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chapter 26 outline (part 1)

by: Anna Perry

chapter 26 outline (part 1) BIOSC 0160

Marketplace > University of Pittsburgh > Biology > BIOSC 0160 > chapter 26 outline part 1
Anna Perry
GPA 3.5
Foundations of Biology 2
Dr. Swiganova

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About this Document

Basically what she said in class so far and an outline of the book of what we've gotten through. Ill continue after we cover more
Foundations of Biology 2
Dr. Swiganova
Class Notes
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This 9 page Class Notes was uploaded by Anna Perry on Sunday February 22, 2015. The Class Notes belongs to BIOSC 0160 at University of Pittsburgh taught by Dr. Swiganova in Winter2015. Since its upload, it has received 48 views. For similar materials see Foundations of Biology 2 in Biology at University of Pittsburgh.


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Date Created: 02/22/15
Chapter 26 Evolutionary Processes Analyzing Change in Allele Frequencies The HardyWeinberg Principle A The HardyWeinberg principle serves as a mathematical null hypothesis for the study of evolutionary processes 1 In a large randomly breeding population allelic frequencies will remain the same from generation to generation assuming that there is no mutation gene migration selection or genetic drift 2 Used to determine genetic frequencies in a population a If no evolution is present it stays constant B Hardy and Weinberg were engaged in population thinking A population is a group of individuals from the same species that live in the same area at the same time and can interbreed and that vary in the traits they possess C The Gene Pool Concept 1 To analyze the consequences of matings among all of the individuals in a population Hardy and Weinberg invented a novel approach a 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 2 To determine which genotypes would be present in the next generation and in what frequencies HampW calculated what happens when two gametes are plucked at random out of the gene pool many times and each of these gamete pairs then combine to form offspring a This could predict the genotypes of the offspring that would be produced and the frequency of each genotype A1 and A2 b p is used to symbolize the frequency of A1 alleles in the gene pool c q is used to symbolize the frequency of A2 alleles in the same gene pool D The HW Principle Makes 2 General Claims 1 If the frequencies of alleles A1 and A2 in a population are given by p and q then the frequencies of genotypes AJAJ AJAZ and A2A2 will be given by p2 2pq and q2 respectively for generation after generation a Allele frequencies p q 1 b Genotype frequencies p2 2pq q2 1 When alleles are transmitted via meiosis and random combinations of gametes their frequencies do not change over time a For evolution to occur some other factors must come into play E The HW Model Makes Important Assumptions 1 These assumptions helped to de ne the four processes of evolution that can be acting on the population In addition the model assumes that mating is random with respect to the gene in question Five assumptions that must be met a Random Mating b No Natural Selection c No Genetic Drift Random Allele Frequency Changes d No Gene Flow e No Mutations How Does the HW Principle Serve as a Null Hypothesis 1 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 HW principle functions as a null hypothesis a A null hypothesis predicts there are no differences among the treatment groups in an experiement b If biologists observe the genotype frequencies that do not conform to the HW prediction it means that something interesting is going on i Nonrandom mating is occurring ii Allele frequencies are changing for some reason Case Study 1 Are MN Blood Types in Humans in HW Proportions a The MN gene codes for a protein found on the surface of red blood cells i The Maee codes for the M version of blood ii The Naee codes for the N version iii The Mand Naees are codominant b Researchers could determine whether individuals are MM MN or NN by treating blood samples with antibodies to each protein c To estimate the frequency of each genotype in a population geneticists obtain data from a large number of individuals and then divide the number of individuals with each genotype by the total number of individuals in the sample d The analysis is based on 4 steps i Step 1 Estimate genotype frequencies by observation in this case by testing many blood samples for the M and N alleles Observed ii Step 2 Calculate observed allele frequencies from the observed genotype frequencies In this case the frequency of the Mallele is the frequency of the MM homozygotes plus half the frequency of MN heterozygotes iii Step 3 Use the observed allele frequencies to calculate the genotypes expected according to the HW principle Under the null hypothesis of random mating and no evolution the expected genotype frequencies are p22pqq2 iv Step 4 Compare the observed and expected values Researchers use statistical tests to determine whether the differences between the observed and expected genotype frequencies are small enough to be due to chance or large enough to reject the null hypothesis of no evolution and random mating For every population surveyed genotypes at the MN gene are in HW proportions they can conclude that the assumptions for the HW model are valid for this locus 3 Case Study 2 Are HLA Genes ln Humans ln HW Equilibrium a HLA genes code for proteins that help immune C system cells recognize and destroy invading bacteria and viruses Previous work had shown that different alleles exist at both the HLAA and HLAB genes and that the alleles at each code for proteins that recognize proteins from slightly different diseasecausing organisms They are codominant The research group hypothesized that individuals who are heterozygous may have a strong tness advantage i This would be because heterozygous people posses a wider variety of HLA proteins so their immune systems can recognize and destroy more ii They should be healthier and have more offspring than homozygotes d To test this hypothesis they used their data on observed genotype frequencies to determine the frequency of each allele present i There were many more heterozygotes and many fewer homozygotes than expected under HW conditions ii This supported their prediction and indicated that one of the assumptions behind the HW principle was being violated e They argued that mutation gene ow and drift are negligible in this case because i Mating may not be random with respect to the HLA genotype ii Heterozygous individuals may have higher tness ll Nonrandom Mating A The most intensively studied form of nonrandom mating is inbreeding the mating between relatives 1 2 Individuals that inbreed are likely to share alleles they inherited from their common ancestor Self fertilization is the most extreme form of inbreeding B How Does Inbreeding Affect Allele and Genotype Frequencies 1 Inbreeding increases homozygosity a It takes alleles from heterozygotes and puts them into homozygotes Inbreeding itself doesn t cause evolution because allele frequencies don t change in the population as a whole Nonrandom mating changes only genotype frequencies not allele frequencies so is not an evolutionary process itself C How Does Inbreeding In uence Evolution 1 Although inbreeding doesn t cause evolution directly because it don t change allele