Lesson 3 Chapter 26
Lesson 3 Chapter 26 05260
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Date Created: 01/20/16
Evolutionary Processes KEYCONCEPTS The Hardy-Weinberg principle acts as a null hypothesis when researchers want to test whether evolution or nonrandom mating is occurring at a particular gene. Each of the four evolutionary mechanisms has different consequences. Only natural selection produces adaption. 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 is usually stronger on males than females. 26.1 Analyzing Change in Allele Frequencies: The Hardy-Weinberg Principle The Hardy-Weinberg principle played an important role in the synthesis of Mendelian genetics and Darwinian evolution. The Hardy-Weinberg principle can serve as a null hypothesis in evolutionary studies because it predicts what genotype and allele frequencies are expected to be if mating is random with respect to the gene in question and if none of the four evolutionary processes is operating on that gene. You should be able to write out the Hardy-Weinberg equations for allele and genotype frequencies and calculate the genotype frequencies expected if the frequency of A1 is 0.2 26.2 Nonrandom Mating Nonrandom mating changes only genotype frequencies, not allele frequencies, so is not an evolutionary process itself. Inbreeding, or mating among relatives, is a form of nonrandom mating. It leads to an increase in homozygosity. Inbreeding can accelerate natural selection and can cause inbreeding depression. You should be able to predict how extensive inbreeding during the 1700’s and 1800’s affected the royal families of Europe. 26.3 Natural Selection Natural selection is the only evolutionary processes that produces adaptation. Directional selection favors phenotypes at one end of the distribution. It decreases allelic diversity in populations. Stabilizing selection eliminates phenotypes with extreme characteristics. It makes intermediate types more common. It too decreases allelic diversity. Disruptive selection favors extreme phenotypes and thus maintains genetic variation in populations. Disruptive selection sometimes leads to new species forming. Balancing selection occurs when no single phenotype is favored; there is a balance among alleles in terms of fitness and frequency. Balancing selection preserves genetic variation. Sexual selection is a type of natural selection that leads to the evolution of traits that help individuals attract mates. It usually has a stronger effect on males than females. You should be able to explain why natural selection violates the Hardy-Weinberg principle. 26.4 Genetic Drift Genetic drift causes random changes in allele frequencies. Genetic drift is particularly important in small populations, and it tends to reduce overall genetic diversity. Genetic drift can result from random fusion of gametes at fertilization, founder events and population bottlenecks. You should be able to suggest how genetic drift could be important to the management of endangered species 26.5 Gene Flow Gene flow equalizes allele frequencies between populations. Gene flow can introduce alleles from one population to another when individuals migrate among populations. The introduced alleles may have beneficial, neutral or deleterious effect. You should be able to suggest how gene flow could be important to the management of endangered species. 26.6 Mutation Mutation is the only evolutionary process that creates new alleles. They may be beneficial, neutral or deleterious. Mutation occurs too infrequently to be a major cause of change in allele frequency alone, but it is important when combined with natural selection, genetic drift and gene flow. You should be able to suggest how mutation could be important to the management of endangered species. LEARNING OBJECTIVES Test whether evolution or nonrandom mating is occurring at a particular gene, using the Hardy-Weinberg principle. Describe the 4 evolutionary mechanisms, and list whether each one causes adaptation, introduces new alleles, acts randomly or causes genetic variation to increase or decrease. Explain how nonrandom mating can effect genotype frequencies without causing evolution. Define sexual selection. Explain which sex is more strongly affected by sexual selection, and why. 26.1 1 Analyzing Change in Allele Frequencies: The Hardy-Weinberg Principle G.H. Hardy and Wilhelm Weinberg, as well as the general population, believed that allele frequencies occur randomly and don’t change as time goes on. They imagined that all of the alleles from all the gametes produced in each generation go into a single group called the gene pool and then combine at random to form offspring. Two alleles observed were A1 and A2. P represents the frequency of A1 and Q represents the frequency of A2. P + Q = 1 ALWAYS. To calculate the genotype frequencies for A1A1 you square P (PxP). For hybrids A1A2 you multiple 2 times p times q (2pq) for A2A2 you square q ( Q x Q). When you add up all these genotype frequencies, the answer would be 1. Apart from the assumptions seen in the graphic below, the Hardy- Weinberg principle assumes that frequencies do NOT change over time. Here are all the assumptions the Hardy Weinberg model makes: 1. Random mating: Individuals do not choose their own mates based on preferences. 2. No natural selection: Assumed all members survived to go on to reproduce and contributed equal numbers of gametes to the gene pool. 3. No genetic drift: Values were never caused by chance and the frequencies never changed. 4. No gene flow: No new alleles were added or lost by the movement of individuals. 