arrived on the islandswould not have survived to establish populations. After all, the Galápagos are not a veryhospitable place, particularly for mammals. Scientists think that a successfulcolonization by a reptile or mammal species takes place only once every 100,000 years.However, over time, enough organisms would have survived to populate the Galápagos.

The first plants and animals to colonize the Galápagos were not unique to theseislands, because they had merely floated from the mainland of Central or SouthAmerica. However, over time, these species changed. From one species of bird evolvedmany different species of birds. Different kinds of land tortoises evolved as well, eachwith a uniquely shaped shell. Of course, when Darwin arrived on the Galápagos, no oneunderstood the origin of these different species. How such a variety of species had cometo exist on a tiny group of isolated islands was a puzzling question—a question Darwineventually answered with his theory of evolution.

Today, many of the Galápagos Islands are protected as biological reserves. Scientistsare currently conducting research on the plants and animals and trying to find ways toprotect them. Thanks to their dry, inhospitable climate, some of the islands remainuntouched by human development. Essentially, they are the same Galápagos Islandsthat Darwin saw on his significant voyage so long ago.

Evaluation Answer the following questions on a separate sheet of paper.

1. The Galapagos Islands are home to a species of penguin called the Galápagospenguin. Penguins are usually associated with cold climates. How might penguinsbe able to survive at the equator?

2. The Galápagos Islands support a variety of animal species. Darwin’s finches, inparticular, are a diverse and highly specialized group—the beak of each finchspecies is particularly adapted to eating a certain food. How do you think the tinyGalápagos Islands can support this variety of animal species?

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Chapter 15 Darwin’s Theory of Evolution Enrichment

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Chapter 15 Darwin’s Theory of Evolution

Comparing Adaptations of Birds

IntroductionWhen Charles Darwin explored the Galápagos Islands, he noted thegreat variety of beak shapes on the finches there. It was laterdetermined that Darwin’s finches made up 13 separate species. Thesimilarities among the species suggested a common ancestor: A singlespecies of finch that came from the mainland of South America. Howdid these birds evolve into genetically unique groups? With few nativecompetitors and a wide variety of food sources, the newcomers wereable to establish niches based on variations in beak shape and size.Finches with beaks that could exploit a particular food source—insectsin the bark of trees, for example—established their own niche in thenew habitat. Natural selection thus favored beak specializationbecause it enabled many birds to coexist within the same ecosystem.Many birds also show specialization of other body parts, includinglegs, feet, wings, and eyes. In this investigation, you will examinesome bird characteristics. What are the survival benefits of each bird’sadaptations?

ProblemWhat adaptations have evolved among modern birds that enable themto survive in diverse habitats?

Pre-Lab DiscussionRead the entire investigation. Then, work with a partner to answer the following questions.

1. What can you learn from observing and comparing specific physicaltraits of different birds?The physical traits constitute evidence that can be used to make inferences about the behaviors and habitats of

the birds.

2. What are some terms you can use to describe the size and shape of abird’s beak?Size can be described using terms such as long, short, large, or small. Shape can be described using

straight, curved, hooked, pointed, flat, narrow, or broad.

3. How does a bird’s beak help you identify its habitat?The size and shape indicate the types of foods the bird eats. The bird must live, or at least hunt, in a

habitat that includes those foods.

4. A certain bird ordinarily lives in or near water and spends much of itstime swimming. Based on Data Table 1, what type of feet would youexpect this bird to have?Webbed and paddlelike feet that enable the bird to swim well.

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You may want to refer students toSection 15–3 in the textbook for adiscussion of adaptations.Time required: 50 minutes

5. Birds that spend much time perching have three front toes and oneback toe. Based on Data Table 1, how is this different from the feetof birds that are better adapted for climbing than for perching?The feet of climbing birds have two front toes and two back toes. The back toes help keep the bird from falling

when climbing upward.

Materials (per group)

specimens or illustrations of birds detailing their physical adaptations,field guides to birds

Procedure1. Look carefully at the diagrams of birds in Figure 1. Notice the

details of each bird’s feet and beak. Using the diagrams, completeData Table 1 by filling in one or more examples of birds that exhibiteach of the beak and foot adaptations described.

2. Inferring In Data Table 2 on page 134 consider the adaptations ofthe birds listed and then describe their likely habitat. Check youranswers using a field guide.

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It might be helpfulto have severaldifferent types offield guides forstudents to use asreferences if theyare having difficultywith the lab or wishto check theirresponses.Mountedspecimens may beavailable on loanfrom local naturecenters, museums,or colleges.

If time permits,suggest thatstudents use fieldguides or othersources to comparewing length andshape, and havestudents discussthe adaptive benefitof these features.

Common snipe

Cardinal

Osprey

Woodpecker

Purple martin

Great horned owl

Figure 1 (Diagrams are drawn in approximate, not exact, scale.)

Great blue heron

Mallard duck

PelicanOstrich

Hummingbird

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Figure 1 continued

Data Table 1

Body structure Adaptation for Type of bird

Short, wide, pointed beak cracking seeds and nuts cardinal

Medium-length, chisel-shaped beak breaking into tree bark for insects woodpecker

Long spear-shaped beak with wide base spearing fish heron

Sharply hooked beak tearing animal tissue osprey/owl

Beak with pouch holding fish pelican

Long, very narrow beak probing flowers for nectar hummingbird

Long, straight, medium-wide beak probing for soft-ground food common snipe

Very short, slightly curved beak catching insects in midair purple martin

Large, forward-looking eyes binocular and nightvision for spotting prey owl

Long, thick, powerful legs and two thick toes running; defense (kicking) ostrich

2 front toes/2 back toes holding onto side of treewithout falling back woodpecker

3 short front toes/1 back toe without claws or webbing perching on branches cardinal/purple martin

Long foot with 3 front toes/ wading; walking on mud;1 back toe walking on sand heron/common snipe

3 front webbed toes/1 back toe swimming mallard/pelican

3 front clawed toes/ catching, grasping, and1 back clawed toe carrying prey over distances osprey/owl

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Make sure students understand thata particular bird may be listed more

than once in Data Table 1.

