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Chapter 25

BIO 1M03 Chapter 25

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McMaster University
Jon Stone

Chapter 25: Evolutionary Process  Four mechanisms that shift allele frequency: 1. Natural selection o Increases the frequency of certain alleles – pertaining to those species that survive and reproduce 2. Genetic Drift o Cause allele frequencies to change randomly 3. Gene flow o When one individual leaves a population and then migrate to another population and then breed. o Allele frequency may change i. Arriving individuals introduce alleles to the new population ii. Departing individuals remove alleles from the old population 4. Mutation o Modifies allele frequencies by continually introducing new alleles o Alleles created by mutation may be harmful, beneficial or simply have no effects on the fitness 25.1: Analyzing Change in Allele Frequencies: The Hardy-Weinberg Principle (pg.528)  To study how the four evolutionary processes affect the population, biologists take a three-way approach. 1. Create mathematical models that track the fate of alleles over time 2. Collect data and test the predictions made by the models’ equation 3. Apply the results to solve problems in human genetics conservation of endangered species, or other fields  Hardy and Weinberg examined what happens to a the frequencies of alleles when many individuals in a population mate and produce offspring o Wanted to know all the possible genotypes produced in a entire population  To analyze all the consequences of mating among the population, Hardy and Weinberg invented a novel approach o Gene pool  Imagined all the gametes produced in each generation go into a single group then combined at random to form offspring  Determining genotypes present in the next generation o Hardy and Weinberg calculated what happened when two gametes were removed at random out of the gene pool o Calculations help predict genotype of offspring and genotypic frequency 1 Biology Chapter 25  Frequency of: o A A 1e1otype is p (Homozygous dominant) o A A 1e2otype is 2pq (2eterozygous) o A A 2e2otype is q (Homozygous recessive)  The genotype frequencies in the offspring generation must add up to 1. Therefore, p + 2pq + q = 1 o Dominant allele do not increase in frequency  Ex. A i1 dominant to A , 2t does not increase in the frequency o Hardy-Weinberg Principle 1. If the frequencies of alleles A and A are given, then the 1 2 fr2quencie2 of genotypes A A 1 A1A ,1an2 A A wi2l 2e given by p , 2pq, q for generations after generations 2. Frequencies of alleles do not change when they are transmitted according to the rules of Mendelian inheritance. In order for evolution to occur, some other factor or factors must come into play If given the Genotype Frequencies and you want to find Allele Frequencies (P&Q) and Expected value is not given, you: o Take the observed values of PP, PQ, QQ and plug it into the following formula  P= PP + ½ (PQ)  Q = QQ + ½ (PQ) o To find expected value:  PP = (P )  PQ = 2(PQ) 2  QQ = (Q ) The Hardy-Weinberg Model Males Important Assumptions (pg. 528)  Five conditions that must be met in respect to the gene in question: 1. No natural selection at the gene in question 2. No genetic drift, or random allele frequency changes, affecting the gene in question 3. No gene flow o No new alleles were added by immigration or lost through emigration anywhere. Refer to figure 25.1 pg. 529 4. No mutations 5. Random mating with respect to the gene in question  This principle tells us what to expect if the first 4 conditions are not affecting a gene, and if mating is random with respect to that gene Biology Chapter 25 2 How Does the Hardy-Weinberg Principle Serve as a Null Hypothesis? (pg. 530)  Biologist wants to test whether natural selection is acting upon a particular gene, nonrandom mating is occurring, or one of the other evolutionary mechanisms is at work  If biologists observe genotype frequencies that do not follow Hardy-Weingberg prediction, it means that something is going on. o Either nonrandom mating is occurring, or allele frequencies are changing  ****REFER TO MN BLOOD GROUP OF HUMANS FIGURE 25.1 PG. 530****  Principle is used to test the hypothesis that currently no evolution is occurring at a particular gene and that mating was random with respect to the gene in question Are HLA Genes in Humans in Hardy-Weinberg Equilibrium? (pg. 531)  Past work has displayed that different alleles exist at both the HLA-A and HLA-B genes  Alleles at each gene code for proteins that recognize slightly different disease- causing organisms  HLA alleles are CODOMINANT o Meaning that both are expressed and create the phenotype  A hypothesis was devised that individuals who are heterozygous at one or both of these genes may have a strong fitness advantage  Based on the observation in figure 25.2, there is a assumption that one of Hardy- Weinberg principle was being violated o Researchers argued that “mutation, migration and drift negligible (not worth considering)  Two factors are being consider for the results for figure 25.1 1. Mating may not be random with respect to the HLA genotype. o People may subconsciously prefer mates opposite to their genotype. Ex. Male-HLA Females-HLB. Therefore increase in heterozygous offspring. 