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Chapter 25

Chapter 25 Textbook Notes - Evolutionary Processes

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Fiona Rawle

Notes From Reading CHAPTER 25:E VOLUTIONARY P ROCESSES (PGS.527-550) Key Concepts - The Hardy-Weinberg principle acts as a null hypothesis when researchers want to test whether evolution or nonrandom mating is affecting a particular gene. - There are four evolutionary mechanisms, and each has different consequences. Only natural selection produces adaptation. Genetic drift causes random fluctuations in allele frequencies. Gene flow equalizes allele frequencies between populations. Mutation introduces new alleles. - Inbreeding changes genotype frequencies but does not change allele frequencies. - Sexual selection leads to the evolution of traits that help individuals attract mates. It usually affects the traits of males more strongly than those of females. Introduction - The four mechanisms that cause evolution are natural selection, genetic drift, gene flow, and mutation. - Natural selection increases the frequency of certain alleles. - Genetic drift causes allele frequencies to change randomly. - Gene flow occurs when individuals immigrate into or emigrate from a population. Allele frequencies may change when gene flow occurs. - Mutation modifies allele frequencies by continually introducing new alleles. 25.1 Analyzing change in Allele Frequencies: The Hardy-Weinberg Principle - To study how the four evolutionary processes affect populations, in 1908 G. H. Hardy and Wilhelm Weinberg developed a mathematical model to analyze the consequences of matings among all of the individuals in a population. - To do this, they imagined that all of the gametes produced in each generation go into a single group called a gene pool and then combine randomly. - Their calculations predict the genotypes of the offspring that the population would produce, as well as the frequency of each genotype. The Hardy-Weinberg Principle - They started with the simplest situation, a gene with two alleles, A and A 1 2 - The frequency of A is1represented by p and the frequency of A is repres2nted by q. Because there are only two alleles, p + q = 1. - In this situation, three genotypes are possible: A A ,1A 1 , 1n2 A A . W2a2 will the frequency of these genotypes be in the next generation? - The model predicts that the frequency of the A A geno1y1e in the new generation will be 2 2 p , that of the A 2 2enotype will be q , and that of the A A gen1t2pe will be 2pq. - Because all individuals in the new generation must have one of the three genotypes, the 2 2 sum of the three genotype frequencies must equal 1 (100% of the population): p + 2pq + q = 1. This is the Hardy-Weinberg equation. Notes From Reading CHAPTER 25:E VOLUTIONARY P ROCESSES (PGS.527-550) - When allele frequencies are calculated for this new generation, the frequency of A is still p1 and the frequency of A is 2till q. - The Hardy-Weinberg principle makes two fundamental claims: - (1) if the frequencies of alleles A 1nd A in2a population are given by p and q, then the frequencies of genotypes A A , 1 1 , 1n2 A A wil2 b2 given by p , 2pq, and q for generation after generation; - (2) when alleles are transmitted according to the rules of Mendelian inheritance, their frequencies do not change over time. For evolution to occur, some other factor or factors must come into play. The Hardy-Weinberg Principle: Important Assumptions - The Hardy-Weinberg principle holds when the following five conditions are met with respect to the gene in question: - (1) no natural selection - (2) no genetic drift or random allele frequency changes - (3) no gene flow - (4) no mutation - (5) random mating Hardy-Weinberg Principle: A Null Hypothesis - The Hardy-Weinberg principle serves as a null hypothesis for determining whether evolution is acting on a particular gene in a population. - When genotype frequencies do not conform to Hardy-Weinberg proportions, evolution or nonrandom mating is occurring in that population. - Let’s look at two examples of the use of the Hardy-Weinberg principle as a null hypothesis. Are MN Blood Types in Humans in Hardy-Weinberg Equilibrium? - Most human populations have two alleles for the MN blood group. - The genotype of a person can be determined from blood samples. - Analysis to determine if the Hardy-Weinberg principle holds requires four steps: - (1) estimate genotype frequencies by observation (or testing), - (2) calculate observed allele frequencies from the observed genotype frequencies, - (3) use the observed allele frequencies to calculate the genotypes expected according to the Hardy-Weinberg principle, and - (4) compare the observed and expected values. - The observed and expected MN genotype frequencies were almost identical. - Since the genotypes at the MN locus are in Hardy-Weinberg proportions, evolutionary processes do not currently affect MN blood groups, and mating must be random with respect to this trait. Notes From Reading CHAPTER 25:E VOLUTIONARY PROCESSES (PGS.527-550) Are HLA Genes in Humans in Hardy-Weinberg Equilibrium? - The HLA genes code for proteins that are important in the function of the immune system. To test the hypothesis that heterozygotes for the HLA-A and HLA-B genes might be more fit than homozygotes, researchers used the genotypes of 125 Havasupai tribe members to estimate population allele frequencies. - The expected genotype frequencies did not match the observed frequencies. - Therefore, at least one of the Hardy-Weinberg assumptions must be violated for these alleles in this population. - Mutation, migration, and genetic drift are negligible in this case. - There are two possible explanations for this result: - (1) mating is not random with respect to the HLA genotype, or - (2) heterozygous individuals have higher fitness. - Research continues on this, with no answer apparent yet. 25.2 Types of Natural Selection - Natural selection occurs in a wide variety of patterns. - For example, heterozygote advantage is a pattern of natural selection in which heterozygous individuals have higher fitness than homozygous individuals do. This pattern maintains genetic variation in a population. - Genetic variation refers to the number and relative frequency of alleles that are present in a particular population. - Maintaining genetic variation is important because lack of variation can make populations less able to respond successfully to changes in the environment. Directional Selection - Directional selection is a pattern of natural selection that increases the frequency of one allele. - This type of selection reduces a population’s genetic diversity over time (Figure 25.3). See the example of cliff swallow body size. - If directional selection continues over time, the favored alleles eventually reach a frequency of 1.0, or 100%. Disadvantageous alleles will reach a frequency of 0.0. - Alleles that reach a frequency of 1.0 are said to be fixed. Alleles that are no longer found in the population are said to be lost. Stabilizing Selection - The pattern of natural selection called stabilizing selection occurs when individuals with intermediate traits reproduce more than others, thereby maintaining intermediate phenotypes in a population. Notes From Reading CHAPTER 25:E VOLUTIONARY PROCESSES (PGS.527-550) - An example is the percentage of newborns with various birth weights compared with their mortality rates, as shown in the figure. Those with birth weights in the middle of the range were most likely to survive. - Stabilizing selection reduces a population's genetic variation over time but does not change the average value of a trait over time. Disruptive Selection - In contrast to stabilizing selection, the pattern of natural selection called disruptive selection occurs when intermediate phenotypes are selected against and extreme phenotypes are favored. -
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