Organisms tend to be well suited to where they live and what theydo. Figure 7 shows a chameleon (kuh MEEL ee uhn) capturing aninsect. Insects are not easy to catch, so how does the chameleon doit? Chameleons can change the color and pattern of their skin, andthen blend into their backgrounds. Their eyes are raised on little,mobile turrets that enable the lizards to look around without mov-ing. An insect is unlikely to notice such an animal sitting motion-less on a branch. When the insect moves within range, thechameleon shoots out an amazingly long tongue to grab the insect,while the chameleon’s big hind feet hold it securely to the branch.

Evolution by Natural SelectionHow do organisms become so well suited to their environments?In 1859, English naturalist Charles Darwin proposed an answer.Darwin observed that organisms in a population differ slightly fromeach other in form, function, and behavior. Some of these differencesare hereditary (huh RED i TER ee)—that is, passed from parent tooffspring. Darwin proposed that the environment exerts a stronginfluence over which individuals survive to produce offspring. Someindividuals, because of certain traits, are more likely to survive andreproduce than other individuals. Darwin used the term

to describe the unequal survival and reproduction thatresults from the presence or absence of particular traits.

Darwin proposed that over many generations natural selectioncauses the characteristics of populations to change. A change inthe genetic characteristics of a population from one generation tothe next is known as evolution.

selectionnatural

Objectives� Explain the process of evolution by

natural selection.� Explain the concept of adaptation.� Describe the steps by which a

population of insects becomesresistant to a pesticide.

Key Termsnatural selectionevolutionadaptationartificial selectionresistance

S E C T I O N 2

Evolution

GeologyConnection to

Darwin and Fossils In the1800s, fossil hunting was a popu-lar hobby. The many fossils thatpeople found started argumentsabout where fossils came from.Darwin’s theory of evolution pro-posed that fossils are the remainsof extinct species from whichmodern species evolved. When hisbook on the theory of evolutionwas first published in 1859, itbecame an immediate bestseller.

Figure 7 � A chameleon catches anunsuspecting insect that has strayedwithin range of the lizard’s long, fast-moving tongue.

Section 2 Evolution 97

www.scilinks.orgTopic: EvolutionSciLinks code: HE4039

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Nature Selects Darwin thought that nature selects for certaintraits, such as sharper claws or lighter feathers, because organismswith these traits are more likely to survive and reproduce. Overtime, the population includes a greater and greater proportion oforganisms with the beneficial trait. As the populations of a givenspecies change, so does the species. Table 1 summarizes Darwin’s

98 Chapter 4 The Organization of Life

Evolution by Natural Selection

1. Organisms produce more In nature, organisms have the ability to produce more offspring than can offspring than can survive. survive to become adults.

2. The environment is hostile and The environment contains things and situations that kill organisms, and contains limited resources. the resources needed to live, such as food and water, are limited.

3. Organisms differ in the traits The organisms in a population may differ in size, coloration, resistance to they have. disease, and so on. Much of this variation is inherited.

4. Some inherited traits provide Some inherited traits give organisms an advantage in coping with organisms with an advantage. environmental challenges. These organisms are more likely to survive

longer and produce more offspring; they are “naturally selected for.”

5. Each generation contains Because organisms with more advantageous traits have more offspring, proportionately more organisms each generation contains a greater proportion of offspring with these with advantageous traits. traits than the previous generation did.

Table 1 �

Darwin’s Finches

Before Charles Darwin formulatedhis theory of evolution, he sailedaround the coast of South America.The plants and animals he saw hada great effect on his thinking abouthow modern organisms had origi-nated. He was surprised by theorganisms he saw on islandsbecause they were often unusualspecies found nowhere else.

He was particularly impressedby the organisms in the GalápagosIslands, an isolated group of vol-canic islands in the Pacific Oceanwest of Ecuador. The islands contain13 unique species of birds, whichhave become known as Darwin’sfinches. All the species look gener-

Princeton University biologistsPeter and Rosemary Grant havespent 25 years studying Darwin’sfinches on Daphne Major, one ofthe Galápagos Islands. Here, one

ally similar, but each species has aspecialized bill adapted to eating adifferent type of food. Some specieshave large, parrotlike bills adaptedto cracking big seeds, some specieshave slim bills that are used to sipnectar from flowers, and somespecies have even become insecteaters. Darwin speculated that allthe Galápagos finches had evolvedfrom a single species of seed-eatingfinch that found its way to theislands from the South Americanmainland. Populations of thefinches became established on thevarious islands, and those finchesthat survived were able to eat whatthey found on their island.

� Notice the beaks in the twospecies of Darwin’s finches. Whatdo you think the finches eat?

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theory of evolution by natural selection. An example of evolution isshown in Figure 8, in which a population of deer became isolated ina cold area. Many died, but some had genes for thicker, warmerfur. These deer were more likely to survive, and their young withthick fur were also more likely to survive to reproduce. The deer’sthick fur is an an inherited trait that increases an organ-ism’s chance of survival and reproduction in a certain environment.

adaptation,

Section 2 Evolution 99

These deer live in a warm climate. Some have thicker fur than others.

Some deer become separated from the rest of the group.

In the cold mountain climate, deer with thicker fur are more likely to survive.

As years pass, eachgeneration has agreater proportionof deer with thick fur. After many generations, most deer have thick fur.

Figure 8 � These steps show theevolution of thicker fur in a popula-tion of deer.

species, the medium ground finch,has a short, stubby beak and eatsseeds as well as a few insects. Theground finches have few predators.The Grants found that the main fac-tor that determined whether a finchlived or died was how much foodwas available. During a longdrought in 1977, many plants diedand the small seeds that the finches

These weather patterns pro-duce alternating periods of verywet and dry weather in a relativelyshort period of time. The weatherdetermines which plants live andwhich plants die. Then, this effectexerts selective pressure on the ani-mals that depend on particularplants for food or for places toreproduce.