2. Heterozygous individuals may have a higher fitness o When two spouses share common related alleles, there is a higher rate of abortion and decrease in pregnancy than spouses who have different alleles. o Balancing selection occurs when heterozygous individuals have a higher fitness than homozygous individuals  Based on these two explanations for the results in figure 25.1, it is possible that both explanations are correct. Using the Hardy-Weinberg principle as a null hypothesis allows biologists to detect an interesting pattern in a natural population Biology Chapter 25 3 25.2: Types of Natural Selection (pg. 532)  Natural selection occurs when individuals with certain phenotypes produce more offspring than individuals with other phenotypes do o Favoured alleles increase in frequency while unflavoured alleles decrease in frequency o Heritable variation leads to success in survival and reproduction  Figure 25.2 is a pattern of natural selection called balancing selection/heterozygote advantage o Heterozygote advantage is a pattern of natural selection that favours heterozygotes individuals compared to homozygotes  Genetic Variation o Refers to the number and relative frequency of alleles that present on a particular population  If the genetic variation is low and the environment undergo changes such as new diseases, virus, change in climate, resources (food), then it is unlikely that any alleles will be present that have a high fitness under the new conditions Directional Selection pg. 532  The antibiotic resistance bacterium and the change in the beak shape and body size of finches (Refer to chapter 24) are examples of Directional selection o Directional selection is a type of natural selection that changes the population of a average phenotype in one direction o Tends to reduce the genetic diversity of populations o If directional selection continues over time, the favoured allele will eventually reach a frequency of 1 while unflavoured allele will reach a frequency of 0  Referring to Figure 25.3a illustrates how the directional selection works when the trait in question has a bell-shaped, normal distribution in all population  Purifying selection is said to occur when unflavoured alleles decline in frequency o Allele frequency of 1 is said to be “fixed” o Allele frequency of 0 is said to be “lost”  It is common to find that a cause of directional selection on a trait is evened out by a different factor that causes selection in the opposite direction Stabilizing Selection pg. 533  Pertaining to cliff swallow in Figure 25.4a, they were exposed to cold weather. Selection greatly reduced one extreme in the range of phenotypes and resulted in a directional change in the average characteristics of the population Biology Chapter 25 4  However selection can affect both extremes in a population refer to Figure 25.4a o This selection is known as stabilizing selection  Favouring phenotypes near the average value and eliminating extreme phenotypes. This is known as stabilizing selection  Stabilizing selection has 2 important consequences 1. There is no change in the average value of a trait over time 2. Genetic variation in the population is reduced Disruptive Selection pg. 534  Disruptive selection has the opposite effect of stabilizing selection o It eliminates phenotypes near the average value and favours extreme phenotypes. (Refer to Figure 25.5a) o Genetic variation in a population is maintained  Disruptive selection plays a role in speciation o Means the formation of new species  Ex. When two small beaked birds mate to form offspring with small beaks while at the same time, large beaked birds mate and form offspring with large beaks. Over time a new specie may be formed Biology Chapter 25 5 25.3: Genetic Drift (pg. 535)  Natural selection is not random. It is directed by the environment and results in adaptation  Genetic drift is in contrast is undirected o It describes that the change in allele in a given population is all due to chance  Sampling error is the accidental selection of a nonrepresentative sample from a larger population, due to chance o When genetic drifts occur, allele frequencies change due to “blind luck”  Random chance causes a change of allele frequencies in a population. This will cause evolution to occur  Genetic drift is random with respect to fitness. The allele frequency changes it produces are not adaptive  Genetic drift is most pronounced in small populations o The smaller the sample, the larger the sampling error  When random lost or fixation occurs, genetic variation in population declines Experimental Studies of Genetic Drift (pg.536)  Genetic marker is a specific allele that causes a distinctive phenotype  As predicted, genetic drift decreased genetic variation within populations and increased genetic differences between populations  Is genetic drift important in natural po
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