Benchmarks—SC.F.2.3.3 (pp. 495–496, 500): SC.G.1.3.2 (pp ... · Ideas About Evolution Figure 1 By studying fossils, scientists have traced the hypothesized evolution of the camel

492 CHAPTER 17 Adaptations over Time

Early Models of Evolution Millions of species of plants, animals, and other organisms

live on Earth today. Do you suppose they are exactly the same asthey were when they first appeared—or have any of themchanged? A species is a group of organisms that share similarcharacteristics and can reproduce among themselves to producefertile offspring. Many characteristics of a species are inheritedwhen they pass from parent to offspring. Change in these inher-ited characteristics over time is evolution. Figure 1 shows howthe characteristics of the camel have changed over time.

■ Describe Lamarck’s hypothesis of acquired characteristics and Darwin’s theory of natural selection.

■ Identify why variations in organ-isms are important.

■ Compare and contrast gradual-ism and punctuated equilibrium.

The theory of evolution suggestswhy there are so many different living things.

Ideas About Evolution

Figure 1 By studying fossils,scientists have traced thehypothesized evolution of thecamel. Discuss the changes you observein camels over time.

Protylopus56 mya

Poebrotherium35 mya

Procamelus23 mya

Small hump

CamelusPresent day

Benchmarks—SC.F.2.3.3 (pp. 495–496, 500): The student knows that generally organisms in a populationlive long enough to reproduce because they have survival characteristics; SC.G.1.3.2 (pp. 495–496): thatbiological adaptations include … that enhance reproductive success …; SC.H.1.3.2 (p. 494): Study …that led scientists to discoveries can provide information about the inquiry process and its effects.

Also covers: SC.F.2.3.2 Annually Assessed (pp. 492, 496), SC.F.2.3.4 (p. 491), SC.H.1.3.1 AnnuallyAssessed (pp. 494–495, 498), SC.H.1.3.6 (p. 493), SC.H.1.3.7 Annually Assessed (p. 493), SC.H.2.3.1 (pp. 496–497), SC.H.3.3.3 (p. 493), SC.H.3.3.5 (p. 493)

Review Vocabularygene: a section of DNA that

contains instructions for makingspecific proteins

New Vocabulary• species

• evolution

• natural selection

• variationadaptation

• gradualism

• punctuated equilibrium

FCAT Vocabulary

SECTION 1 Ideas About Evolution 493

Hypothesis of Acquired Characteristics In 1809, JeanBaptiste de Lamarck proposed a hypothesis to explain howspecies change over time. He suggested that characteristics, ortraits, developed during a parent organism’s lifetime are inher-ited by its offspring. His hypothesis is called the inheritance ofacquired characteristics. Scientists collected data on traits thatare passed from parents to offspring. The data showed that traitsacquired during a parent’s lifetime, such as large muscles fromhard work or exercise or good manners, are not passed on to off-spring. The evidence did not support Lamarck’s hypothesis.

What was Lamarck’s explanation of evolution?

Darwin’s Model of EvolutionIn December 1831, the HMS Beagle sailed from England on

a journey to explore the South American coast. On board was ayoung naturalist named Charles Darwin. During the journey,Darwin recorded observations about the plants and animals hesaw. He was amazed by the variety of life on the GalápagosIslands, which are about 1,000 km from the coast of Ecuador.Darwin hypothesized that the plants and animals on theGalápagos Islands originally must have come from Central andSouth America. But the islands were home to many species hehad not seen in South America, including giant cactus trees,huge land tortoises, and the iguana shown in Figure 2.

Figure 2 This map shows theroute of Darwin’s voyage on the HMS Beagle. Darwin noticed manyspecies on the Galápagos Islandsthat he had not seen along thecoast of South America, includingthe marine iguana. This species isthe only lizard in the world knownto enter the ocean and feed onseaweed.

Azores

Canary Is.Cape Verde Is.

AscensionSt. Helena

MauritiusRio de Janeiro

Bahia

Montevideo

Falkland Is.

Galápagos Is.

ValparaisoTahiti

Sydney

HobartKing George I.

Cocos Is.

