Chapter 25: Population Genetics

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1 Chapter 25: Population Genetics Student Learning Objectives Upon completion of this chapter you should be able to: 1. Understand the concept of a population and polymorphism in populations. 2. Apply the Hardy-Weinberg equation to calculate the frequency of alleles and genotypes in a population. 3. Understand microevolution and the factors that affect it. 4. Distinguish between the various patterns of natural selection. 5. Understand the concept of genetic drift. 6. Know how migration and nonrandom mating change allele frequencies in a population and how they influence Hardy-Weinberg equilibrium. 7. Know the various mechanisms by which organisms acquire new genetic variations Genes in Populations and the Hardy-Weinberg Equation The majority of the text has examined the relationship between genes and the individual. This chapter explores the bigger picture of population genetics. In the first section we explore some of the general features of populations and gene pools, as well as the mathematical calculations of allele and genotype frequencies. In order to study population genetics we need to first develop a mathematical model that examines the allele frequencies of a population that is stable from one generation to the next. Once that is established it is possible to assess the influences of factors that may alter the allele frequency of a population. This is the basis of the Hardy-Weinberg equilibrium. While the Hardy-Weinberg formula (page 625) appears too mathematical, in reality it is a theoretical model of a stable population. As you will observe in the next section, stable populations are rare in the natural world since there are many factors that influence the frequencies of alleles in a population. However, an understanding of the equation is needed in order to assess the influence of these factors. Outline of Key Terms Population genetics Gene pool Population Local population (deme) Polymorphism Polymorphic Monomorphic Single-nucleotide polymorphism (SNP) Allele frequencies Genotype frequencies Hardy-Weinberg equation Equilibrium Disequilibrium Calculations of allele and genotype frequencies (pages ) Discussion of Hardy-Weinberg equilibrium (pages ) 317

2 Exercises and Problems For each of the following, match the term to its correct definition. 1. Population 2. Gene pool 3. Allele frequencies 4. Genotype frequencies a. The sum of all of the genes within a population. b. A group of members of the same species that can interbreed with one another. c. The percent that an allele is represented in a population. d. The percent of individuals that are homozygous recessive, homozygous dominant or heterozygous in a population. Match each of the following Hardy-Weinberg equation terms with its correct definition. 5. p 2 a. The genotype frequency of homozygous dominant individuals. 6. q 2 b. The genotype frequency of heterozygous individuals. 7. 2pq c. The genotype frequency of homozygous recessive individuals. For questions 8 and 9, use the following information: In a certain population of 200 people, the number of individuals with AA, Aa, and aa genotypes is 60, 100, and 40, respectively. 8. What is the frequency of AA individuals? 9. What is the frequency of the A allele? For questions 10 and 11, use the following information: In a human population, a recessive autosomal disorder occurs in approximately 1 in 10,000 births. 10. What is the frequency of the recessive allele? 11. What is the percentage of carriers? 25.2 of Microevolution The term microevolution describes changes in a population s gene pool from generation to generation. Such change is based on two related phenomena: 1) introduction of new genetic variation into a population, and 2) actions of mechanisms that alter existing genetic variation in a population. The previous section introduced the fact that a stable population possesses certain characteristics under the Hardy-Weinberg equilibrium. Yet in the natural world, few populations are in equilibrium. The factors that contribute to disequilibrium are outlined in this section. 318