Data Table 2

Bird Habitat

Osprey shorelines

Great horned owl coniferous woods near open fields

Woodpecker forests; parks; suburban areas

Purple martin fields near streams/rivers

Great blue heron marshes; shores of lakes, ponds, and rivers

Common snipe bogs; marshes; shorelines

Mallard duck creeks; rivers; ponds; lakes

Pelican shorelines; estuaries

Cardinal wooded and suburban areas

Hummingbird wherever appropriate plants are found, including suburban areas

Ostrich savanna; grassy plain

Analysis and Conclusions

1. Comparing and Contrasting Each pair of birds mentioned belowshares some characteristics. Yet they each have their own niche; theydo not compete directly for the same resources. For each pair,describe how the birds are similar and how they are different. Howmight their differences enable them to occupy different niches?

a. osprey and great horned owlBoth have similar beaks and claws adapted for hunting; the owl has large eyes that enable it to hunt at night.

b. woodpecker and purple martinBoth eat insects. The woodpecker has a longer beak that can probe. The purple martin’s beak helps it

catch and trap insects as it flies.

c. great blue heron and common snipeBoth have feet adapted for wading; the heron has longer legs, enabling it to wade more deeply.

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justify with evidence and logic.

2. Inferring Birds have hollow bones. Explain how this is adaptive. It is a structural adaptation that lessens their weight and enables them to fly.

3. Observing The hummingbird’s beak is adapted to getting its food.Describe another adaptation of the hummingbird that enables it toget the food it needs. Its tiny, lightweight body and its ability to beat its wings rapidly for hovering over flowers assist the

hummingbird in obtaining nectar.

4. Inferring Birds such as Darwin’s finches are adapted to occupy highlyspecific niches. Would this adaptation make it easy or difficult for suchbirds to adapt to environmental change? Explain your answer.It would be difficult for them to adapt because they are so specialized. For example, they might not be able

to adjust to an environmental change such as loss of a food source.

5. Formulating Hypotheses Why might a particular bird be found in morethan one habitat?Birds might hunt or feed in one habitat but nest or seek shelter in another.

6. Inferring Birds are well-adapted to their habitats, yet some birdsare considered members of endangered species. Infer why certainbirds are endangered.Students may infer that certain birds are now endangered because their habitats are being destroyed.

Going FurtherBased on the results of this investigation, develop a hypothesis aboutthe physical characteristics and habitat requirements of birds that livein hot, dry climates such as the desert. When developing a hypothesis,consider sources of food, water, shelter, nesting materials, and otherfactors. To test your hypothesis, with your teacher’s permission, usevarious resources to learn about the physical characteristics and thehabitat of one or more desert-dwelling birds.

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Teaching Resources /Chapter 15 191

Concept Map

Using information from the chapter, complete the concept map below. If there isnot enough room in the concept map to write your answers, write them on aseparate sheet of paper.

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Chapter 15 Darwin’s Theory of Evolution Graphic Organizer©

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Darwin’s Theory of Evolution

In nature, organisms differ from one another.

Some of this variation is inherited.

1.

2.

3.

causes

includes

causes

leads to

resultsin

includes

Members of the samespecies must competefor limited resources.

Species change over time.New species arise, and other species disappear.

Matching On the line provided, write the letter of the definition that bestmatches each term on the left.

1. evolution

2. fossil

3. natural variation

4. struggle for existence

5. fitness

6. adaptation

7. natural selection

8. common descent

9. homologous structures

10. vestigial organ

Defining Terms On the lines provided, write a definition for each of thefollowing terms.

11. theory

12. artificial selection

13. survival of the fittest

14. descent with modification

Multiple Choice On the line provided, write the letter of the answer that bestcompletes the sentence.

15. Cows that give more milk than other cows are anexample of

a. natural variation. c. survival of the fittest.

b. natural selection. d. struggle for existence.

16. The practice of breeding dogs to produce offspringwith specific traits is an example of

a. natural variation. c. common descent.

b. natural selection. d. artificial selection.

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a. change over time

b. differences among individuals within a species

c. preserved remains of an ancient organism

d. survival of the fittest

e. all species are derived from common ancestors

f. structures that develop from the same embryonictissues, but have different mature forms

g. ability of an individual to survive and reproducein a specific environment

h. organ with little or no function

i. competition for food, space, and other resourcesamong members of a species

j. inherited characteristic that increases anorganism’s chance of survival

17. Natural selection is the same as

a. struggle for existence. c. artificial selection.

b. survival of the fittest. d. descent with modification.

18. The diagrams of the limbs below show evidence of

a. natural variation. c. common descent.

b. natural selection. d. artificial selection.

19. The bones in the diagram above are examples of

a. homologous structures. c. adaptation.

b. fitness. d. struggle for existence.

20. A human’s appendix and a skink’s legs are examples of

a. vestigial organs. c. natural selection.

b. fitness. d. artificial selection.

Short Answer On the lines provided, answer the following questions.

21. How did Darwin interpret the fossil record?

22. What conditions force organisms to compete in a struggle for existence?

23. How do successful adaptations improve an organism’s fitness?

24. What does descent with modification imply about the relationships among the different species on Earth today?

Teaching Resources /Chapter 15 189

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Turtle Alligator Bird Mammals