Tierra del Fuego

New Zealand

GalápagosIslands

CulpepperWenman

Pinta

Marchena Equator

Isabela

Fernandina San Salvador

Santa CruzBaltra

San CristóbalSanta

FéSanta Maria

Española

Genovesa

Barbera Cushman/DRK Photo


Darwin’s Observations Darwin observed 13 species offinches on the Galápagos Islands. He noticed that all 13 specieswere similar, except for differences in body size, eating habits, andbeak shape, as shown in Figure 3. He also noticed that all theGalápagos finch species were similar to one finch species he hadseen on the South American coast.

Darwin reasoned that the Galápagos finches must have hadto compete for food. Finches with beak shapes that allowedthem to eat available food survived longer and produced moreoffspring than finches without those beak shapes. After manygenerations, these groups of finches became separate species.

How did Darwin explain the evolution of the dif-ferent species of Galápagos finches?

Natural SelectionAfter the voyage, Charles Darwin returned to England and

continued to think about his observations. He collected moreevidence on inherited traits by breeding racing pigeons. He alsostudied breeds of dogs and varieties of flowers. In the mid 1800s,Darwin developed a theory of evolution that is accepted by mostscientists today. He described his ideas in a book called On theOrigin of Species, which was published in 1859.

Figure 3 Darwin observed that the beak shape of eachspecies of Galápagos finch is related to its eating habits.

Finches with medium-sized beakseat a variety of foods includingseeds and insects.

Finches that feed on insects havelong, slender beaks for probingbeneath tree bark.

Finches that eat nuts and seedshave short, strong beaks for breaking hard shells.

Topic: Darwin’s FinchesVisit fl8.msscience.com for Weblinks to information about thefinches Darwin observed.

Activity In your Science Journal,describe the similarities and dif-ferences of any two species ofGalápagos finches.

LA.B.2.3.4

LA.A.1.3.4

(l c)Tui De Roy/Minden Pictures, (r)Tim Davis/Photo Researchers


Darwin’s Theory Darwin’s observations led manyother scientists to conduct experiments on inheritedcharacteristics. After many years, Darwin’s ideas becameknown as the theory of evolution by natural selection.Natural selection means that organisms with traits bestsuited to their environment are more likely to surviveand reproduce. Their traits are passed to more offspring.All living organisms produce more offspring than sur-vive. Galápagos finches lay several eggs every few months.Darwin realized that in just a few years, several pairs offinches could produce a large population. A population isall of the individuals of a species living in the same area.Members of a large population compete for living space,food, and other resources. Those that are best able to sur-vive are more likely to reproduce and pass on their traitsto the next generation.

The principles that describe how natural selectionworks are listed in Table 1. Over time, as new data wasgathered and reported, changes were made to Darwin’soriginal ideas about evolution by natural selection. Histheory remains one of the most important ideas in thestudy of life science.

Alejandro raises tropical fish as a hobby.Could the observations that he makes

over several weeks illustrate the principles of natural selection?

Identifying the ProblemAlejandro keeps a detailed journal of his

observations, some of which are given in thetable to the right.

Solving the ProblemRefer to Table 1 and match each of

Alejandro’s journal entries with the princi-ple(s) it demonstrates. Here’s a hint: Someentries may not match any of the principles ofnatural selection. Some entries may matchmore than one principle.

Does natural selection take place in a fish tank?

Fish Tank ObservationsDate Observation

June 6 6 fish are placed in aquarium tank.

July 22 16 new young appear.

July 24 3 young have short or missing tail fins. 13 young have normal tail fins.

July 28 Young with short or missing tail fins die.

August 1 2 normal fish die—from overcrowding?

August 12 30 new young appear.

August 15 5 young have short or missing tail fins. 25 young have normal tail fins.

August 18 Young with short or missing tail fins die.

August 20 Tank is overcrowded. Fish are divided equally into two tanks.

Table 1 The Principles of Natural Selection

1. Organisms produce more offspring than can survive.

2. Differences, or variations, occur among individuals of a species.

3. Some variations are passed to offspring.

4. Some variations are helpful. Individuals with helpful variations survive and reproduce better than those without these variations.

