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Chapter 25

Unit 5 - Chapter 25 Bio 1M03 .docx

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McMaster University
James S Quinn

Bio 1AA3 Unit Five: Evolutionary Processes and Patterns Chapter 25: Evolutionary Processes Key Concepts  Population – a group of individuals from the same species that live in the same area and regularly interbreed  Hardy-Weinberg Principle – a null hypothesis that projects what genotype frequencies should be given known allele frequencies  Factors that affect allele and/or genotype frequencies that can cause departures from HWE: o Natural Selection – produces adaptation; increases or decreases the frequency of certain alleles o Genetic Drift – produces stochastic fluctuations in allele frequencies; causes allele frequencies to change stochastically o Gene Flow (migration) – equalized allele frequencies between populations; introduces alleles from another population o Mutation – introduces new alleles; modifies allele frequencies by continually introducing new alleles, even deleterious ones o Biased Mating  Types of biases mating: o Inbreeding o Sexual Selection 25.1 Analyzing Change in Allele Frequencies: The Hardy-Weinberg Principle  To study how these factors affect populations, in 1908 G. H. Hardy and Wilhelm Weinberg developed a mathematical model to analyze allele frequencies of individuals in a population  To do this, they imagined that all of the gametes produced in each generation go into a single group called a gene pool and then combine  Their calculations predict the genotypes of the offspring that the population would produce, as well as the frequency of each genotype The Hardy-Weinberg Principle: Important Assumptions  The Hardy-Weinberg principle holds when the following five assumptions/conditions are met with respect to the gene in question 1. No natural selection 2. No genetic drift (stochastic allele frequency changes) 3. No gene flow (migration) 4. No mutation 5. No biased mating  They started with the simplest situation, a gene with two alleles, 1 and A2  The frequency of A i1 represented by p and the frequency of A is2 represented by q o Because there are only two alleles, p + q = 1  In this situation three genotypes are possible, A1 1, 1 2 , and 2 2 A Numerical Example of the Hardy-Weinberg Principle  Allele frequencies to HWE genotypes (Picture and Equation) Bio 1AA3 o Remember these are the genotype frequencies ONLY if the population in in HWE  Genotypes to Allele Frequencies o If the population is HWE, p = √p 2  If the population is not in HWE o Cannot assume p = √p 2  The model predicts that, in a population at HWE, the frequency of the A A gen1 1pe in the new generation will be p , that of the A2 2genotype will be q and that of the A A 1 2otype will be 2pq  Because all individuals in the new generation must have one of the three genotypes, the sum of the three genotype frequencies must equal 1 (100% of the population) o True for a biallelic locus, whether the population is in HWE or not 2 2  Hardy-Weinberg Equation for a population: p + 2pq + q = 1  The Hardy-Weinberg principle makes two fundamental claims 1. If the frequencies of alleles A1and A 2n a population are given by p and q, then the frequencies of genotypes A A , A A and A A will be given by p , 2pq and q for generation after generation 1 1 1 2 2 2 2. 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 Bio 1AA3  The Hardy-Weinberg principle serves as a model for generating predictions about genotype frequencies that are consistent with the null hypothesis of random association of alleles  When genotype frequencies do not conform to Hardy-Weinberg proportions, some factor is affecting that population and causing non-random association of alleles Are MN Blood Types in Humans in Hardy-Weinberg Equilibrium  Most human populations have two alleles for the MN blood group  Analysis to determine if the Hardy-Weinberg principle holds requires four steps 1. Estimate genotype frequencies by observation 2. Calculate observed allele frequencies from the observed genotype frequencies 3. Use the observed allele frequencies to calculate the expected genotypes assuming HWE 4. Compare the observed and expected values o The observed and expected MN genotype frequencies were almost identical  Since the genotypes at the MN locus are in Hardy-Weinberg proportions, we can conclude that the factors (selection, drift, migration, mutation and biased mating) do not substantially affect genotype frequencies of MN blood groups  This doesn’t mean that MN blood types have always been in HWE – one generation of random mating can generate HWE genotypes for an autosomal locus (a gene not located on the sex chromosome)  Another point worth noting (not in text) is that allele frequencies of different human populations are different  This is suggestive of population structure between localities even though mating within localities appears random HLA Locus and Immune Function  The human leukocyte antigen (HLA) system is a name for the major histocompatibility locus in humans  HLA genes are a set of genes on chromosome 6 that binds to antigens and present them on the outside of cells so the cells can be recognized by the immune system  Could Mate Choice Encourage Heterozygosity at HLA o Step 1: Ask some male students to not change their t-shirts for a few days o Step 2: Ask some female students to smell their t- shirts and rate them o Step 3: compare preferences to HLA genotypes o Null hypothesis of this experiment Bio 1AA3  There should be no difference in mate preference  Could There be an Advantage to Heterozygosity at HLA o Null Hypothesis – no difference in the interval between the couples that share and do not share the HLA locus  Is the HLA locus of Humans in Hardy-Weinberg Equilibrium? o To test the hypothesis that heterozygotes for the HLA-A and HLA-B genes might be more fit than homozygotes, researchers used genotypes of 125 Havasupai tribe members to estimate population allele frequencies o If people tend to reproduce with partners with different HLA genotypes, what is the expectation with respect to HWE  The expected genotype frequencies did not match the observed frequencies did not match the observed frequencies  More specifically there were more heterozygotes observed than expected by chance (i.e. than expected if the population were in HWE)  Therefore, at least one of the Hardy-Weinberg assumptions must be violated for these alleles in this population o Mutation, Migration and Genetic Drift are negligible in this case  There are two possible explanations for this result o Mating is biased with respect to the HLA genotypes  Individuals may be more attracted to mates with HLA genotype unlike their own – producing more heterozygotes o Heterozygotes individuals have higher fitness 25.2 Types and Patterns of Natural Selection  Natural Selection occurs in a wide variety of patterns  Heterozygote Advantage – a pattern of natural selection in which heterozygous individuals have higher fitness than homozygous individuals do Bio 1AA3 o This pattern maintains genetic variation in a population and could explain the excess of heterozygotes observed at the HLA locus of humans o Alleles in a heterozygote must be “co-dominant” in that they have a phenotype that differs from either homozygote o Eg/ Sickle Cell Anemia Directional Selection  Average phenotype of the populations changed in one direction  Tends to reduce the genetic diversity of populations  Favored alleles will eventually reach a frequency of 1.0 – said to be fixed  Disadvantageous alleles will reach a frequency of 0.0 – said to be lost  When disadvantageous alleles decline in frequency – purifying selection occur  1996 Swallows o Six-day period of exceptional cold and rainy weather o Cliff swallows feed by catching mosquitos and other insects in flight – insects disappeared during cold snap o The bodies of 1853 swallows were recovered that died of starvation o Measured the bodies of 1027 survivors from the same population – the average body size was much larger than the body size of those who died of starvation Stabilizing Selection  Selection that reduces both extremes in a population  Two important consequences: o There is no change in the average value of a trait over time o Genetic variation in the population is reduced  1950’s Babies o Babies of average size survived best o Large or small babies had a high mortality rate o Alleles associated with high birth weight or low birth weight were subject to purifying selection Disruptive Selection  Opposite of Stabilizing Selection  Eliminates phenotypes near
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