chapter 26 outline (full)
chapter 26 outline (full) BIOSC 0160
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This 14 page Class Notes was uploaded by Anna Perry on Monday February 23, 2015. The Class Notes belongs to BIOSC 0160 at University of Pittsburgh taught by Dr. Swiganova in Winter2015. Since its upload, it has received 45 views. For similar materials see Foundations of Biology 2 in Biology at University of Pittsburgh.
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Date Created: 02/23/15
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 b 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 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 Evolution a change in allele frequencies in a population occurs due to genetic drift 2 Computer Simulations a C When a computer stimulates the process of random combinations in gametes over time the program uses the allele frequencies to create a new gene pool It showed i Striking differences between the effects of drift in small versus large populations ii The consequences for genetic variation when alleles drift to xation or loss Given enough time drift can be an important factor even in large populations 3 Key Points About Genetic Drift a Genetic drift is random with respect to tness i The changes in allele frequency that it produces are not adaptive Genetic drift is most pronounced in small populations Over time genetic drift can lead to the random loss or xation of alleles i When random loss or xation occurs genetic variation in the population declines C Experimental Studies of Genetic Drift 1 Kerr and Wright did an experiment to determine how drift works in practice a They started with a large lab population of fruit ies that contained a genetic marker a speci c aee that causes a distinctive phenotype i The marker was the morphology of bristles fruit ies have bristles on their bodies that can either be straight or bent ii This difference depends on a single gene iii The normal strait allele was designated AN iv The forked bent allele was designated AF b The trait is sexlinked so males have only one allele i In females AF is recessive to AN 2 They studied drift in these aees by a Setting up 96 small populations in a lab each consisting of 4 adult females and 4 adult males i They chose individuals ies to begin these experimental populations so that the frequency of each allele was 05 ii The two alleles do not affect the tness of ies in the lab environment so they could be con dent that if changes in the frequencies occurred they wouldn t be do to natural selection b In the offspring F1 generation they randomly chose 4 males and 4 females from each of the 96 offspring populations and allowed them to breed and produce the next generation c They repeated this until all 96 populations had undergone a total of 16 generations d They then counted the number of populations that had both alleles still present only the normal allele present or only the forked allele present 3 During the entire course of the experiment no migration from one population to another occurred 4 As predicted genetic drift decreased genetic variation within populations and increased genetic differences between populations D What Causes Genetic Drift in Natural Populations 1 The random sampling process that takes place during fertilization occurs in every population in every generation in every species that reproduces sexually 2 Because genetic drift is caused by genetic sampling error it can occur by any process or event that involves sampling not just the sampling of gametes that occurs during fertilization or the loss of unlucky individuals due to accidents 3 Founder Effects on the Green lguanas of Anguilla a When a group of individuals immigrates to a new geographic area and establishes a new population a founder event is said to occur i If the new population is small enough the allele frequencies in the new population are almost guaranteed to be different from those in the source population ii This is due to sampling error b A change in allele frequencies that occurs when a new population is established is called a founder effect c Each time a founder event occurs a founder effect is likely to accompany it changing allele frequencies through genetic drift 4 Genetic Bottleneck on Pingelap Atoll a If a large population experiences a sudden reduction in size a population bottleneck is said to occur i Disease outbreaks natural catastrophes such as oods or res or storms or other events can cause population bottlenecks b Genetic bottlenecks follow population bottlenecks just as founder effects follow founder events c A genetic bottleneck is a sudden reduction in the number of alleles in a population i Genetic drift occurs during genetic bottlenecks and causes a change in allele frequencies V Gene Flow A When an individual leaves on population joins another and breeds gene ow is said to occur the movement of alleles between populations 1 As an evolutionary process gene ow usually has one outcome It equalizes allele frequencies between the source population and the recipient population a When alleles move from one population to another the populations tend to become more alike B Measuring Gene Flow Between Populations 1 The presence or absence of gene ow has particularly important implications for the conservation of threatened and endangered species a Numerous studies have documented the decline of gene ow between wild populations that have been isolated from one another b Others have documented the effects of gene ow between wild populations and captive populations i Data demonstrated that gene ow occurred from the captivebred population to the wild population and caused a reduction in tness in the population C Gene Flow ls Random with Respect to Fitness 1 It is not true that gene ow always reduces tness in the receiving population a If a population has lost alleles due to genetic drift then the arrival of new alleles via gene ow should increase genetic diversity b If increased genetic diversity results in better resistance to infections gene ow would increase the average tness of individuals 2 Gene ow is random with respect to tness the arrival or departure of alleles can increase or decrease average tness depending on the situation 3 But in every case movement of alleles between populations tends to reduce their genetic differences a Because individuals rom different cultural and ethnic groups are intermarrying and having offspring allele frequencies are becoming more similar across human populations Vl Mutation A Entirely new alleles come from mutation These can occur in a number of ways 1 Point mutations If a change in nucleotide sequence occurs in a stretch of DNA that codes for a protein the new allele may result in a polypeptide with a novel amino acid sequence 2 Chromosomelevel mutations A consequence of this is gene duplication If duplicated genes diversify via point mutations they can lose their function or create new alleles 3 Lateral gene transfer The transfer of genes from one species to another rather than from parent to offspring may be more important for genetic variation B Mutation is inevitable and is an evolutionary process that increases genetic diversity in populations a Although it leads to an increase in genetic diversity it is random with respect to tness b Changes in the makeup of chromosomes or in speci c DNA sequences do not occur in ways that tend to increase or decrease tness i Mutation just happens c Mutations can result in deleterious alleles which lower tness but tend to be eliminated by purifying selection d On rare occasions mutations can produce a bene cial allele an allele that allows individuals to produce more offspring i Bene cial alleles should increase in frequency in the population due to natural selection Mutation as an Evolutionary Process 1 As an evolutionary process mutation is slow compared with selection genetic drift and gene ow Experimental Studies of Mutation 1 Experimental Evolution a Researchers measured the tness of descendent populations relative to ancestral populations i Relative tness values greater than 1 meant that recentgeneration cells outnumbered oldergeneration cells following the competition 2 Fitness Increased in Fits and Starts a Cells that happen to have a bene cial mutation grow rapidly and come to dominate the population b After a bene cial mutation occurred the tness of the population stabilized until another random but bene cial mutation occurred and produced another jump in tness Studies of Mutation in Natural Populations 1 Several forms of mutation have combined with natural selection to create color variations or polymorphisms in pea aphids 2 Lateral Gene Transfer a Pea aphids are insects that feed on plant sap and occur in red and green b The color of aphids is determined by carotenoid pigments Usually what insects eat would determine this however researchers found that they were generating their own carotenoids c This was possible by mutation more speci cally lateral gene transfer from the genome of a fungal symbiont to the genome of a recent ancestor of the aphids d This was clear evidence of LGT from one eukaryote to another 3 Gene Duplication a A comparison of aphid and fungal genome sequences suggests that after the fungal genes for enzymes were transferred to aphids they underwent further mutations duplications sequence diversi cation by point mutations and deletions b This proves that sometimes a loss of function mutation can be adaptive F Take Home Messages 1 2 3 Mutation is the ultimate source of genetic variation lf mutation did not occur evolution would eventually stop Mutation alone is usually inconsequential in changing allele frequencies at a particular gene
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