5. Over time, the offspring of individuals with helpful variations make up more of a population and eventually may become a separate species.


Variation and AdaptationDarwin’s theory of evolution by natural selection empha-

sizes the differences among individuals of a species. These dif-ferences are called variations. A variation is an inherited traitthat makes an individual different from other members of itsspecies. Variations result from permanent changes, or muta-tions, in an organism’s genes. Some gene changes produce smallvariations, such as differences in the shape of human hairlines.Other gene changes produce large variations, such as an albinosquirrel in a population of gray squirrels or fruit without seeds.Over time, more and more individuals of the species mightinherit these variations. If individuals with these variations con-tinue to survive and reproduce over many generations, a newspecies can evolve. It might take hundreds, thousands, or mil-lions of generations for a new species to evolve.

Some variations are more helpful than others. An adaptationis any variation that makes an organism better suited to its envi-ronment. The variations that result in an adaptation can involvean organism’s color, shape, behavior, or chemical makeup.Camouflage (KA muh flahj) is an adaptation. A camouflagedorganism, like the one shown in Figure 4, blends into its environ-ment and is more likely to survive and reproduce.Figure 4 Variations that

provide an advantage tend toincrease in a population overtime. Variations that result ina disadvantage tend todecrease in a population over time.

Albinism can prevent an organism from blending into its environment.Infer what might happen to an albino lemur in its natural environment.

Camouflage allows organisms to blend into their environments. Infer how its coloration gives this scorpion fish a survival advantage.

Evolution of EnglishIf someone fromShakespeare’s time wereto speak to you today,you probably would not understand her.Languages, like species,change over time. In yourScience Journal, discusssome words or phrasesthat you use differentlyfrom how your parents or teachers use them.

(l)Gregory G. Dimijian, M.D./Photo Researchers, (r)Patti Murray/Animals Animals


Changes in the Sources of Genes Over time, the geneticmakeup of a species might change its appearance. For example,as the genetic makeup of a species of seed-eating Galápagosfinch changed, so did the size and shape of its beak. Many kindsof environmental factors help bring about changes. When indi-viduals of the same species move into or out of an area, theymight bring in or remove genes and variations. Suppose a fam-ily from another country moves to your neighborhood. Theymight bring different foods, customs, and ways of speaking withthem. In a similar way, when new individuals enter an existingpopulation, they can bring in different genes and variations.

Geographic Isolation Sometimes mountains, lakes, orother geologic features isolate a small number of individualsfrom the rest of a population. Over several generations, varia-tions that do not exist in the larger population might begin to bemore common in the isolated population. Also, gene mutationscan occur that add variations to populations. Over time, the twopopulations can become so different that they no longer canbreed with each other. The Key deer, shown in Figure 5, evolvedbecause of geographic isolation about 4,000–10,000 years ago.

Figure 5 During the last ice age, Virginiawhite-tailed deer moved south ahead of anadvancing ice sheet. When ice sheets meltedworldwide about 4,000–10,000 years ago,ocean levels rose. Some deer were isolated on achain of islands and evolved into a new sub-species, the Key deer. Key deer are found onlyon approximately 30 islands in the subtropicallower keys.

Key deer can be 24 inches to 28 inches tall atthe shoulder.Infer why the Key deer is smaller than theVirginia white-tailed deer.

Virginia white-tailed deer can be 36 inchesto 42 inches tall at the shoulder.

Modeling EvolutionProcedure1. On a piece of paper, print

the word train.2. Add, subtract, or change

one letter to make a newword.

3. Repeat Step 2 with thenew word.

4. Repeat Step 2 and Step 3two more times.

5. Make a “family tree” thatshows how your first wordchanged over time.

AnalysisHow is this process similar to evolution by natural selection?

SC.H.2.3.1

(l)George McCarthy/CORBIS, (r)Arthur Morris/Visuals Unlimited


The Speed of EvolutionScientists do not agree on how quickly evolution occurs.

Many scientists hypothesize that evolution occurs slowly, per-haps over tens or hundreds of millions of years. Other scientistshypothesize that evolution can occur quickly. Most scientistsagree that evidence supports both of these models.

