Study Guide Chapter 23

The Evolution of Populations Interactive Question 23.1 In a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous recessive (bb). a. The allele frequencies of this population are __ B allele __ b allele. The 98 BB mice contribute 196 B alleles, and the 84 Bb mice contribute 84 B alleles to the gene pool. These 84 Bb mice also contribute 84 b alleles, and the 18 bb mice contribute 36 b alleles. Of a total of 400 alleles, 280 are B and 120 are b. Allele frequencies are 0.7 B and 0.3 b. b. The genotype frequencies of this population are __ BB __ Bb __ bb. The frequencies of genotypes are 0.49 BB, 0.42 Bb, and 0.09 bb (98/200, 84/200, 18/200). Interactive Question 23.2 Use the allele frequencies you determined in Interactive Question 23.1 to predict the genotype frequencies of the next generation. Frequencies of B (p) = _0.7_ b (q) = _0.3_ BB (p) = _0.49_ Bb (2pq) = _0.42_ bb (q) = _0.09_ Interactive Question 23.3 Practice using the Hardy-Weinberg equation so that you can easily determine genotype frequencies from allele frequencies and vice versa. a. The allele frequencies in a population are A = 0.6 and a = 0.4. Predict the genotype frequencies for the next generation. 0.36; 0.48; 0.16. Plug p (0.6) and q (0.4) into the expanded binomial: p + 2pq + q. b. What would the allele frequencies be for the generation you predicted above in part a.? 0.6; 0.4. Add the frequency of the homozygous dominant genotype to the frequency of the heterozygote for the frequency of p. For q, add the homozygous recessive frequency and the heterozygote frequency. Alternatively, to determine q, take the square root of the homozygous recessive frequency if you are sure the population is in Hardy-Weinberg equilibrium. The frequency of p is then 1 q. Interactive Question 23.4 a. What is a major source of genetic variation for bacteria and viruses? Mutation, with some recombination. b. What is the major source of genetic variation for plants and animals? Sexual recombination. c. Explain why your answers to a. and b. are different. Bacteria and viruses have very short generation times and a new beneficial mutation can increase in frequency rapidly in an asexually reproducing bacterial population. Although mutations are the source of new alleles, they are so

infrequent that their contribution to genetic variation in a large, diploid population is minimal. However, sexual recombination in the production and union of gametes produces zygotes with fresh combinations of alleles each generation. Interactive Question 23.5 Fill in the following concept map that summarizes three causes of microevolution. Better still, create your own concept map to help you review the ways in which a populations genetic composition may be altered. a. Natural Selection b. Genetic Drift c. Gene Flow d. Better Reproductive Success e. Small Population f. Bottleneck Effect g. Founder Effect h. Genetic Variation Between Populations Interactive Question 23.6 a. Do humans have more or less genetic variation than most species? Less; we have about a tenth of the variability found in fruit fly populations. b. Two humans differ by about what percentage of their nucleotide bases? Humans have about a 0.1% nucleotide diversity. Interactive Question 23.7 a. A gene locus has two alleles, B and b. The genotype BB has a relative fitness of 0.5 and bb has a relative fitness of 0.25. What is the relative fitness of the genotype Bb? The two given fitness values are less than 1, so Bb must produce the most offspring and have a relative fitness of 1. b. What is the relative fitness of a sterile animal? 0 Interactive Question 23.8 a. Why is the highly deleterious sickle-cell allele still present in the gene pool of the U.S. population? Diploidy The sickle-cell allele is hidden from selection in heterozygotes. b. Why is this allele at such a relatively high frequency in the gene pool of some African populations? Heterozygote advantage Heterozygotes are resistant to malaria and have a selective advantage in areas where malaria is a major cause of death. Suggested Answers to Structure your Knowledge 1. a. What is the Hardy-Weinberg theorem? The Hardy-Weinberg theorem states that allele frequencies within a population will remain constant from one generation to the next as long as only Mendelian segregation and sexual recombination of alleles are involved. An equilibrium in which neither allele nor genotype frequencies will change from generation to

