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What is thetheory ofevolution bynaturalsetectiow?

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By the end of this lesson, youshould be able to describethe role of genetic andenvironmental factors inthe theory of evolution bynatural selection.

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a Fur color can help prevent ananimal from being eaten.

D D ^e amount of available foodcan affect an organism'ssurvival.

D D Your parents' characteristicsare not passed on to you.

Q Q A species can go extinct if itshabitat is destroyed.

fciUActiwR&adiHfl3 Synthesize You can often define an unknown

word by clues provided in the sentence. Usethe sentence below to make an educated guessabout the meaning of the word artificial.Example sentence:Many people prefer real sugar to artificialsweeteners made by humans.

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Quick Labs• Model Natural Selection• Analyzing Survival Adaptations• The Opposable ThumbExploration lab• Environmental Change and Evolution

2 Infer How do you think this bird and this flowerare related? Explain your answer. it

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Vocabulary Terms• evolution • mutation• artificial selection • adaptation• natural selection • extinction• variation

4 Apply As you learn the deflnition of eachvocabulary term In this lesson, createyour own definition or sketch to help youremember the meaning of the term.

Lesson 2 Theory of Evolution by Natural Selection 15

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Darwin's VoyageWhat did Darwin observe?Charles Darwin was born in England in 1809. Wlien he was22 years old, Darwin graduated from college with a degreein theology. But he was also interested in plants and animals.Darwin became the naturalist—a scientist who studies nature—

on the British ship HMS Beagle.During his voyage, Darwin observed and collected many

living and fossil specimens. He made some of his most importantobservations on the Galapagos Islands of South America. Hekept a log that was later published as The Voyage of the Beagle.With the observations he made on this almost five-year journey,Darwin formed his idea about how biological evolution couldhappen.

In biology, evolution refers to the process by whichpopulations change over time. A population is all of theindividuals of a species that live in an area at the same time.A species is a group of closely related organisms that can mate toproduce fertile offspring. Darwin developed a hypothesis, whicheventually became a theory, of how evolution takes place.

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5 Explore Trace Darwin's route onthe map, and choose one of thefollowing stops on his journey:Galapagos Islands, AndesMountains, Australia. Do researchto find out what plants andanimals live there. Then write an

entry in Darwin's log to describewhat he might have seen.

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Differences among SpeciesDarwin collected birds from the GalapagosIslands and nearby islands. He observedthat these birds differed slightly from thoseon the nearby mainland of South America.And the birds on each island were differentfrom the birds on the other islands. Carefulanalysis back in England revealed thatthey were all finches! Eventually, Darwinsuggested that these birds may have evolvedfrom one species of finch.

Darwin observed differences in beaksize among finches from different islands.Many years later, scientists confirmed thatthese differences related to the birds diets.Birds with shorter, heavier beaks could eatharder foods than those with thinner beaks.

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What other ideas influenced Darwin?The ideas of many scientists and observations of the natural worldinfluenced Darwin's thinking. Darwin drew on ideas about Earth'shistory, the growth of populations, and observations of how traitsare passed on in selective breeding. All of these pieces helped himdevelop his ideas about how populations could change over time.

Organisms Pass Traits On toOffspringFarmers and breeders have been producingmany kinds of domestic animals and plants forthousands of years. These plants and animals Jhave traits that the farmers and breeders desire. A Etrait is a form of an inherited characteristic. For

example, the length of tail feathers is an inheritedcharacteristic, and short or long tail feathers arethe corresponding traits. The practice by whichhumans select plants or animals for breedingbased on desired traits is artificial selection.

Artificial selection shows that traits can change.Traits can also spread through populations.

th'if chicken has been bredto have large tail -feather?anJ a big red comb.

7 List Darwin studied artificial selection in thepigeons that he bred. List three other domesticanimals that have many different breeds.

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8 Identify As you read, underline the names of otherimportant thinkers who influenced Darwin's ideas.

Organisms Acquire TraitsScientist Jean-Baptiste Lamarck thought thatorganisms could acquire and pass on traits theyneeded to survive. For example, a man coulddevelop stronger muscles over time. If themuscles were an advantage in his environment,Lamarck thought the man would pass on thistrait to his offspring. Now we know that acquiredtraits are not passed on to offspring because thesetraits do not become part of an organism's DNA.But the fact that species change, and the ideathat an organism's traits help it survive, shapedDarwin's ideas.

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9 Apply Explain why the size of your muscles ispartly an acquired trait and partly dependent onDNA.

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Earth Changes over TimeThe presence of different rock layers, such as those in thephoto, show that Earth has changed over time. GeologistCharles Lyell hypothesized that small changes in Earthssurface have occurred over hundreds of millions of years.Darwin reasoned that if Earth were very old, then therewould be enough time for very small changes in life formsto add up.

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A Struggle for Survival ExistsAfter his journey, Darwin read an essay aboutpopulation growth by economist ThomasMalthus. The essay helped Darwin understandhow the environment could influence whichorganisms survive and which organisms die.All populations are affected by factors that limitpopulation growth, such as disease, predation,and competition for food. Darwin reasonedthat the survivors probably have traits that helpthem survive and that some of these traits couldbe passed on from parent to offspring.

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What are the four parts of natural selection?Darwin proposed that most evolution happens through the natural selection ofadvantageous traits. Natural selection is the process by which organisms that inheritadvantageous traits tend to reproduce more successfully than other organismsdo. There are four parts that contribute to the process of evolution by naturalselection—overproduction, genetic variation, selection, and adaptation.