5. No mutation: The model does take into account the possibility of A3 or A4. Biologists often want to test whether nonrandom mating is occurring, natural selection is acting on a particular gene, or one of the other evolutionary processes is at work. In addressing questions like these, the Hardy- Weinberg principle functions as a null hypothesis. This is because it predicts the genotype frequencies without taking into account events that affect these levels and assume random mating is occurring. If the actual levels differ then either nonrandom mating is occurring or allele frequencies are changing for some other reason. Find the values for observed allele frequencies and expected genotype frequencies for the Ainu people of Japan. MM MN NN M N Observed: 0.179 0.502 0.319 0.43 0.57 Expected: 0.185 0.49 0.325 Analyze whether a gene suspected of causing hypertension in humans is in Hardy- Weinberg proportions. In one study, the observed genotype frequencies were A1A1 0.574, A1A2 0.339 and A2A2 0.087. Note that a difference larger than 3 percent is statistically significant. Frequency of A1 is 0.744 and for A2 it is 0.256. Given these allele frequencies, the expected amount for A1A1 is 0.554, a1a2 IS 0.381 and f0r A2A2 its 0.066. Given these calculations, there is most likely something acting to change the frequencies in the population as they are not in Hardy-Weinberg amounts. 26.2 Nonrandom Mating The most intensively studied form of nonrandom mating is called inbreeding, mating between relatives. 1. Inbreeding increases homozygosity. 2. Inbreeding itself does not cause evolution, because allele frequencies do not change in the population as a whole. 3. It only changes genotype frequencies, not allele frequencies, so it is not an evolutionary process itself. Explain how observed genotype and allele frequencies should differ from those expected under the Hardy-Weinberg principle when inbreeding is occurring. The proportions of homozygotes should increase, and the proportions of heterozygotes should decrease, but allele frequencies will not change if no selection is occurring at this locus.4 Even though it does not cause evolution directly-because it does not change allele frequencies-it can speed the rate of evolutionary change. More specifically, it increases the rate at which natural selection eliminates recessive deleterious alleles that lower fitness from the population. Inbreeding depression is the decline in average fitness that takes place when homozygosity increases and heterozygosity decreases in a population. This results from 2 causes: 1. Many recessive alleles represent loss-of-function mutations: Normally few homozygous recessive individuals in a population with these alleles. In heterozygotes they appear to be normal but inbreeding increases the frequency of homozygous recessive individuals. They are usually deleterious or even lethal when homozygous. 2. Many genes-especially those involved in fighting disease-are under intense selection for heterozygote advantage, a selection process that favor genetic diversity: Fitness declines for homozygotes at these genes. This can cause these genes and those individuals to die out. When they are the majority like in inbreeding, the outcomes can be devastating. Another form is sexual selection where a mate, usually the female, chooses a partner based on her own preferences and not at random with respect to certain genes. 26.3 Natural Selection If certain alleles are associated with the favored phenotypes, they increase in frequency while the other alleles decrease in frequency. The result is evolution-a violation of the assumption of hardy-weinberg. Natural selection affects genetic variation in a variety of ways. Directional Selection: The average phenotype shifts to one direction away from the normal distribution. Individuals at the other end of the distribution experience poor reproductive success. It tends to reduce the genetic diversity of populations. If it continues over time, the frequency will approach 1, while disadvantageous alleles will approach 0. Stabilizing Selection: This reduces both extremes in the population and favors an intermediate type. There is no change in the average value of a trait over time, and genetic variation in the population is reduced. The two extremes will experience poor reproductive success. Disruptive Selection: Has the opposite effect of stabilizing selection as it favors the extreme phenotypes, eliminating those at the average. When this occurs the overall amount of genetic variation is increased in the population. It is important as it sometimes plays a part in speciation, or the formation of new species. Balancing Selection: Occurs when no single allele has a distinct advantage. Instead there is a balance among several alleles in terms of their fitness and frequency. This occurs when: 1. Heterozygotes have higher fitness levels than those who are homozygotes. 2. The environment varies over time or in different geographic areas occupied by a population-meaning that certain alleles are favored by selection at different times or in different places, as a result the overall genetic variation is maintained or increased. 