Gradualism Darwin hypothesized that evolution takes placeslowly. The model that describes evolution as a slow, ongoingprocess by which one species changes to a new species is knownas gradualism. According to the gradualism model, a continu-ing series of mutations and variations over time will result in anew species. Look back at Figure 1, which shows the evolutionof the camel over tens of millions of years. Fossil evidence showsa series of intermediate forms that indicate a gradual changefrom the earliest camel species to today’s species.

Punctuated Equilibrium Gradualism doesn’t explain theevolution of all species. For some species, the fossil record showsfew intermediate forms—one species suddenly changes toanother. According to the punctuated equilibrium model, rapidevolution comes about when the mutation of a few genes resultsin the appearance of a new species over a relatively short periodof time. The fossil record gives examples of this type of evolu-tion, as you can see in Figure 6.

Figure 6 The hypothesized evo-lution of bears illustrates the punc-tuated equilibrium model ofevolution. Discuss how the six species on thefar right are explained better bypunctuated equlibrium.

Common ancestor

about 40 millionyears ago

15-20 millionyears ago

2 millionyears ago

RaccoonRed

pandaGiantpanda

Polarbear

Spectacledbear

Slothbear

Blackbear

Brownbear

Sunbear

(l)Joe McDonald/Animals Animals, (c)Tom McHugh/Photo Researchers, (r)Tim Davis/Photo Researchers


Self Check1. Compare Lamarck’s and Darwin’s ideas about how evo-

lution takes place.

2. Explain why variations are important to understandingchange in a population over time.

3. Discuss how the gradualism model of evolution differsfrom the punctuated equilibrium model of evolution.

4. Describe how geographic isolation contributes to evolution.

5. Think Critically What adaptations would be helpful foran animal species that was moved to the Arctic?

6. Concept Map Use information given in Figure 6 tomake a map that shows how raccoons, red pandas,giant pandas, polar bears, and black bears are relatedto a common ancestor.

SummaryEarly Models of Evolution

• Evolution is change in the characteristics of aspecies over time.

• Lamarck proposed the hypothesis of inheritedacquired characteristics.

Natural Selection

• Darwin proposed evolution by natural selec-tion, a process by which organisms bestsuited to their environments are most likely tosurvive and reproduce.

• Organisms have more offspring than can sur-vive, individuals of a species vary, and manyof these variations are passed to offspring.

Variation and Adaptation

• Adaptations are variations that help an organ-ism survive or reproduce in its environment.

• Mutations are the source of new variations.

The Speed of Evolution

• Evolution may be a slow or fast processdepending on the species under study.

7. Use Percentages The evolution of the camel can betraced back at least 56 million years. Use Figure 1 toestimate the percent of this time that the moderncamel has existed.

Punctuated Equilibrium Today Evolution by the punctu-ated equilibrium model can occur over a few thousand or mil-lion years, and sometimes even faster. For example, manybacteria have changed in a few decades. The antibiotic penicillinoriginally came from the fungus shown in Figure 7. But manybacteria species that were once easily killed by penicillin nolonger are harmed by it. These bacteria have developedresistance to the drug. Penicillin has been in use since1943. Just four years later, in 1947, a species of bacteriathat causes pneumonia and other infections already haddeveloped resistance to the drug. By the 1990s, several dis-ease-producing bacteria had become resistant to peni-cillin and many other antibiotics.

How did penicillin-resistant bacteria evolve soquickly? As in any population, some organisms have vari-ations that allow them to survive unfavorable living con-ditions when other organisms cannot. When penicillinwas used to kill bacteria, those with the penicillin-resist-ant variation survived, reproduced, and passed this trait totheir offspring. Over a period of time, this bacteria popu-lation became penicillin-resistant.

Figure 7 The fungus growing inthis petri dish is Penicillium, theoriginal source of penicillin. It pro-duces an antibiotic substance thatprevents the growth of certainbacteria.

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Benchmarks—SC.F.2.3.3 (pp. 495–496, 500): SC.G.1.3.2 (pp ... · Ideas About Evolution Figure 1 By studying fossils, scientists have traced the hypothesized evolution of the camel