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Biology Chapter 25.docx

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McMaster University
Laura Parker

Chapter 25: Evolutionary Processes Bernard Ho January 15, 2011  Natural selection is not the only process that causes evolution  There are four mechanisms that shift allele frequencies in populations  Natural selection o Increases frequency of certain alleles, the ones that contribute to success in survival and reproduction  Genetic drift o Causes alleles frequencies to change randomly o In some cases, drift may even cause alleles that decrease fitness to increase in frequency  Gene flow o Occurs when individuals leave one population, join another and breed o Allele frequencies may change when gene flow occurs because arriving individuals introduce alleles to their new population and departing individuals remove alleles from their old population  Mutation o Modifies allele frequencies by continually introducing new alleles o Alleles created by mutation may be beneficial or detrimental or have no effect on fitness Analyzing Change in Allele Frequencies: The Hardy-Weinberg Principle  To study how four evolutionary processes affect populations, G. H. Hardy and Wilhelm Weinberg developed a mathematic-al model to analyze the consequences of matings among all individuals in a population  At the time it was commonly believed that changes in allele frequency occurred simply as a result of sexual reproduction (meiosis followed by the random fusion of gametes) to form offspring  Some biologists claimed that dominant alleles inevitable increase in frequency  Others predicted that two alleles of the same gene inevitable reach a frequency of 0.5  To analyze the consequences of mating among all of the individuals in a population, Hardy and Weinberg invented a novel approach o They imagined that all of the gametes produced in each generation go into a single group called the gene pool and then combine at random to form offspring o Ex. Clams and sea stars release their gametes into the water, where they mix randomly with gametes from other individuals in the population and combine to form zygotes  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 combined to form offspring  These calculations would predict the genotypes of the offspring that would be produced, as well as the frequency of each genotype  The researchers began by analyzing the simplest situation possible, that just two alleles of a particular gene exist in a population o Call these alleles A a1d A and 2 to symbolize the frequency of A and q to 1 symbolize the frequency of A 2 o Because there are only two alleles, the two frequencies must add up to 1 (p + q = 1) o Suppose that the initial frequency of A is10.7 and that of A is20.3 o In this case, 70% of the gametes in the gene pool carry A and130% carry A 2 o Because only two alleles are present, three genotypes are possible, A A , A A1 1 1 2 A A 2 2 o The frequency of these three genotypes in the next generation would be  A 1 1enotype is p = 0.49  A 1 2enotype is 2pq 2 0.42  A 2 2enotype is q = 0.09 o The genotype frequencies in the offspring generation must add up to 1, which 2 2 means that p + 2pq + q = 1 (0.49 + 0.42 + 0.09 = 1) o The frequency of allele A in1the next generation is still 0.7 and the frequency of allele 2 is still 0.3  Frequency of allele A 1 0.49 + ½(0.42) = 0.7  Frequency of allele A = ½(0.42) + 0.09 = 0.03 2  The frequency of 2pq is halved in both calculations because half the gametes will carry the A a1lele and the other half will carry the A a2lele o No allele frequency changes occurred o Even if A i1 dominant to A , it2does not increase in frequency, nor do the two trend toward a frequency of 0.5 o These results are called the Hardy-Weinberg principle and it makes two fundamental claims  If the frequencies of alleles A 1nd A in2a population are given by p and q, then the frequencies of genotypes A A ,1 1A ,1 2A w2 2 be given by p , 2 2 2pq and q for generation after generation  When alleles are transmitted according to the rules of Mendelian inheritance, their frequencies do not change over time, for evolution to occur, some other factor or factors must come into play o Genotype frequencies will be given by p : 2pq : q as long as all Hardy-Weinberg principles are met  The Hardy-Weinberg model makes important assumptions o For a population to conform to the HWP, none of the four mechanisms of evolution can be acting on the population o In addition, the model assumes that mating is random WRT gene in question o Five conditions that must be met  No natural selection at the gene in question  Model assumes that all members of the parental generation survived and contributed equal numbers of gametes to the gene pool, no matter what their genotype (step 2)  No genetic drift or random allele frequency changes affecting the gene in question  We avoided this type of allele frequency change by assuming that we drew alleles in their exact frequencies p and q and not at some different values by chance (step 4)  Ex. Allele 1 did not “get lucky” and get drawn more than 70% of the time (no random changes due to luck occurred)  No gene flow  No new alleles were added by immigration or lost through emigration  As a result, all of the alleles in the offspring population came from the original population’s gene pool  No mutation  We didn’t consider that new A s or A s or other, new alleles might 1 2 be introduced into the gene pool (step 2 and step 5)  Random mating WRT gene in question  We enforced this condition by picking gametes from the gene pool at random (step 3)  We did not allow individuals to choose a mate based on their genotype  How the HWP serves as a null hypothesis o The HWP serves as a null hypothesis o Given a set of allele frequencies, it predicts what genotype frequencies will be when natural selection, mutation, genetic drift and gene flow are not affecting the gene and when mating is random WRT to that gene o If biologists observe genotype frequencies that do not conform to the HW prediction, it means that either non-random mating is occurring or allele frequencies are changing for some