CRITICAL THINKING

1. Making Inferences What is theshortest period in which a popula-tion of Darwin’s finches can evolve?

2. Analyzing RelationshipsWould you expect that the finchesthat evolved bigger beaks in thisstudy might one day evolvesmaller beaks?

eat became scarce. Only finchesthat had large beaks survived. Largebeaks allowed them to eat largerseeds from the larger plants thathad survived the drought.

The finches that survived the drought passed their genes for large beaks to their offspring. Two years later, the Grants found that the beaks of medium ground finches on Daphne Majorwere nearly 4 percent larger, onaverage, than they were before the drought. The Grants hadobserved evolution occurring inbirds over a short period of time,something that had seldom beenseen before.

The Galápagos Islands are wellsuited for research on evolutionbecause the islands are stronglyinfluenced by El Niño and La Niñaweather patterns.

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Coevolution Organisms evolve adaptations to other organisms aswell as to their physical environment. The process of two

species evolving in response to long-term interactions witheach other is called coevolution (koh EV uh LOO shuhn).One example is shown in Figure 9. The honeycreeper’sbeak is long and curved, which lets it reach the nectar atthe base of the long, curved flower. The flower hasevolved structures that ensure that the bird gets pollen

from the flower on its head as it sips nectar. When thebird moves to another flower, some of the pollen will rub

off. In this way, the bird helps lobelia plants reproduce. Thehoneycreeper’s adaptation is a long, curved beak. The plant has

two adaptations. One is sweet nectar, which attracts the birds.The other is a flower structure that forces pollen onto a bird’shead when the bird sips nectar.

Evolution by Artificial SelectionMany populations of plants and animals do not live in the wildbut instead are cared for by humans. People control how theseplants and animals reproduce and therefore how they evolve. Thewolf and the Chihuahua in Figure 10 are closely related species.Over thousands of years, humans bred the ancestors of today’swolves to produce the variety of dog breeds we now have. Theselective breeding of organisms by humans for specific character-istics is called

The fruits, grains, and vegetables we eat were also producedby artificial selection. Humans saved the seeds from the largest,sweetest fruits and most nutritious grains. By selecting for thesetraits, farmers directed the evolution of crop plants. As a result,crops produce fruits, grains, and roots that are larger, sweeter,and often more nutritious than their wild relatives do. NativeAmericans cultivated the ancestor of today’s corn from a grasslikeplant in the mountains of Mexico. Modern corn is very differentfrom the wild plant that was its ancestor.

artificial selection.

100 Chapter 4 The Organization of Life

MATHPRACTICEPlumper PumpkinsEach year a farmer saves and plants only the seeds from his largest pumpkins. If he startswith pumpkins that average 5 kgand each year grows pumpkins 3percent more massive, on average,than those he grew the year before,what will be the average mass ofhis pumpkins after 10 years?

Figure 10 � As a result of artificialselection, the Chihuahua on the rightlooks very different from its wolfancestor on the left.

Figure 9 � This Hawaiian honey-creeper is using its curved beak to sipnectar from a lobelia flower.

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Evolution of ResistanceSometimes humans cause populations of organisms to evolveunwanted adaptations. You may have heard about insect peststhat are resistant to pesticides and about bacteria that are resist-ant to antibiotics. What is resistance, and what does it have todo with evolution?

is the ability of one or more organisms to toleratea particular chemical designed to kill it. An organism may beresistant to a chemical when it contains a gene that allows it tobreak the chemical down into harmless substances. By trying tocontrol pests and bacteria with chemicals, humans promote theevolution of resistant populations.

Pesticide Resistance Consider the evolution of pesticideresistance among corn pests, as shown in Figure 11. A pesticideis sprayed on corn to kill grasshoppers. Most of the grasshop-pers die, but a few survive. The survivors happen to have agene that protects them from the pesticide. The survivinginsects pass on the gene to their offspring. Each time the corn issprayed, the insect population changes to include more andmore resistant members. After many sprayings, the entire popu-lation may be resistant, making the pesticide useless. The fasteran organism reproduces, the faster its populations can evolve.

Resistance

Section 2 Evolution 101

1. Explain what an adaptation is, and provide threeexamples.

2. Explain the process of evolution by natural selection.

3. Describe one way in which artificial selection canbenefit humans.

4. Explain how a population of insects could becomeresistant to a pesticide.

CRITICAL THINKING5. Understanding Concepts Read the description of

evolution by natural selection in this section anddescribe the role that the environment plays in thetheory.

6. Identifying Relationships A population of rabbitsevolves thicker fur in response to a colder climate. Isthis an example of coevolution? Explain your answer.

READING SKILLS

S E C T I O N 2 Review

FIELD ACTIVITY FIELD ACTIVITY Artificial Selection Look aroundyour school grounds and the areaaround your home for possibleexamples of artificial selection.Observe and report on any exam-ples you can find.

Dogs are one example of artifi-cial selction mentioned in thischapter, but you will probablyfind many more plant examples.Record your observations in your EcoLog.

Figure 11 � The evolution of resist-ance to a pesticide starts when thepesticide is sprayed on the corn.Most of the insects are killed, but afew resistant ones survive. After eachspraying, the insect population con-tains a larger proportion of resistantorganisms.

The survivorspass the trait forinsecticide resistanceto their offspring.

When the same insecticide is usedagain, more insects survive becausemore of them are resistant.

Insect pests are sprayed withan insecticide. Only a few resistant insects survive.

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