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Chapter 6

Chapter 6- Population Genetics I.docx

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University of Waterloo
BIOL 359
Jonathan Witt

BIOL 359 Lecture 6 Janice Wong Evolution Population Genetics part I Chapter 6: Mendelian Genetics in Populations I- Selection and Mutation as Mechanisms of Evolution 5.4| Measuring Genetic Variation in Natural Populations How Much Genetic Diversity Exists in a Typical Population • Early efforts to study allelic diversity in populations were based on allozyme electrophoresis • Isolating proteins from a large sample of individuals, separating the proteins in an electrophoresis gel, and then staining the gel to visualize the proteins produced by a particular gene • If alleles in a population were different enough that their proteins products had different sizes or charges, then proteins would migrate differently in the gel and would show up as different bands • Demonstrated that most natural populations have substantial genetic varation • Not as accurate • Recent studies have shown that in most populations, many alleles are present at every gene in the genome. Genetic variation is extensive • Genetic diversity is maintained by natural variation 6.1| Mendelian Genetics in Populations: The Hardy-Weinberg Equilibrium Principle • Population genetics begins with a model of what happens to allele and genotype frequencies in an idealized population • Once we know how Mendelian genes behave in the idealized population, we will be able to explore how they behave in real populations A Stimulation • Gene pool: the set of all copies of all alleles in a population that could potentially be contributed by the members of one generation to the members of the next generation • Starting with the eggs and sperm that constitute the gene pool, our model traces alleles through zygotes and adults and into the next generation’s gene pool • When the alleles frequencies from the start are different from the end of the generation, then the population has evolved • Genetic Drift: change in frequencies of alleles in a population resulting from sampling error in drawing gametes from the gene pool to make zygotes and from chance variation in the survival and/or reproductive success of individuals; results in nonadaptative evolution A Numerical Calculation • Punnett squares can be used to predict genotypes of offspring by making crosses between two heterozygotes • Can also calculate genotype frequencies among the zygotes by multiplying allele frequencies BIOL 359 Lecture 6 Janice Wong Evolution Population Genetics part I The Hardy Weinberg equilibrium principle yields two fundamental conclusions 1. The allele frequencies in a population will not change, generation after generation 2. If the allele frequencies in a population are given by p and q, then genotype frequencies 2 2 will be given by p , 2pq, q Hardy Weinberg Assumptions 1. There is no selection. • All members have survival at equal rates and contribute equal numbers of gametes to the gene pool (equal reproductive success) • When violated: the frequencies may change from one generation to the next 2. There is no mutations • No copies of existing alleles were converted by mutation, no new alleles created • When violated: some alleles with higher mutations rates, allele frequencies may change from one generation to the next 3. There is no migration • No individuals moved into or out of the model population' • When violated: individuals carrying some alleles move into or out of the population at higher rates than individual carrying other alleles, allele frequencies may change from on e generation to the next 4. There are no chance events • No genetic drift, and the model population is indefinitely large • When violated: if some individuals contribute more alleles to the next generation than others, allele frequencies may change from one generation to the next 5. Mating is random • When violated: genotype frequencies may change. Shifts in genotype frequency and in combination of the other 4 assumptions, can influence the evolution of populations 6.2| Selection • Selection happens when individuals with particular phenotypes survive to reproductive age at higher rates than other phenotypes • Or when individuals with particular phenotypes produce more offspring during reproduction than individuals with other phenotypes • Selection can lead to evolution when phenotypes that exhibit difference in reproductive success are heritable Adding Selection to the Hardy-Weinberg Analysis: Changes in Allele Frequencies • Selection can cause allele frequencies to change across generations • Violation of the no-selection assumption has resulted in violation of conclusion 1 of the Hardy-Weingberg analysis • The numerical examples shows that when individuals with some genotypes survive at higher rates than individuals with other genotypes, allele frequencies can change from one generation to the next • Natural selection causes change in allele frequencies, causes evolution • Fig 6.12 BIOL 359 Lecture 6 Janice Wong Evolution Population Genetics part I o Each curve shows the change in allele frequency over time under a particular selection intensity Adding Selection to the Hardy-Weinberg Analysis: The Calculation of Genotype Frequencies • Selection can change genotype frequencies so that they cannot be calculated by multiplying the allele frequencies • Natural selection can also drive genotype frequencies away from the values predicted under the Hardy-Weinberg equilibrium principle • In this experiment, there was a strong natural selection for homozygotes • Genotype frequencies have changed among the adult survivors • Violation of the no-selection assumption has resulted in violation of conclusion 2 • The discovery that genotype frequencies in a population are not in HW equilibrium may be a clue that natural selection is at work 6.3| Patterns of Selection: Testing Predictions of Population Genetic Theory Selection on Recessive and Dominant Alleles • Fig 5.16 • The curves predict that evolution will be rapid at first but will slow as the experiment proceeds • Dominance and allele frequency interact to determine the rate of evolution • When a recessive allele is common (and a dominant allele is rare) evolution by natural selection is rapid • When the recessive allele is rare, and the dominant allele is common, evolution by natural selection is slow • A) The decline in frequency of a lethal recessive allele. As the allele becomes rare, the rate of evolution slows dramatically • B) The increase of the corresponding dominant allele. • Natural selection is m
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