WIXLIB

NameClassCHAPTER 7DateThe Evolution of Living ThingsSECTION2 How Does Evolution Happen?National ScienceEducation StandardsBEFORE YOU READAfter you read this section, you should be able to answerthese questions:LS 2a, 2b, 2e, 3d, 5a, 5b Who was Charles Darwin? What ideas affected Darwin’s thinking? What is natural selection?Who Was Charles Darwin?In 1831, Charles Darwin graduated from college.Although he eventually earned a degree in religion, Darwinwas most interested in the study of plants and animals.Darwin’s interest in nature led him to sign on for afive-year voyage around the world. He was a naturalist onthe HMS Beagle, a British ship. A naturalist is someonewho studies nature. During the trip, Darwin made observations that helped him form a theory about how evolution happens. These ideas caused scientists to change theway they thought about the living world.DARWIN’S JOURNEYOn the trip, Darwin observed plants and animals frommany parts of the world. One place Darwin found interestingwas the Galápagos Islands. These islands are located about1,000 km west of Ecuador, a country in South America. Manyunusual organisms live on the Galápagos Islands.STUDY TIPSummarize After you readthis section, make a chartshowing the four steps ofnatural selection. In the chart,explain what happens ateach step.Math Focus1. Convert About how farare the Galápagos Islandsfrom Ecuador in miles?1 km 0.62 miThis line shows the course of the HMS Beagle.TAKE A LOOK2. Describe Which continentare the Galápagos Islandsclosest to?Darwin studied plants and animals on the Galápagos Islands.Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook101The Evolution of Living Things

NameSECTION 2ClassDateHow Does Evolution Happen? continuedDARWIN’S FINCHESREADING CHECK3. Describe What didDarwin observe about thefinches on the GalápagosIslands?Darwin observed that the animals and plants onthe Galápagos Islands were similar to those in Ecuador.However, they were not identical. For example, Darwinclosely observed birds called finches. The finches on theGalápagos Islands were slightly different from the finchesin Ecuador. In addition, the finches on each island in theGalápagos differed from the finches on the other islands.Darwin hypothesized that the island finches weredescendents of South American finches. He thought thefirst finches on the islands were blown there from SouthAmerica by a storm. He suggested that over many generations, the finch populations evolved adaptations thathelped them survive in the different island environments.For example, the beaks of different finch species areadapted to the kind of food the species eat.The large ground finch has a wide,strong beak. It can easily crack openlarge, hard seeds. Its beak workslike a nutcracker.The cactus finch has a tough beak. Ituses its beak to eat cactus parts andinsects. Its beak works like a pair ofneedle-nose pliers.Critical Thinking4. Infer What can you guessabout the environment inwhich the cactus finch livesbased on the information inthe figure? Explain your answer.The warbler finch has a small,narrow beak. It can catch smallinsects with its beak. Its beakworks like a pair of tweezers.Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook102The Evolution of Living Things

NameSECTION 2ClassDateHow Does Evolution Happen? continuedHow Did Darwin Develop the Theory ofEvolution by Natural Selection?After Darwin returned to England, he spent many yearsthinking about his experiences on the trip. In 1859, Darwinpublished a famous book called On the Origin of Species byMeans of Natural Selection. In his book, Darwin proposedthe theory that evolution happens by natural selection.Natural selection happens when organisms thatare well adapted to their environment survive, but lesswell-adapted organisms do not. When the better-adaptedorganisms reproduce, they pass their useful traits on totheir offspring. Over time, more members of the population have these traits. Darwin combined ideas aboutbreeding, population, and Earth’s history to come up witha theory to explain his observations.READING CHECK5. Define What is naturalselection?IDEAS ABOUT BREEDINGIn Darwin’s time, farmers and breeders had producedmany kinds of farm animals and plants. They learned thatif they bred plants or animals that had a desirable trait,some of the offspring might have the trait. A trait is aform of an inherited characteristic. The practice in whichhumans select plants or animals for breeding based ondesired traits is called selective breeding.Selective breeding showed Darwin that the traits oforganisms can change and that certain traits can spreadthrough populations. For example, most pets, such as thedogs below, have been bred for a variety of desired traits.Over the past 12,000 years, people have selectively breddogs to produce more than 150 breeds.READING CHECK6. Explain How did ideasabout selective breedingaffect Darwin’s thinkingabout evolution?People have selectively bred dogs for different traits. Today, there areover 150 dog breeds.Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook103The Evolution of Living Things

