AP Biology Reading Guide Name Chapter 23: The Evolution of Populations This chapter begins with the idea that we focused on as we closed the last chapter: Individuals do not evolve! Populations evolve. The Overview looks at the work of Peter and Rosemary Grant with Galápagos finches to illustrate this point, and the rest of the chapter examines the change in populations over time. As in the last chapter, first read each concept to get the big picture and then go back to answer these questions. Don t lose sight of the conceptual understanding by getting lost in the details! Overview 1. What is microevolution? 2. What are the three main mechanisms that can cause changes in allele frequency? (1) (2) (3) 3. Which is the only mechanism that is adaptive, or improves the match between organisms and their environment? Explain. Concept 23.1 Mutation and sexual reproduction produce the genetic variation that makes evolution possible 4. What is the ultimate source of new alleles? 5. Mutations are any change in the nucleotide sequence of an organism s DNA. These mutations provide the raw material from which new traits may arise and be selected. Multicellular organisms may accumulate many mutations over their lifetime; however, it is likely that none of them may be passed on to their offspring. Explain why this is so. 1
6. Summarize how each of the following processes can affect evolutionary change. For example, is it likely to cause change? If so, how? If not, why not? Then, provide an example. Process Affect on Evolutionary Change & Example Point mutations Mutations that alter gene numbers or sequences Mutation rates 7. Much of the genetic variation that makes evolution possible comes through sexual reproduction. What are the three mechanisms by which sexual reproduction shuffles existing alleles? (1) (2) (3) Concept 23.2 The Hardy-Weinberg equation can be used to test whether a population is evolving 8. What is a population? 2
9. What is a gene pool? 10. The greater the number of fixed alleles, the lower the species diversity. What does it mean to say that an allele is fixed? 11. The Hardy-Weinberg principle is used to describe a population that is not evolving. What does this principle state? 12. If the frequency of alleles in a population remains constant, the population is at Hardy-Weinberg equilibrium. There are five conditions for Hardy-Weinberg equilibrium. It is very important for you to know these conditions, so enter them neatly into the box below. The first one has been done for you. 1. No Mutations 2. 3. 4. 5. CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM Concept 23.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population 13. There are 3 major factors that affect allele frequencies directly and cause most evolutionary change. Identify the 3 major factors below and provide an explanation of how each can affect allele frequencies. Factor Explanation 3
14. Which factor(s) above (#13) results in a random, nonadaptive change in allelic frequencies? Explain. 15. Which factor(s) above (#13) tends to reduce the genetic differences between populations and make populations more similar? Explain. 16. Of the 3 factors you listed above (#13), only one results in individuals that are better suited to their environment. Which is it? 17. Let s go back and revisit genetic drift. Explain what happens in each of type of genetic drift and provide an example. Type of Drift Explanation & Example Founder Bottleneck 18. Studies on populations that have been affected by genetic drift have led to 4 main conclusions. These key points are important to remember for the AP Exam. Briefly summarize each key point below. 4 Key Points Learned from Genetic Drift Studies (1) (2) (3) (4) 4
Concept 23.4 Natural selection is the only mechanism that consistently causes adaptive evolution 19. Although we often refer to the relative fitness of a genotype, remember that the phenotypes of the organism are subjected to natural selection, not the underlying genotypes. Thus, natural selection acts upon the genotype indirectly, via how the genotype affects the phenotype. Natural selection can alter the frequency of distribution of heritable traits in 3 ways, depending on which phenotypes are favored in a population. Figure 23.13 helps explain the three modes of phenotypic selection. Summarize each type of selection, then draw and label a sketch of the corresponding graph. Type of Selection Explanation Graph 20. Explain what is often the result of sexual selection? 21. a. What is the difference between intrasexual selection and intersexual selection? b. Give an example of each type of selection. 5
22. It is impossible for natural selection to eliminate all unfavorable genotypes through directional and stabilizing selection because there are mechanisms in place that also help preserve or restore genetic variation. One such mechanism is the preservation of recessive alleles in heterozygous individuals. Discuss what is meant by heterozygote advantage, and use sickle-cell anemia as an example. 23. Recall that natural selection can cause adaptive evolution, which is evolution resulting in a better match between organisms and their environment, but not necessarily a perfect match. Even though some alleles are consistently favored over others because they allow for better matches to the environment, it is all of the alleles in a surviving organism that get passed along to the offspring, whether beneficial or not. Therefore, as our final concept, summarize 4 reasons why natural selection cannot fashion perfect organisms. Reason Explanation 6