reason o Further research is needed it determine which of the five HW conditions is being violated o Are MN blood types in humans in HW equilibrium  Most humans have two alleles, designated M and N at this gene  Because gene codes for a protein found on the surface of red blood cells, researchers could determine whether individuals are MM, MN or NN by treating blood samples with antibodies to each protein  Analysis to determine if HWP holds requires four steps  Estimate genotype frequencies by observation or testing  Calculate observed allele frequencies from the observed genotype frequencies o In this case, the frequency of the M allele is the frequency of MM homozygotes plus half the frequency of MN heterozygotes (same with N allele, except NN homozygotes)  Use the observed allele frequencies to calculate the genotypes expected according to the HWP o Under the null hypothesis of no evolution and random 2 m2ting, the expected genotype frequencies are p : 2pq : q  Compare the observed and expected values  Results show that observed and expected frequencies for all three genotypes were almost identical  Since genotypes of MN locus are in HW proportions, implies that the M and N alleles in these populations were not being affected by the four evolutionary mechanisms and that mating was random WRT to this gene o Are HLA genes in humans in HW equilibrium  Biologists were studying two genes that are important in the functioning of the human immune system  More specifically, the genes that they analyzed code for proteins that help immune system cells recognize and destroy invading bacteria and viruses  Previous work had shown that different alleles exist at both the HLA-A and HLA-B genes and that the alleles at each gene code for proteins that recognize slightly different disease-causing organisms  HLA alleles are codominant like the M and N alleles so both are expressed and create the phenotype  As a result, the research group hypothesized that individuals who are heterozygous at one or both of these genes may have a strong fitness advantage  The logic is that heterozygous people have a wider variety of HLA proteins so their immune systems can recognize and destroy more types of bacteria and viruses  To test this hypothesis, researchers used data on observed genotype frequencies to determine the frequency of each allele present  When they used these allele frequencies to calculated the expected number of each genotype according to the HWP, they found that there are many more heterozygotes and many fewer homozygotes than expected under HW conditions  These results supported the team’s prediction and indicated that one of the assumptions behind the HWP was being violated  Researchers argued that mutation, migration and drift are negligible in this case and offered two competing explanations for their data  Mating may not be random WRT the HLA genotype o Specifically, people may subconsciously prefer mates with HLA genotypes unlike their own and thus produce an excess of heterozygous offspring o If this is true, then non-random mating would lead to an excess of heterozygotes compared with the proportion expected under HW  Heterozygous individuals may have higher fitness o Data showed that married women who have the same HLA-related alleles as their husbands have more trouble getting pregnant and experience higher rates of spontaneous abortion than do women with HLA-related alleles different from those of their husband o Data suggests that homozygous foetuses have lower fitness than do foetuses heterozygous at these genes o If this were true, selection would lead to an excess of heterozygotes relative to HW expectations Types of Natural Selection  Balancing Selection o AKA heterozygote advantage o When balancing selection occurs, heterozygous individuals have higher fitness than homozygous individuals do o The consequence of this pattern is that genetic variation is maintained in populations o Genetic variation refers to the number and relative frequency of alleles that are present in a particular population o Biologists generally focus on genetic variation when analyzing the consequences of different patterns of selection o If genetic variation is low and the environment changes, perhaps due to the emergence of a new disease-causing virus, a rapid change in climate or a reduction in the availability of a particular food source, it is unlikely that any alleles will be present that have high fitness under the new conditions o As a result, average fitness of the population will decline o If the environmental change is severe enough, the population may even be faced with extinction  Directional Selection o Is when the average phenotype of the populations changes in one direction o Ex. Natural selection has increased the frequency of drug-resistant strains of bacteria and causes changes in beak shapes of finches o When many genes influence a trait, the distribution of phenotypes in the population tends to form a bell-shaped curve o Directional selection causes the bell shaped to move to the right and to thin out on both sides (change in average value), where the right side is individuals with high fitness and the left side is individuals with low fitness o In such cases, directional selection is acting on many different genes at once (except in the case of selection on drug resistance, where selection was acting on a single gene) o Most often, directional selection tends to reduce the genetic diversity of populations o If directional selection continues over time, the favoured alleles will eventually reach a frequency of 1.0, while disadvantageous alleles will reach a frequency of 0.0 o Alleles that reach a frequency of 1.0 are said to be fixed and those that reach a frequency of 0.0 are said to be lost o When disadvantageous alleles decline in frequency, purifying selection is said to occur  Stabilizing Selection o When selection reduces both extremes in a population o It has two important consequences  There is no change in the average value of a trait over time  Genetic variation in the populations is reduced o On a bell-shaped curve, both ends represent low fitness, while the peak represents