NameSECTION 2ClassDateHow Does Evolution Happen? continuedIDEAS ABOUT POPULATIONREADING CHECK7. Identify According toThomas Malthus, what aretwo things that can limit thesize of human populations?During Darwin’s time, a scientist named ThomasMalthus was studying human populations. He observedthat there were more babies being born than there werepeople dying. He thought that the human populationcould grow more rapidly than food supplies could grow.This would result in a worldwide food shortage. Malthusalso pointed out that the size of human populations islimited by problems such as starvation and disease.Darwin realized that Malthus’s ideas can apply to allspecies, not just humans. He knew that any species canproduce many offspring. He also knew starvation, disease, competition, and predation limited the populationsof all species. Only a limited number of individuals livelong enough to reproduce.Darwin reasoned that the survivors had traits thathelped them survive in their environment. He alsothought that the survivors would pass on some of theirtraits to their offspring.IDEAS ABOUT EARTH’S HISTORYREADING CHECK8. Explain How did CharlesLyell’s book change howscientists thought aboutEarth’s history?New information about Earth’s history also affectedDarwin’s ideas about evolution. During Darwin’s time,most geologists thought that Earth was very young.But important books, such as Principles of Geologyby Charles Lyell, were changing ideas about the Earth.Lyell’s book gave evidence that Earth is much older thananyone once thought.Darwin thought that evolution happens slowly. Darwinreasoned that if Earth was very old, there would beenough time for organisms to change slowly.IdeaHow it contributed to Darwin’s theorySelective breedinghelped Darwin realize that not all ofan organism’s offspring will survive toreproducehelped Darwin realize that slow changescan produce large differences over a longperiod of timeTAKE A LOOK9. Describe Fill in the blankspaces in the table.Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook104The Evolution of Living Things

NameSECTION 2ClassDateHow Does Evolution Happen? continuedHOW NATURAL SELECTION WORKSNatural selection has four steps: overproduction,inherited variation, struggle to survive, and successfulreproduction.Say It1Overproduction A tarantula’s egg saccan hold 500 to 1,000 eggs. Some ofthe eggs will survive and develop intoadult spiders. Some will not.2Inherited Variation Everyindividual has its own combinationof traits. Each tarantula is similar,but not identical, to its parents.Give Examples The figureshows one example of howthe four steps of naturalselection can work. In agroup, talk about three ormore other examples of hownatural selection can affectpopulations.TAKE A LOOK3Struggle to Survive Some tarantulas may have traits that make itmore likely that they will survive.For example, a tarantula may bebetter able to fight off predators,such as this wasp.4Successful Reproduction Thetarantulas that are best adaptedto their environment are likelyto survive and reproduce. Theiroffspring may inherit the traits thathelp them to survive.10. Identify Why are sometarantulas more likely tosurvive than others?GENETICS AND EVOLUTIONDarwin knew that organisms inherit traits, but nothow they inherit traits. He also knew that there is greatvariation among organisms, but not how that variationhappens. Today, scientists know that genes determinethe traits of an organism. These genes are exchanged andpassed on from parent to offspring.Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook105The Evolution of Living Things

NameClassSection 2 ReviewDateNSESLS 2a, 2b, 2e, 3d, 5a, 5bSECTION VOCABULARYnatural selection the process by which individualsthat are better adapted to their environmentsurvive and reproduce more successfully thanless well adapted individuals do; a theory toexplain the mechanism of evolutionselective breeding the human practice ofbreeding animals or plants that have certaindesired traitstrait a genetically determined characteristic1. Explain How did the ideas in Charles Lyell’s book affect Darwin’s thinkingabout evolution?2. Identify In what way are the different finch species of the Galápagos Islandsadapted to the different environments on the islands?3. Compare How is natural selection different from selective breeding?4. Describe How did Darwin apply Malthus’s ideas about human populations to thetheory of evolution by natural selection?5. List What are the four steps of natural selection?Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook106The Evolution of Living Things