3. Certain alleles are favored when they are rare, but not when they are common. As a result overall genetic variation in the population is maintained or increased. No matter how natural selection occurs, though, its fundamental attribute is the same: It increases fitness and leads to adaptation. You should be able to predict how genotype frequencies differ from Hardy Weinberg proportions under directional selection. There will be an excess of the observed genotypes containing the favored allele compared to the proportion expected in the Hardy-Weinberg principle. Sexual Selection: A form on nonrandom mating. In most species, females invest much more in their offspring than do males. If male fitness is limited by access to mates, then any allele that increases a male’s attractiveness to females or success in male-male competition should increase rapidly in the population, violating the assumptions of Hardy-Weinberg. Thus, sexual selection should act more strongly on males than on females. In many species, females prefer to mate with males that care for young or that provide the resources required to produce eggs. Because sexual selection tends to be more intense in males than females, males tend to have many more traits that function only in courtship or male-male competition. These traits that differ between males and females are called sexual dimorphism. They include things like antlers that males use to fight over females. You should be able to define the fundamental asymmetry of sec and explain why males are usually the sex with exaggerated traits used in courtship. Sperm are small and much easier to produce, whereas eggs take a lot of resources to produce. Sperm are so easy to produce that the male’s success is dependent on their ability to find mates and they develop traits that help them solely for that purpose. 26.4 Genetic Drift Genetic drift is defined as any change in allele frequencies in a population that is due to chance. When drift occurs, allele frequencies change due to blind luck- known as a sampling error in statistics. Drift occurs in every population, in every generation, but especially in small populations. Genetic drift is more pronounced in small populations than in large ones. 1. Genetic drift is random with respect to fitness. The changes it produces in allele frequency that it produces are not adaptive. 2. Genetic drift is most pronounced or apparent in small populations. The changes in allele frequency seem time in a population of several thousand but can be enormous in a population of only four individuals. 3. Over time, genetic drift can lead to the random loss or fixation of alleles. In the computer simulation we saw that it took at most 20 generations for an allele to be either lost entirely or be fixated on, meaning almost every individual had that trait. This is especially important in endangered species as a loss of genetic diversity would make their dark situation even darker. This can be seen in an experiment where they take flies and mate them repeatedly in a controlled environment and measure the amount of genetic variation. They started off the group with a very small population at first as that would greater the chance that genetic drift would cause alleles to change and shift towards one end. As predicted, genetic drift decreased genetic variation within populations and increased genetic differences between populations. The two causes of genetic drift in natural populations are the founder effect and the genetic bottleneck. The founder effect occurs after a founder event. It is a change in allele frequencies that occurs when a new population is established. A genetic bottleneck is a sudden reduction in the number of alleles in a population. The results of both are likely to have frequencies occurring that are different from the source they came from or originally were. Explain why genetic drift leads to a random loss or fixation of alleles. When the frequencies fluctuate randomly, sooner or later one of the frequencies will hit either 1 or 0. Thus, one is fixated on and the other is lost. Explain why genetic drift is particularly important as an evolutionary force in small populations. In tiny populations, sampling errors are large. The accidental death of a few individuals would have an enormous impact on allele frequencies. 26.5 Gene Flow Gene flow is the movement of alleles between populations, whether individuals leave or go. As an evolutionary process, gene flow usually has one outcome: it equalizes allele frequencies between the source population and the recipient population. When the alleles move from one population to another, those two groups tend to become more alike. Gene flow is random with respect to fitness-the arrival or departure of alleles can increase or decrease average fitness, depending on the situation. But in every case, movement of alleles between populations tends to reduce their genetic differences. 26.6 Mutation Mutations occur in a number of ways: 1. Point mutations: a stretch of DNA will code for an entirely new amino acid sequence and affect the expression of other alleles. 2. Chromosome-level mutations: Gene duplication can occur and result in another chromosome. This can lead to a loss of function or the creation of new alleles. 3. Lateral gene transfer: transfer of genes from one species to another rather than from one parent to offspring. Mutation is an evolutionary process that increases genetic diversity in populations. Changes in the makeup of chromosomes or in specific DNA sequence do not always increase fitness or decrease it. Mutation is random and just happens. As an evolutionary process, mutations is slow compared with selection, genetic drift and gene flow. However, it can have a very large effect on evolution when combined with genetic drift, gene flow and selection. In a lab we saw that fitness increased in fits and starts and resembled a stair like pattern. 1. Mutation is the ultimate source of genetic variation. It is the only source of new alleles. 2. If mutation did not occur, evolution would eventually stop. 3. Mutation alone is usually inconsequential in changing allele frequencies at a particular gene.
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