frequencies it can speed the rate of evolutionary change a It increases the rate at which natural selection eliminates recessive deleterious alleles alleles that lower tness from a population Inbreeding depression is the decline in average tness that takes place when homozygosity increases and heterozygosity decreases in a population It results from two causes a Many recessive alleles represent lossoffunction mutations b Many genes especially those involved in ghting disease are under intense selection for heterozygote advantage a selection process that favors genetic diversity 3 Offspring of inbred matings are expected to have lower tness than the offspring of outcrossed matings a Several studies show similar results on inbreeding depression in human populations around the world 4 In many other animals where inbreeding is uncommon females often don t mate at random but actively choose certain males a This form of nonrandom mating is fundamentally different from inbreeding because it does lead to changes in allele frequencies in the population and thus is a form of natural selection called sexual selection b Sexual selection violates the quotno natural selectionquot HW hypothesis lll Natural Selection A Evolution by natural selection occurs when heritable variation leads to differential success in survival and reproduction 1 If certain alleles are associated with the favored phenotypes they increase in frequency while other alleles decrease in frequency 2 The result is evolution a violation of the assumptions of the HW model B How Does Selection Affect Genetic Variation 1 Genetic variation is the number and relative frequency of alleles that are present in a particular population a Lack of genetic variation in a population is usually a bad thing 2 Natural selection occurs in a wide variety of patterns or modes each with different consequences to genetic variation 3 Mode 1 Directional Selection changes average value of trait a When directional selection occurs the average phenotype of a population changes in one direction Selection against an extreme Population after selection Grig i il rCI DUl aiming b Directional selection tends to reduce the genetic diversity of a population i If it continues over time the favored alleles will eventually approach a frequency of 10 while disadvantageous alleles will approach a frequency of 00 ii This is a clear violation of the HW model iii Alleles that reach 10 are said to be xed iv Alleles that reach 00 are said to be lost c When disadvantageous alleles decline in frequency purifying selection is said to occur 4 Mode 2 Stabilizing Selection a Stabilizing selection functions to reduce both extremes in a population Selection agai i both extremes Pooolo tmon alter selection Dr igin oi population b It has two important consequences i There is no change in the average value of a trait over time ii Genetic variation in the population is reduced 5 Mode 3 Disruptive Selection a Disruptive selection has the opposite effect of stabilizing selection Instead of favoring phenotypes near the average value and eliminating extreme phenotypes it eliminates phenotypes near the average value and favors extreme phenotypes Selection against the mean Population salechow Gri g inal 39 l npulaiiar n b When it occurs the overall amount of genetic variation in the population is maintained c It is important because it sometimes plays a part in speciation or the formation of new species 6 Mode 4 Balancing Selection a Balancing selection occurs when no single allele has a distinct advantage Instead there is a balance among several alleles in terms of their tness and frequency b It occurs when i Heterozygous individuals have higher tness than homozygous individuals do a pattern called heterozygote advantage ii 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 places iii Certain alleles are favored when they are rare but not when they are common a pattern known as frequencydependent selection 7 No matter how natural selection occurs its most fundamental attribute is the same it increases tness and leads to adaptation 8 It results in allele frequencies that deviate from those predicted by the HW principle because selection favors certain alleles over others C Sexual Selection 1 Selection based on success in courtship is a mechanism of evolutionary change a lntersexual selection is the selection of an individual of one gender for mating by an individual of the other gender b Selection within a gender is referred to as intrasexual selection 2 Theory The Fundamental Asymmetry of Sex a Pattern Sexual selection usually acts on males much more strongly than on females Traits that attract members of the opposite sex are much more highly elaborated in males b Process The energetic cost of creating a large egg is enormous whereas a sperm contains few energetic resources c In most species females invest much more in there offspring than males d The BatemanTrivers theory of sexual selection makes strong predictions i If females invest a great deal in each offspring then they should protect that investment by being choosy about their mates ii Conversely if males invest little in each offspring then they should be willing to mate with almost any female iii If there are an equal number of males and females in the population and if males are trying to mate with any female possible then males will compete with others for mates iv lf male tness 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 violation the HW assumptions v Thus sexual selection should act more strongly on males than on females Female Choice for quotGood Allelesquot a Colorful beaks and feathers in birds carry the message quotI m healthy and well fed because I have good alleles Mate with mequot Female Choice for Paternal Care a In many species females prefer to mate with males that care for young or that provide the resources required to produce eggs b Females choose mates on the basis of i Physical characteristics that signal male genetic quality ii Resources or parental care provided by males MaleMale Competition a In some species females don t have the luxury of choosing a male lnstead competition among males is the primary cause of sexual selection Sexual Dimorphism Results from Sexual Selection a Because sexual selection tends to be much more intense in males than females males tend to have many more traits that function only in courtship or malemale competition b Sexual dimorphism refers to any trait that differs between males and females D Female choice and malemale competition illustrate how selection can favor certain phenotypes in a population E Sexual selection is just one type of natural selection All other types referred to as ecological selection favor traits that enable organisms to do things other than obtain mates such as survive in their physical and biological environments Genetic Drift A Genetic drift is de ned as any change in allele frequencies in a population that is due to chance It causes allele frequencies to drift up and down randomly overtime 1 When drift occurs allele frequencies change due to blind luck what is known as sampling error a Sampling error occurs when the allele frequencies of a chosen subset of a population are different from those in the total population by chance 2 Drift occurs in every population in every generation but especially in small populations B Stimulation Studies of Genetic Drift 1 C


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