high fitness  Average value does not change, but curve thins out o Biologists who analyzed birth weights and mortality in babies found that babies of average size survived best o Mortality was high for very small babies and very large babies o This is evidence that birth weight was under strong stabilizing selection in this population o Alleles associated with high birth-weight or low birth-weight were subject to purifying selection  Disruptive Selection o Has the opposite effect of stabilizing selection o Instead of favouring phenotypes near the average value and eliminating extreme phenotypes, it eliminates phenotypes near the average value and favours extreme phenotypes o On a bell-shaped curve, both ends represent high fitness, while the peak represents low fitness  Average value decreases, forming two smaller peaks on either side o When disruptive selection occurs, the overall amount of genetic variation in the population is maintained o Ex. The beaks of black-bellied seedcrackers  Individuals with either very short or very long beaks survive best and that birds with intermediate phenotypes are at a disadvantage  This is because where they live, only two sizes of seeds are available, small and large  Small birds can easily eat small seeds and large birds can easily eat large seeds  Intermediate birds have trouble with both so alleles associated with medium-sized beaks are subject to purifying selection  Disruptive selection maintains high overall variation in this population o Disruptive selection is important because it sometimes plays a part in speciation (the formation of new species) o If small-beaked seedcrackers began mating with other small-beaked individuals, their offspring would tend to be small-beaked and would feed on small seeds o If large-beaked seedcrackers began mating with other large-beaked individuals, their offspring would tend to be large-beaked and would feed on large seeds o In this way, selection would result in two distinct populations o Under some conditions, the populations may eventually form two new species Genetic Drift  Natural selection is not random, it is directed by the environment and results in adaptation  In contrast, genetic drift is undirected  It is defined as any chance in allele frequencies in a population that is due to chance  When drift occurs, allele frequencies change due to luck, what is formally known as sampling error  Genetic drift causes allele frequencies to drift up and down randomly over time  Example of founders of Pitcairn Island  Genetic drift is random with respect to fitness o 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  Over time, genetic drift can lead to the random loss or fixation of alleles o When random loss or fixation occurs, genetic variation in the population declines  However, given enough time, drift can be an important factor even for large populations  Experimental studies of genetic drift o Biologists (Kerr and Wright) started with a large laboratory population of fruit flies that contained a genetic marker, a specific allele that causes a distinctive phenotype o In this case, the marker was the morphology of bristles o Fruit flies have bristles that can be either straight or bent o This difference in bristle phenotype depends on a single gene o Their lab population contained just two alleles, normal and forked o To begin their experiment, the researchers set up 96 cages in their lab o They placed four adult females and four adult males in each o They chose individual flies to begin these experimental populations so that the frequency of the normal and forked alleles in each of the 96 starting populations was 0.5 o The two alleles do not affect the fitness of flies in the lab environment so the researchers would be confident that if changes in the frequency of normal and forked phenotypes occurred, they would not be due to natural selection o After the first-generation adults bred, the biologists reared their offspring o They randomly chose four males and four females from each of the 96 offspring populations and allowed them to breed and produce the next generation o Kerr and Wright repeated this procedure until all 96 populations had undergone a total of 16 generations o During the entire course of the experiment, no migration from one population to another occurred o Previous studies have shown that mutations from normal to forked are rare o Thus, the only evolutionary process operating during the experiment was genetic drift o Their results showed that after 16 generations, the 96 populations fell into three groups  Forked bristles were found on all of the individuals in 29 of the experimental populations  Due to drift, the forked allele had been fixed in these 29 populations and the normal allele had been lost  In 41 other populations, the opposite was true  All individuals had normal bristles  In these populations, the forked allele had been lost due to chance  Both alleles were still present in 26 of the populations o In 73% of the experimental populations (70 out of 96), genetic drift had reduced allelic diversity at this gene to zero o As predicted, genetic drift decreased genetic variation within populations and increased genetic differences between populations  Genetic drift in natural populations o Because drift is caused by sampling error, it can occur by any process or event that involves sampling, not just the sampling of gametes that occurs during fertilization o How founder effects cause drift  When a group of individuals immigrates to a new geographic area and establishes a new population, a founder event is said to occur  If the group is small enough, the allele frequencies in the new population are almost guaranteed to be different from those in the source population, due to sampling error  A change in allele frequencies that occurs when a new population is established is called a founder effect  Fisherman on the island of Anguilla recently witnessed a founder event involving green iguanas  A few weeks after hurricanes swept through the region, a large raft composed of downed logs tangled with other debris floated
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