3 Outline of Key Terms Microevolution Factors that govern microevolution (Table 25.1) Exercises and Problems Complete the following sentences with the most appropriate term(s): 1. Microevolution describes changes in a population s from generation to generation. 2. Random mutations can introduce new alleles into a population, but at a rate. 3. is the phenomenon in which the environment favors individuals that possess certain traits. 4. The change in genetic variation from generation to generation due to random sampling error is called. 5. is the phenomenon in which individuals select mates based on their phenotypes or genetic lineage Natural Selection The theory of evolution by natural selection was proposed in the 1850s by Charles Darwin and Alfred Russell Wallace. The theory posits that the conditions found in nature result in the survival and reproduction of individuals that are best suited to their environment. This process will lead to changes in allele frequencies from one generation to the next. This is based on the concept of Darwinian fitness, which is the relative likelihood that one genotype will contribute to the gene pool of the next generation compared with other genotypes. Note that Darwinian fitness does not necessarily mean physical fitness. Rather, it is a measure of reproductive success. An extremely fertile genotype may have a higher Darwinian fitness than a less fertile genotype that appears more physically fit. Geneticists have found that natural selection can occur in several ways, depending on the relative fitness values of the different genotypes and on the variation of environmental effects. In this section, we will consider four different patterns of natural selection: directional, stabilizing, disruptive, and balancing selection. We will also examine an ongoing example of natural selection a change in the beak size of Darwin s finches due to drought conditions. 319

4 Outline of Key Terms Natural selection Darwinian fitness Directional selection Mean fitness of the population Stabilizing selection Disruptive selection Balancing selection Heterozygote advantage Selection coefficient (s) Negative frequency-dependent selection Directional selection (Figures 25.6 and 25.8); stabilizing selection (Figure 25.9); disruptive selection (Figure 25.10); and balancing selection (pages ) Exercises and Problems For questions 1 to 6, choose the model of selection from the diagram below: Favors the survival of individuals with intermediate phenotypes. Separates a population into distinct phenotypic classes. Favors one extreme of the phenotype. Occurs in species that occupy diverse environments. Resistance to antibiotics typically occurs in this manner. Clutch size in birds is an example of this type of selection. 320

5 For questions 7 to 11, complete the following sentences with the most appropriate term(s): 7. Darwinian fitness values are denoted by the variable. 8. For genetic variation involving a single gene, balancing selection may arise when the heterozygote has a higher fitness that either corresponding homozygote. This situation is called or. 9. Balancing selection may also be caused by, a pattern in which the fitness of a genotype decreases when its frequency becomes higher. 10. The change in beak size in the Galapagos finches that the Grants observed is an example of natural selection, and most likely due to selection. 11. A heterozygote for the sickle-cell allele, Hb A Hb S, has a higher level of fitness compared to the Hb A Hb A homozygote, because the heterozygote has a better chance of survival if infected with the malarial parasite, Genetic Drift The concept of random genetic drift, or simply genetic drift, was developed in the 1930s by the geneticist Sewall Wright. It refers to changes in allelic frequencies in a population due to random fluctuations. Over the long run, genetic drift can lead to the loss or fixation of a particular allele. The rate at which this occurs depends on the population size and on the initial allele frequencies (Figure 25.16). In nature, geography and population size can influence how genetic drift affects the genetic composition of a species in different ways. In general, small isolated populations tend to be more genetically disparate in relation to other populations. Changes in population size may influence genetic drift in one of two main ways: 1) the bottleneck effect, where a population is reduced dramatically in size by natural disasters or human destruction of habitat; and 2) the founder effect, which involves separation of a small group of individuals from a larger population, and establishment of a colony in a new location. Outline of Key Terms Random genetic drift (genetic drift) Bottleneck effect Founder effect A hypothetical simulation of genetic drift (Figure 25.16) The bottleneck effect, an example of genetic drift (Figure 25.17) 319

6 Exercises and Problems For questions 1 to 4, use the following information: The mutation rate is given by, and the number of individuals in a population is given by N. Assume equal numbers of males and females contribute to the new generation. 1. What is the expected number of new mutations in a given gene? 2. If a new mutation has arisen, what is the probability that the new allele will be fixed due to genetic drift? 3. What is the probability that the new allele will be eliminated? 4. If a population has 1000 breeding members, what is the average number of generations required to achieve fixation? For questions 5 to 6, complete the following sentences with the most appropriate term(s): 5. The African cheetah population has lost a substantial amount of its genetic variation due to a effect. 6. The high frequency of dwarfism in the Old Order Amish of Lancaster County, Pennsylvania, is due to the effect Migration Migration is the movement of individuals from one location to another. Population geneticists are particularly interested in the phenomenon of gene flow. This occurs when individuals migrate from one population to another with different allele frequencies, and the migrants are able to breed successfully with the members of the recipient population. Thus gene flow depends not only on migration, but also on the ability of the migrants alleles to be passed to subsequent generations. Migration can be unidirectional or bidirectional. Depending on its rate, migration tends to reduce differences in allele frequencies between neighboring populations and increase genetic diversity within a population. Outline of Key Terms Conglomerate Gene flow Quantitative discussion of migration (page 639) 320

