GENETICS A Conceptual Approach Benjamin A. Pierce GENETICS A Conceptual Approach FIFTH EDITION CHAPTER 25 Population Genetics © 2014 W. H. Freeman and Company

Rocky Mountain bighorn sheep (Ovis canadensis) Rocky Mountain bighorn sheep (Ovis canadensis). A population of bighorn sheep at the National Bison Range suffered the loss of genetic variation due to genetic drift; the introduction of sheep from other populations dramatically increased genetic variation and the fitness of the sheep.

Figure 25. 1 All organisms exhibit genetic variation Figure 25.1 All organisms exhibit genetic variation. (a) Extensive variation among humans. (b) Variation in the spotting patterns of Asian lady beetles. [Part a: Michael Dwyer/Alamy.]

25.1 Genotypic and Allelic Frequencies Are Used to Describe the Gene Pool Of a Population Calculating genotypic frequencies Number of individuals possessing the genotype divided by total number of individuals in sample f(AA) = # AA individuals/N N: total # of individuals f: frequency each genotype. Calculating allelic frequencies Number of copies of a particular allele present in a sample divided by total number of alleles frequency of an allele = # copies of the alleles/# copies of all alleles at the locus in a population. Loci with multiple alleles and X-linked loci calculated similarly

25.2 The Hardy–Weinberg Law Describes the Effect of Reproduction on Genotypic and Allelic Frequencies Hardyn–Weinberg Law Assumption: population is large, randomly mating, not affected by mutation, migration, or natural selection. Prediction 1: the allelic frequencies of a population do not change. Prediction 2: the genotypic frequencies stabilize. p2: homozygous dominant allele pair frequency q2: homozygous recessive allele pair frequency 2pq: heterozygous allele frequency

Figure 25.2 Random mating produces genotypes in the proportions p2, 2pq, and q2.

25.2 The Hardy–Weinberg Law Describes the Effect of Reproduction on Genotypic and Allelic Frequencies Closer Examination of the Assumptions of the Hardy–Weinberg Law Implications of the Hardy–Weinberg Law

Figure 25.3 When a population is in Hardy–Weinberg equilibrium, the proportions of genotypes are determined by the frequencies of alleles.

25.2 The Hardy–Weinberg Law Describes the Effect of Reproduction on Genotypic and Allelic Frequencies Extensions of the Hardy–Weinberg Law Testing for Hardy–Weinberg Proportions Estimating Allelic Frequencies with the Hardy–Weinberg Law

Concept Check 1 What is the expected frequency of heterozygotes in a population with allelic frequency X and Y that is in Hardy–Weinberg? X + Y XY 2XY (X - Y)2

Concept Check 1 What is the expected frequency of heterozygotes in a population with allelic frequency X and Y that is in Hardy-Weinberg? 2XY X + Y XY (X - Y)2

25.3 Nonrandom Mating Affects the Genotypic Frequencies of a Population Positive assortative mating: a tendency of like individuals to mate. Negative assortative mating: a tendency of unlike individuals to mate. Inbreeding vs. Outcrossing Inbreeding Coefficient Inbreeding depression

Figure 25. 4 Individuals may be homozygous by descent or by state Figure 25.4 Individuals may be homozygous by descent or by state. Inbreeding is a measure of the probability that two alleles are identical by descent.

Figure 25.5 Inbreeding increases the percentage of homozygous individuals in a population.

Figure 25. 6 Inbreeding often has deleterious effects on crops Figure 25.6 Inbreeding often has deleterious effects on crops. As inbreeding increases, the average yield of corn, for example, decreases.

Concept Check 2 What is the effect of outcrossing on a population? Allelic frequency changes. There will be more heterozygotes than predicted by the Hardy–Weinberg law. There will be fewer heterozygotes than predicted by the Hardy–Weinberg law. Genotypic frequencies will equal those predicted by the Hardy–Weinberg law.

Concept Check 2 What is the effect of outcrossing on a population? Allelic frequency changes. There will be more heterozygotes than predicted by the Hardy–Weinberg law. There will be fewer heterozygotes than predicted by the Hardy–Weinberg law. Genotypic frequencies will equal those predicted by the Hardy–Weinberg law.

25.4 Several Evolutionary Forces Potentially Cause Changes in Allelic Frequencies Mutation The effect of mutation on allelic frequencies Reaching equilibrium of allelic frequencies Summary effects Migration (gene flow) The effect of migration on allelic frequencies The overall effect of migration

Figure 25. 8 Recurrent mutation changes allelic frequencies Figure 25.8 Recurrent mutation changes allelic frequencies. Forward and reserve mutations eventually lead to a stable equilibrium.

Figure 25.9 Change due to recurrent mutation slows as the frequency of p drops. Allelic frequencies are approaching mutational equilibrium at typical low mutation rates. The allelic frequency of G1 decreases as a result of forward (G1 → G2) mutation at rate m (0.0001) and increases as a result of reverse (G2 → G1) mutation at rate n (0.00001). Owing to the low rate of mutations, eventual equilibrium takes many generations to be reached.

Figure 25.10 The amount of change in allelic frequency due to migration between populations depends on the difference in allelic frequency and the extent of migration. Shown here is a model of the effect of unidirectional migration on allelic frequencies. The frequency of allele a in the source population (population I) is qI.The frequency of this allele in the recipient population (population II) is qII.

Genetic Drift; changes in allele frequency 25.4 Several Evolutionary Forces Potentially Cause Changes in Allelic Frequencies Genetic Drift; changes in allele frequency The magnitude of genetic drift Causes of genetic drift Founder effect Genetic bottleneck The effects of genetic drift

Figure 25.11 Populations diverge in allelic frequency and become fixed for one allele as a result of genetic drift. In Buri’s study of two eye-color alleles (bw75 and bw) in Drosophila, each population consisted of eight males and eight females and began with the frequency of bw75 equal to 0.5.

Figure 25.13 Genetic drift changes allelic frequencies within populations, leading to a reduction in genetic variation through fixation and genetic divergence among populations. Shown here is a computer simulation of changes in the frequency of allele A2 (q) in five different populations due to random genetic drift. Each population consists of 10 males and 10 females and begins with q = 0.5.

Fitness and the selection coefficient The general selection model 25.4 Several Evolutionary Forces Potentially Cause Changes in Allelic Frequencies Natural Selection Fitness and the selection coefficient The general selection model The results of selection Directional selection Overdominance vs. underdominance Change in allele frequency of a recessive allele due to natural selection

Figure 25.14 Natural selection produces adaptations, such as those seen in the polar bears that inhabit the extreme Arctic environment. These bears blend into the snowy background, which helps them in hunting seals. The hairs of their fur stay erect even when wet, and thick layers of blubber provide insulation, which protects against subzero temperatures. Their digestive tracts are adapted to a seal-based carnivorous diet. [Digital Vision.]

Figure 25.15 Mutation, migration, genetic drift, and natural selection have different effects on genetic variation within populations and on genetic divergence between populations.

Concept Check 3 The average number of offspring produced by three genotypes are: GG = 6; Gg = 3; gg = 2. What is the fitness of Gg? 3 0.5 0.3 0.27

Concept Check 3 The average number of offspring produced by three genotypes are: GG = 6; Gg = 3; gg = 2. What is the fitness of Gg? 3. 0.5 0.3 0.27