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Chapter 25.docx

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McMaster University
Jon Stone

Chapter 25 Population is a group of individuals from the same species that live in the same area and regularly interbreed. Natural selection is not the only process that causes evolution, however. There are actually four mechanisms that shift allele frequencies in populations: 1. Natural selection increases the frequencies of certain alleles—the ones that contribute to reproductive success in a particular environment. 2. Genetic drift causes allele frequencies to change randomly. In some cases, drift may cause alleles that decrease fitness to increase in frequency 3. Gene flow occurs when individuals leave one population, join another, and breed. Allele frequencies may change when gene flow occurs, because arriving individuals introduce alleles to their new pouation and departing individuals remove alleles from their old population. 4. Mutation modifies allele frequencies by continually introducing new alleles. The alleles created by mutation may be beneficial or detrimental or have no effect on fitness Natural selection is not the only agent responsible for evolution, and each of the four evolutionary processes has different consequences. Analyzing change in Allele Frequencies: the hardy-weinberg principle Gene pool: they imagined that al of the gametes produced in each generation go into a single group, and then combine at random to form offspring. To determine which genotypes would be present in the next generation and in what frequency, Hardy and Weinberg simply had to calculate what happened when two gametes were plucked at random out of the gene pool, many times, and each of these gamete pairs was then combine to form offspring. The principle makes two fundamental claims: 1. If the frequencies of alleles A1 and A2 in a population are given by p and q, then the frequencies of genotypes A1A1, A1A2, and A2A2 will be given by p2,2pq, and q2 for generation after generation. 2. When alleles are transmitted via meiosis and random combination of gametes, their frequencies do not change over time. For evolution to occur, some factor or factors must come into play. Specifically, for a population to conform to the Hardy-Winberg principle, none of the four mechanisms of evolution can be acting out the population. In addition, the model assumes that mating is random with respect to the gene in question. 1. No natural selection at the gene in question; 2. No genitive drift, or random allele frequency changes, afftecting the gene in question;3, no gene flow; 4, no mutation; random mating with respect to the gene in question. Biologist often want to test whether natural selection is acting on a particular gene, non-random mating is occurring, or one of the other evolutionary mechanisms is at work. In addressing questions like these, the Hardy-weinberg principle functions as a null hypothesis. Case 1: The Hardy-Weinberg principle is used to test the hypothesis that currently no evolution is occurring at a particular gene and that in the previous generation, mating was random with respect to the gene in question. Types of Natural selection: Direction selection: the average phenotypes of the populations changed in one direction. If directional selection continues over time, the favoured alleles will eventualy approach a frequency of 1.0, while disadvantageous alleles will approach a frequency of 0.0. Aelles that reach a frequency of 1.0 are said to be fixed; those that reach a frequency of 0.0 are said to be lost. When disadvantageous alleles decline in frequency, purifying selection is said to occurred Stabilizing selection: reduce both extremes in a population There is no change in te average value of a trait over time, and genetic variation in the population is reduced. Disruptive selection: opposite effect of stabilizing selection It eliminiates phenotypes near the average vaue and favours extreme phenotypes. The overall amount of genetic variation in the population is maintained. Disruptive section plays a part in speciation, or the formation of new species. Heterozygote advantage,: when selection operates in the say that heterozygous individuals have higher fitness than homozygous individuals do. The consequence of this pattern is that genetic variation is maintained in population. Heterzygote advantage is one mechanism responsible for a more general phenomenon known as balancing selection (No single allele has a distinct advantage and increases in frequency.. There is a balance among several alleles in terms of their fitness and frequency. Occurs when:  the environment varies over time or in different geographic ares occupied by a population--- overall genetic variation in the population is maintained or increased.  Certain alleles are favoured when they are rare, but not when they are common, which is know as a pattern called frequency-dependent selection. No matter how natural selection occurs, though, its more fundamental attribute is the same: it increases fitness and leads to adaptation. Genetic Drift. Natural selection is directed by the environement and results in adapation. Genetic drift, in contrast, is undirected and random. Genetic drift is defined as any change in allele frequencies in a population that is due to chance. The process is aptly named, because it causes alleles frequencies to drift up and down randomy over time. When drift occurs, allel frequencies change due to blind luck---what is formally known as sampling error. Drift occurs in every population, in every generation. Through computer simulation, genetic drift is more pronounced in small population than large population. Key points about genetic drift:  Genetic drift is random with respect to fitness;  Genetic drift is most pronounced in small populations;  Overt
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