7 Exercises and Problems Complete the following sentences with the most appropriate word or phrase: After migration has occurred, the new population is called a (1). To calculate the allele frequencies in this new population, we need two kinds of information. First, we must know the (2) in the donor and recipient populations. Second, we must know the proportion of the (3) population that is due to (4) Nonrandom Mating You may recall that in the first section of this chapter, we stated that one of the conditions required to establish the Hardy-Weinberg equilibrium is random mating. This means that individuals choose their mates regardless of their genotypes and phenotypes. However, in many cases, especially in human populations, this condition is violated frequently. When mating is based on phenotype, the process is called assortative mating. Positive assortative mating occurs when individuals with similar phenotypes choose each other as mates, while in negative assortative mating individuals with dissimilar phenotypes mate preferentially. When mating is based on genotype, inbreeding or outbreeding occurs. The mating of two genetically related individuals is called inbreeding, while outbreeding involves preferential mating between unrelated individuals. Note that nonrandom mating may alter the genotype frequencies that would be predicted by the Hardy-Weinberg equation. In general, positive assortative mating and inbreeding increase homozygosity, whereas negative assortative mating and outbreeding increase heterozygosity. Outline of Key Terms Nonrandom mating Assortative mating Inbreeding Inbreeding coefficient (F) Inbreeding depression Outbreeding A human pedigree containing inbreeding (Figure 25.18) Quantitative discussion of nonrandom mating (pages ) 321

8 Exercises and Problems For questions 1 to 6, match the term to its correct definition. 1. Negative assortative mating 2. Disequilibrium 3. Coefficient of inbreeding 4. Inbreeding 5. Outbreeding 6. Positive assortative mating a. the mating of two genetically related individuals b. has the ability to create hybrids that are heterozygous for many genes c. individuals who mate due to similar phenotypes d. individuals who mate based on dissimilar phenotypes e. allele and genotype frequencies are not in Hardy-Weinberg equilibrium f. the quantification of the degree of inbreeding For questions 7 to 16, choose the most appropriate term for the definition. Note that this exercise offers a review of the first six sections of this chapter. 7. A population is not evolving towards fixation of an allele. 8. The degree to which a genotype is selected against. 9. A fitness calculation in which the terms do not add up to The drastic reduction in size of a population. 11. Random changes in allele frequencies due to chance events. 12. Genetic variation that decreases the average fitness of a population. 13. Occurs due to the movement of individuals between populations. 14. Mating that produces homozygotes that are less fit, thereby decreasing the reproductive success of the population. 15. The relative likelihood that a phenotype will survive and contribute to the next generation s gene pool. 16. Occurs when a small group of individuals establish a new population. a. inbreeding depression f. mean fitness of the population b. founder effect g. balanced polymorphism c. genetic drift h. Darwinian fitness d. bottleneck effect i. genetic load e. gene flow j. selection coefficient 322