generation requires five conditions: The population is large, mutation and migration are negligible, mating is random, and no selective pressure operates. This Hardy-Weinberg equilibrium provides a null hypothesis to allow population geneticists to test for evolution. b. Define the variables of the equation for Hardy-Weinberg equilibrium. Make sure you can use this equation to determine allele frequencies and predict genotype frequencies. p + 2pq + q = 1. In the Hardy-Weinberg equation, p and q refer to the frequencies of two alleles in the gene pool. The frequency of homozygous offspring is (p x p) or p and (q x q) or q. Heterozygous individuals can be formed in two ways, depending on whether the ovum or sperm carries the p or q allele, and their frequency is equal to 2pq. 2. It seems that natural selection would work toward genetic unity; the genotypes that are most fit produce the most offspring, increasing the frequency of adaptive alleles and eliminating less beneficial alleles from the population. Yet there remains a great deal of variability within populations of a species. Describe some of the factors that contribute to this genetic variability. Genetic variation is retained within a population by diploidy and balancing selection. Diploidy masks recessive alleles from selection when they occur in the heterozygote. Thus, less adaptive or even harmful alleles are maintained in the gene pool and are available should selection pressures change. Balanced selection maintains several alleles at a gene locus in a population and leads to balanced polymorphism. In situations in which there is heterozygote advantage, the two alleles will be retained in stable frequencies within the gene pool. Frequencydependent selection, in which morphs present in higher numbers are selected against by predators or other factors, is another cause of balanced polymorphism. Test Your Knowledge MULTIPLE CHOICE: Choose the one best answer. 1. Which of the following determines an organisms fitness? e. number of viable offspring (Page 464) 2. According to the Hardy-Weinberg theorem, a. the allele frequencies of a population should remain constant from one generation to the next if the population is large and only sexual recombination is involved. (Page 456) 3. If a population has the following genotype frequencies, AA = 0.42, Aa = 0.46, and aa = 0.12, what are the allele frequencies? c. A = 0.65 a = 0.35 (Page 457) 4. In a population with two alleles, B and b, the allele frequency of b is 0.4. What would the frequency of heterozygotes if the population is in Hardy-Weinberg equilibrium? c. 0.48 (Page 457) 5. In a population that is in Hardy-Weinberg equilibrium for two alleles, C and c, 16% of the population show a recessive trait. Assuming C is dominant to c, what percent show the dominant trait? d. 84% (Page 457) 6. Genetic drift is likely to be seen in a population

d. that is very small (Page 460) 7. Gene flow often results in e. a reduction of the allele frequency differences between populations. (Page 462) 8. The existence of two distinct phenotypic forms in a species is known as d. polymorphism. (Page 463) 9. The average heterozygosity of Drosophila is estimated to be about 14%, which means that b. on average, 14% of a fruit flys gene loci are heterozygous. (Page 457) 10. Mutations are rarely the cause of microevolution in eukaryotes because e. of all of the above. (Page 459) a. they are most often harmful and do not get passed on. b. they do not directly produce most of the genetic variation present in a diploid population. c. they occur very rarely. d. they are only passed on when they occur in gametes. 11. In a study of a population of field mice, you find that 48% of the mice have a coat color that indicates that they are heterozygous for a particular gene. What would be the frequency of the dominant allele in this population? e. You cannot estimate allele frequency from this information. (Page 457) 12. In a random sample of a population of shorthorn cattle, 73 animals were red (CRCR), 63 were roan, a mixture of red and white (CRCr), and 13 were white (CrCr). Estimate the allele frequencies of CR and Cr, and determine whether the population is in HardyWeinberg equilibrium. c. CR = 0.7, Cr = 0.3; the genotype ratio is close to what would be predicted from these frequencies and the population is in equilibrium. (Page 457) 13. A scientist observes that the height of a certain species of asters decreases as the altitude on a mountainside increases. She gathers seeds from samples at various altitudes, plants them in a uniform environment, and measures the height f the new plants. All of her experimental asters grow to approximately the same height. From this she concludes that d. the height variation she initially observed was an example of nongenetic environmental influence. (Page 464) 14. Sexual selection will a. select for traits that enhance and individuals chance of mating. (Page 468) 15. The greatest source of genetic variation in plant and animal populations is from b. sexual recombination (Page 456) 16. A plant population is found in an area that is becoming more arid. The average surface area of leaves has been decreasing over the generations. This trend is an example of b. directional selection (Page 465) 17. Mice that are homozygous for a lethal recessive allele die shortly after birth. In a large breeding colony of mice, you find that a surprising 5% of all newborns die from this trait. In checking lab records, you discover that the same proportion of offspring have been dying from this trait in this colony for the past three years. (Mice breed several times a year and have large litters.) How might you explain the persistence of this lethal allele at such a high frequency? c. There is some sort of heterozygote advantage and perhaps selection against the homozygous dominant trait. (Page 466)

18. In breeding experiments with Drosophila, you count the offspring produced by each of three different genotypes and determine that flies with the genotype AA have a relative fitness of 1. What does that mean? b. AA flies have a lower fitness than do flies that are Aa or aa. (Page 464) 19. All of the following would tend to maintain balanced polymorphism in a population except b. directional selection. (Page 466) 20. Genetic analysis of a large population of mink inhabiting an island in Michigan revealed an unusual number of loci where one allele was fixed. Which of the following is the most probably explanation for this genetic homogeneity? c. A very small number of mink may have colonized this island, and this founder effect and subsequent genetic drift could have fixed many alleles. (Page 460) 21. Directional selection would be most likely to occur when a. a populations environment has undergone a change. (Page 465) 22. If an allele is recessive and lethal in homozygotes, e. Both c and d are correct. (Page 464) c. the relative fitness of the homozygous recessive genotype is 0. d. the allele will most likely remain in the population at a low frequency because it cannot be selected against when in a heterozygote. 23. Sexual reproduction may be maintained by natural selection because d. maintaining a high variability in a population for traits such as cell surface markers protect against pathogens such as viruses and bacteria. (Page 468) 24. Humans have an estimated 1,000 olfactory receptor genes. This is most likely an example of c. gene duplication. (Page 459) 25. Which of these types of selection is mismatched with its example? e. stabilizing the frequency of A, B, AB, and O blood groups remains constant in a population (Page 465)