17.1 Genes and Variation

Lesson Objectives Define evolution in genetic terms. Identify the main sources of genetic variation in a population. State what determines the number of phenotypes for a trait. Genetics Joins Evolutionary Theory For Questions 1–4, complete each statement by writing the correct word or words. 1. Natural selection works on an organism’s rather than its . 2. A(n) consists of all the genes, including the alleles for each gene, that are present in a population.

3. A gene pool typically contains different for each heritable trait. 4. The number of times that an allele occurs in a gene pool compared with the number of times other alleles for the same gene occur is called the of the population.

Use the circle graph of a sample mouse population to answer Questions 5–8. 5. THINK VISUALLY In the diagram below, use circles to represent the alleles within each segment of the population. Draw the B alleles as solid circles and the b alleles as outline circles. The total number of individuals in this population is ; the total number of alleles is .

6. How many alleles for black fur are in the sample population and what percentage of allele frequency does that represent?

7. How many alleles for brown fur are in the sample population and what percentage of allele frequency does that represent?

8. Describe how a geneticist might be able to tell that this population is evolving.

9. Can you determine whether an allele is dominant or recessive on the basis of the ratio of phenotypes in the population? Explain your answer.

Sources of Genetic Variation 10. What are mutations? When do they affect evolution?

11. How does sexual reproduction affect a population’s genetic variation?

12. Identify two ways in which genes can be recombined during meiosis.

13. What is lateral gene transfer? How does it affect variation?

Single Gene and Polygenic Traits 14. Label the two graphs to show which represents a single-gene trait and which represents a polygenic trait.

For Questions 15–19, write True if the statement is true. If the statement is false, change the underlined word or words to make the statement true. 15. The number of phenotypes produced for a given trait depends on how many genes control the trait. 16. Height in humans is an example of a single-gene trait. 17. Each gene of a polygenic trait often has two or more phenotypes. 18. A single polygenic trait often has many possible genotypes. 19. A symmetrical bell-shaped graph is typical of polygenic traits. 20. Use the Venn diagram to compare and contrast single-gene traits and polygenic traits.

Single-Gene Traits Polygenic Traits Both

Apply the Big idea

21. Why is genetic variation important to the process of evolution?

17.2 Evolution as Genetic Change in Populations

Lesson Objectives Explain how natural selection affects single-gene and polygenic traits. Describe genetic drift. Explain how different factors affect genetic equilibrium. How Natural Selection Works 1. If a trait made an organism less likely to survive and reproduce, what would happen to the allele for that trait?

2. If a trait had no effect on an organism’s fitness, what would likely happen to the allele for that trait?

Use the table showing the evolution of a population of mice to answer Questions 3–5.

Initial Population Generation 10 Generation 20 Generation 30

90% 80% 70% 40%

10% 20% 30% 60%

3. Is the trait for fur color a single-gene trait or a polygenic trait? Explain your answer.

4. Describe how the relative frequency of fur color alleles is changing in this population and propose one explanation for this change.

5. Suppose a mutation causes a white fur phenotype to emerge in the population. What might happen to the mouse population after 40 generations?

6. What effect does stabilizing selection have on variation in a population?

For Questions 7–9, match the type of selection with the correct situation. Type of Selection Situation 7. Directional A. Individuals at the upper and lower ends of the 8. Stabilizing curve have higher fitness than individuals near the middle. 9. Disruptive B. Individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. C. Individuals near the center of the curve have higher fitness than individuals at either end.

10. Draw the missing line in the graph to the right to show how disruptive selection affects beak size. Genetic Drift For Questions 11–13, complete each statement by writing the correct word or words. 11. In small populations, random changes in is called genetic drift. 12. A situation in which allele frequencies change as a result of the migration of a small subgroup of a population is known as the . 13. The is a change in allele frequency following a dramatic reduction in the size of a population. 14. Complete the concept map. Genetic Drift

can result from

Bottleneck Effect

caused by caused by

Evolution Versus Genetic Equilibrium 15. What does the Hardy-Weinberg principle state?