9 25.7 Sources of New Genetic Variation Earlier in the textbook we discussed various mechanisms by which genetic variation may be generated. For example, independent assortment (Chapter 3) and crossing-over (Chapter 6) during sexual reproduction can produce new allelic combinations in the offspring. This section explores additional mechanisms through which organisms acquire new genetic variation. These include: 1) mutations, which involve changes in gene sequences, chromosome structure and/or chromosome number; 2) exon shuffling (by which new genes are created); 3) horizontal gene transfer (by which new genes are acquired); and 4) changes in repetitive sequences. The last part of this section examines the technique of DNA fingerprinting, which is used to identify individuals and to study the genetic relationship between them. Outline of Key Terms Mutation rate Exon shuffling Horizontal gene transfer Repetitive sequences Microsatellite Minisatellite DNA fingerprinting DNA profiling Sources of new genetic variation (Table 25.2) The process of exon shuffling (Figure 25.19) Horizontal gene transfer from bacterium to eukaryote (Figure 25.20) DNA fingerprinting (pages ) Exercises and Problems Complete the following sentences with the appropriate terms(s): 1. DNA fingerprinting is also called. 2. are also called short tandem repeats (STRs). 3. refers to the process by which an exon and its flanking introns are inserted into a gene, thus producing a new gene encoding a protein with an additional domain. 4. In, the repeat unit is 6 to 80 bp in length. 5. In, an organism incorporates genetic material from another organism without being its offspring. 6. The is defined as the probability that a gene will be altered by a new mutation. 7. Repetitive sequences may come from, which are genetic sequences that can move from place to place within a species genome. 323

10 Chapter Quiz 1. The sum of all of the alleles in a population is called what? a. deme b. genotype frequency c. gene pool d. fitness coefficient 2. Individuals who prefer dissimilar phenotypes for mating are said to be exhibiting a. inbreeding. b. positive assortative breeding. c. disequilibrium. d. negative assortative mating. 3. Which of the following is the measure of inbreeding? a. mutation frequency b. Darwinian fitness c. fixation coefficient d. selection coefficient 4. Which of the following forms of selection favors an intermediate phenotype? a. directional selection b. disruptive selection c. stabilizing selection d. none of the above 5. Which of the following is an example of genetic drift in which catastrophic events influence the allele frequencies? a. bottleneck effect b. inbreeding c. founder effect d. migration 6. In a population of animals, 96% have brown bodies (BB or Bb) and only 4% are black-bodied (bb). According to the Hardy-Weinberg equation, what percentage of the total population is expected to be heterozygous? a. 20% b. 32% c. 48% d. 64% e. 80% 7. In shorthorn cattle, individuals can be red (C R C R ), white (C W C W ), or roan (C R C W ), which is a mixture of red and white. A population of Shorthorns was sampled and found to contain 108 red, 48 white, and 144 roan animals. What is the frequency of the C R allele in the population? a. 0.2 b. 0.3 c. 0.4 d. 0.5 e

11 8. A population in which two alleles are NOT evolving towards fixation is said to be an example of which of the following? a. disequilibrium b. balanced polymorphism c. monomorphism d. inbreeding depression 9. The degree to which a genotype is being selected against is called the a. selection coefficient. b. mean fitness of the population. c. Darwinian fitness. d. fixation coefficient. 10. The variation that decreases the average fitness of a population is called the a. genetic load. b. mean fitness of the population. c. Darwinian fitness. d. fixation coefficient. Answer Key for Study Guide Questions This answer key provides the answers to the exercises and chapter quiz for this chapter. Answers in parentheses ( ) represent possible alternate answers to a problem, while answers marked with an asterisk (*) indicate that the response to the question may vary b 2. a 3. c 4. d 5. a 6. c gene pool 2. very low 3. Natural selection 7. b (q) % (2pq) 4. genetic drift 5. Nonrandom mating c 2. b 3. a 4. b 5. a 6. c 7. W 8. heterozygote advantage; overdominance 9. negative-frequency dependent selection 10. directional 11. Plasmodium falciparum 321

12 N 2. 1/2N /2N generations (4N) 5. bottleneck 6. founder conglomerate 2. allele frequencies 3. conglomerate 4. migrants d 2. e 3. f 4. a 5. b 6. c 7. g 8. j 9. f 10. d 11. c 12. i 13. e 14. a 15. h 16. b 17. c 18. b 19. a 20. b DNA profiling 2. Microsatellites 3. Exon shuffling 4. minisatellites 5. horizontal gene transfer 6. mutation rate 7. transposable elements Quiz 1. c 2. d 3. c 4. c 5. a 6. b 7. e 8. b 9. a 10. a 511

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