Go to Section: Interest Grabber Yes, No, or Maybe Some traits, such as a widow’s peak, fall into neat categories: You either have a widow’s peak or you don’t. Other traits, such as height, aren’t so easy to categorize. Section 16-1

Go to Section: Interest Grabber continued 1.Make a list of physical traits that you think are influenced by genes. Then, write next to each trait whether you have the trait or not (e.g., a widow’s peak) or whether there are many variations of the trait (e.g., hair color). 2.Are most of the traits you listed clear-cut or are they mostly traits that have many variations? Which traits in your list are difficult to categorize? 3.Compare your list with that of another student. Did he or she think of any traits that you missed? Why do you think some traits are clear-cut, while others are not? Section 16-1

Go to Section: Section Outline 16–1Genes and Variation A.How Common is Genetic Variation? B.Variation and Gene Pools C.Sources of Genetic Variation 1.Mutations 2.Gene Shuffling D.Single-Gene and Polygenic Traits Section 16-1

Go to Section: Frequency of Phenotype Phenotype (height) Generic Bell Curve for Polygenic Trait Section 16-1

Go to Section: Sample Population 48% heterozygous black 36% homozygous brown 16% homozygous black Frequency of Alleles allele for brown fur allele for black fur Figure 16–2 Relative Frequencies of Alleles Section 16-1

Go to Section: Frequency of Phenotype (%) Widow’s peakNo widow’s peak Phenotype Figure 16–3 Phenotypes for Single-Gene Trait Section 16-1

Go to Section: Interest Grabber... All the Help I Can Get Natural selection operates on traits in different ways. You might be able to predict which traits natural selection would favor if you think about the demands of an organism’s environment. Section Choose an animal that you know something about, such as a deer, and write its name at the top of a sheet of paper. Then, divide your paper into two columns, and write the heading Trait in one column and Advantage in the other. 2.Under Trait, write in several of the animal’s traits. 3.Under Advantage, write in how you think the trait would be helpful to the animal.

Go to Section: Section Outline 16–2Evolution as Genetic Change A.Natural Selection on Single-Gene Traits B.Natural Selection on Polygenic Traits 1.Directional Selection 2.Stabilizing Selection 3.Disruptive Selection C.Genetic Drift D.Evolution Versus Genetic Equilibrium 1.Random Mating 2.Large Population 3.No Movement Into or Out of the Population 4.No Mutations 5.No Natural Selection Section 16-2

Go to Section: Sample of Original Population Founding Population A Founding Population B Descendants Genetic Drift Section 16-2

Go to Section: Sample of Original Population Founding Population A Founding Population B Descendants Genetic Drift Section 16-2

Go to Section: Sample of Original Population Founding Population A Founding Population B Descendants Genetic Drift Section 16-2

Go to Section: Directional Selection Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness Figure 16–6 Graph of Directional Selection Section 16-2

Go to Section: Key Percentage of Population Birth Weight Selection against both extremes keep curve narrow and in same place. Figure 16–7 Graph of Stabilizing Selection Section 16-2 Low mortality, high fitness High mortality, low fitness Stabilizing Selection

Go to Section: Disruptive Selection Largest and smallest seeds become more common. Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Beak Size Figure 16–8 Graph of Disruptive Selection Number of Birds in Population Key Low mortality, high fitness High mortality, low fitness Section 16-2

Go to Section: Interest Grabber Country Cousin/City Cousin What happens when a population or group of living things is divided into two separate groups in two separate environments? To understand what goes on, think about someone who lives in another part of the United States or in another country. Section Make a list of everyday things that this person encounters that you don’t. For example, does he or she eat different kinds of food? Does he or she live in a climate different from yours? 2.All humans are the same species. What might happen if groups of humans were separated for millions of years in very different environments, such as those you have just described?

Go to Section: Section Outline 16–3The Process of Speciation A.Isolating Mechanisms 1.Behavioral Isolation 2.Geographic Isolation 3.Temporal Isolation B.Testing Natural Selection in Nature 1.Variation 2.Natural Selection 3.Rapid Evolution C.Speciation in Darwin’s Finches 1.Founders Arrive 2.Separation of Populations 3.Changes in the Gene Pool 4.Reproductive Isolation 5.Ecological Competition 6.Continued Evolution D.Studying Evolution Since Darwin Section 16-3

Go to Section: Concept Map Section 16-3 results from which include produced by which result in Reproductive Isolation Isolating mechanisms Behavioral isolationTemporal isolation Geographic isolation Behavioral differencesDifferent mating times Physical separation Independently evolving populations Formation of new species

Internet Links from the authors on restricting use of antibiotics Interactive test For links on population genetics, go to and enter the Web Code as follows: cbn For links on speciation, go to and enter the Web Code as follows: cbn Go Online

Section 1 Answers Interest Grabber Answers 1.Make a list of physical traits that you think are influenced by genes. Then, write next to each trait whether you have the trait or not (e.g., a widow’s peak) or whether there are many variations of the trait (e.g., hair color). Students’ answers will include dimples and detached earlobes. 2.Are most of the traits you listed clear-cut or are they mostly traits that have many variations? Which traits in your list are difficult to categorize? Most traits listed likely have many variations. 3.Compare your list with that of another student. Did he or she think of any traits that you missed? Why do you think some traits are clear-cut, while others are not? Some students may suggest that patterns of inheritance for traits with many variations (polygenic) are more complex than for clear-cut (single-gene) ones.

Section 2 Answers Interest Grabber Answers 1. Choose an animal that you know something about, such as a deer, and write its name at the top of a sheet of paper. Then, divide your paper into two columns, and write the heading Trait in one column and Advantage in the other. Animal choices should be sufficiently familiar that students can describe several traits. 2. Under Trait, write in several of the animal’s traits. Students should list traits such as size, color, and specialized behavior. 3. Under Advantage, write in how you think the trait would be helpful to the animal. Students should indicate that adaptive value is clearer for some traits than for others. For example, white-tailed deer raise their tails upon sensing a predator. This may be an alarm signal for other deer, or it may induce the predator to chase the now-conspicuous deer.

Section 3 Answers Interest Grabber Answers 1.Make a list of everyday things that this person encounters that you don’t. For example, does he or she eat different kinds of food? Does he or she live in a climate different from yours? Students’ lists should include several social/environmental factors. 2.All humans are the same species. What might happen if groups of humans were separated for millions of years in very different environments, such as those you have just described? Students may understand that humans would evolve separately in response to different environmental pressures.

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