16. What is genetic equilibrium?

17. List the five conditions that can disturb genetic equilibrium and cause evolution to occur.

18. Explain how sexual selection results in non-random mating.

19. Suppose a population of insects live in a sandy habitat. Some of the insects have tan bodies and some have green bodies. Over time, the habitat changes to a grass-filled meadow. Use the ideas of natural selection to explain how and why the insect population might change.

11. How does sexual reproduction affect a population’s genetic variation?

12. Identify two ways in which genes can be recombined during meiosis.

13. What is lateral gene transfer? How does it affect variation?

Single Gene and Polygenic Traits 14. Label the two graphs to show which represents a single-gene trait and which represents a polygenic trait.

For Questions 15–19, write True if the statement is true. If the statement is false, change the underlined word or words to make the statement true. 15. The number of phenotypes produced for a given trait depends on how many genes control the trait. 16. Height in humans is an example of a single-gene trait.

270 17. Each gene of a polygenic trait often has two or more phenotypes. 18. A single polygenic trait often has many possible genotypes. 19. A symmetrical bell-shaped graph is typical of polygenic traits. 20. Use the Venn diagram to compare and contrast single-gene traits and polygenic traits.

Single-Gene Traits Polygenic Traits Both

Apply the Big idea

21. Why is genetic variation important to the process of evolution?

17.3 The Process of Speciation

Lesson Objectives Identify the types of isolation that can lead to the formation of new species. Describe the current hypothesis about Galápagos finch speciation.

Isolating Mechanisms 1. What is speciation?

2. What does it mean for two species to be reproductively isolated from each other?

3. What must happen in order for a new species to evolve?

4. List three ways that reproductive isolation occurs.

5. When does behavioral isolation occur?

6. When does geographic isolation occur?

7. What is an example of temporal isolation?

8. Suppose a seamount forms from an underwater volcano. Birds on the mainland colonize the island. How might this lead to speciation?

Speciation in Darwin’s Finches For Questions 9–13, complete each statement by writing the correct word or words. 9. Peter and Rosemary Grant spent years on the Galápagos Islands studying changes in populations. 10. Many finch characteristics appear in bell-shaped distributions typical of traits. 11. The ancestors of the Galápagos Island finches originally came from the continent of . 12. The populations of finches on separate islands are isolated from one another by large stretches of open water. 13. Big-beaked finches that prefer to mate with other big-beaked finches are isolated from small-beaked finches living on the same island.

14. Write a paragraph that summarizes how speciation likely occurred in the Galápagos finches. Use the following terms in your response: geographic isolation, gene pools, behavioral isolation, and competition.

Apply the Big idea 15. Explain why reproductive isolation must occur for separate populations of the same species to evolve into different species.

17.4 Molecular Evolution

Lesson Objectives Explain how molecular clocks are used. Explain how new genes evolve. Describe how Hox genes may be involved in evolutionary change

Timing Lineage Splits: Molecular Clocks 1. What is a molecular clock?

2. Why are only neutral mutations useful for molecular clocks?

3. Why are there many molecular clocks in a genome instead of just one?

Use the diagram of an ancestral species to answer Questions 4–5. Each picture in the diagram represents a gene. Each shaded portion of a gene represents a mutation. 4. Which species is most closely related to Species B? Explain your answer.

5. How can you tell that Species C is probably not a descendant of the organism with Gene 2?

Gene Duplication For Questions 6–7, write the letter of the correct answer on the line at the left. 6. Multiple copies of a duplicated gene can turn into a group of related genes called A. globins. B. duplicates. C. a Hox gene. D. a gene family. 7. A chromosome may get several copies of the same gene during the process of A. crossing-over. B. gene mutation. C. gene expression. D. artificial selection.

8. Complete the flowchart to show how a new gene can evolve from a duplicated gene.

Original gene

Exact copy of original gene

New gene evolves new function. Developmental Genes and Body Plans 9. What genetic factors might be responsible for a change in an organism’s body plan?

Apply the Big idea

10. How can Hox genes help reveal how evolution occurred?