OverproductionWhen a plant or animal reproduces, it usuallymakes more offspring than the environmentcan support. For example, a female jaguarmay have up to four pups at a time. Acrossthe jaguar population, only some of them willsurvive to adulthood, and a smaller number ofthem will successfully reproduce.

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11 Infer What might be a natural reason for lowsurvival rates of jaguar cubs?

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Genetic VariationWithin a population there are natural differences,or variations, in traits. For example, in the twojaguar skulls to the left, one has a larger jawthan the other. This difference results from adifference in the genetic material of the jaguars.Genetic variations can be passed on from parentto offspring. An important source of variation isa mutation, or change in genetic material.

With each new generation, genetic variationintroduces changes to the traits of a population.Greater genetic variation in a populationincreases the chance that some individualswill have traits that can help them surviveenvironmental changes or diseases. Traits thatenhance an individuals ability to reproduce willalso increase the chance of survival of a species.

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g,^^^sIndividuals try to get the resources they needto survive. These resources include food, water,space and, in most cases, mates for reproduction.About 11,000 years ago, jaguars faced a shortageof food because the climate changed and manyprey species died out. A genetic variation in jawsize then became important for survival. Jaguarswith larger jaws could eat hard-shelled reptileswhen other prey were hard to find.

Individuals with a particular trait, such as alarge jaw, are more likely to survive long enoughto reproduce. As a result, the trait is "selected" for,becoming more common in the next generationof offspring.

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M^^-^An inherited trait that helps an organism surviveand reproduce in its environment is an adaptation.Adaptation is the selection of naturally occurringtrait variations in populations. Jaguars withlarger jaws were able to survive and reproducewhen food was hard to find. As natural selectioncontinues, adaptations grow more commonin the population with each new generation,and new adaptations may arise. Over time,the population becomes better adapted to theenvironment.

13 Explain In the table below, explain how each part of natural selection works.

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How do species change over time?In order for a population to change, some individuals have to begenetically different from other members of the population. Mutationsare one of the main sources of genetic variation. Offspring sometimesinherit a gene that has a slight mutation, or change, from the genethe parent has. Mlutations can be harmful, helpful, or have no effect.Beneficial mutations help individuals survive and reproduce.

Over Generations, AdaptationsBecome More Common

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Adaptations are inherited traits that helporganisms survive and reproduce. At first,an adaptation is rare. For example, imagine a.population of birds in which some birds haveshorter beaks. If more birds with shorter beakssurvive and reproduce than birds with longerbeaks, more birds in the next generation willprobably have short beaks. The number ofindividuals with the adaptation would continueto increase. Some adaptations, such as a duckswebbed feet, are physical. Other adaptations areinherited behaviors that help an organism findfood, protect itself, or reproduce.

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Genetic Differences Add UpParents and offspring often have small geneticdifferences between them. Over many generations,the small differences can add up. These differencesaccumulate so that organisms alive now are oftenvery different from their ancestors. As a result,there is great diversity among organisms. Forexample, the antibiotic penicillin was able to killmany types of bacteria in the 1950s. Today, someof those species of bacteria are now completelyresistant to penicillin. The genetic makeup ofthese bacterial populations has changed. Newfossil discoveries and new information aboutgenes add to scientists' understanding of naturalselection and evolution.

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What happens to species as theenvironment changes?

& Certain environments favor certain traits. Consider a snake:%^r,n1flt-ion with either brown- or green-colored snakes. In a forest

riiathas many dead leaves on the ground, brown snakes will blend^ in better than green snakes will. But in an area with more grass,the

snakes may be better at hiding from predators. Changes inenvironmental conditions can affect the survival of organisms with

a particular trait. Environmental changes can also lead to diversityof organisms by increasing the number of species.

Can Allow a Species Some Species May BecomeExtinct

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g| plants have xylem tissue that carries water upfrom the roots to the rest of the plant.

If the environment changes, a species ismore likely to survive if it has genetic variation.For example, imagine a species of grass in whichsome plants need less water than others. If the

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If no individuals have traits that help themto survive and reproduce in the changedenvironment, a species will become extinct.Extinction occurs when all members of aspecies have died. Greater competition, newpredators, and the loss of habitat are examplesof environmental changes that can lead toextinction. Some extinctions are caused bynatural disasters. Because a natural disaster candestroy resources quickly,, organisms may die nomatter what adaptations they have. The fossilrecord shows that many species have becomeextinct in the history of life on Earth.

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21 Infer How does the environment influence natural selection?

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Lesson ReviewVocabularyUse a term from the lesson to complete thesentences below.

1 The four parts of natural selection areoverproduction,

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selection, and adaptation.2 is the process

by which populations change over time.3 The hollow bones of birds, which keep birds

lightweight for flying, is an example ofa(n).

Key Concepts4 Summarize Describe Darwin's observations

on the Galapagos islands during his voyage onthe HMS Beagle.

Critical ThinkingUse the diagram to answer the following question. |

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5 Explain How does environmental changeaffect the survival of a species?

6 Compare Why are only inherited traits, notacquired ones, involved in the process ofnatural selection?

I 7 Describe What is the relationship betweenJ, mutation, natural selection, and adaptation?£

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9 Infer What might happen to a population ofrabbits in a forest if a new predator moved tothe forest?

Lesson 2 Theory of Evolution by Natural Selection 25