Chapter 24-2 - 204-325 1 Summary of the Last Lecture...

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Unformatted text preview: 204-325 1 Summary of the Last Lecture Population genetics is the study of the genetic structure of populations and its evolution. Gene pool of a population is described by allelic frequencies. Distribution of alleles is described by genotypic frequencies. Diversity of a population is described by proportion of polymorphism and by heterozygosity. The Hardy-Weinberg law states that if a large, randomly mating population is free from mutation, migration and selection, then the allelic frequencies p and q do not change, and the genotypic frequencies are p2, 2pq and q2. 2 Today's Topics 1. Effect of Mutation 2. Genetic Drift 3. Gene Flow due to Migration 4. Natural Selection 5. Non-random Mating 3 Topic 1 Effect of Mutation: Raw Materials for Evolution 4 Mutation and Genetic Structure u= A to a mutation rate p = v x q u x p v= a to A mutation rate p+q=1 If mutation is the only violation of Hardy-Weinberg At equilibrium p = 0 p= q= v u+v u u+v 5 Slow to Reach Equilibrium due to Mutation Alone p = v x q u x p u= A to a mutation rate ~ 10-4 to 10-9 v= a to A mutation rate ~ 10-4 to 10-9 p< 1 q<1 p ~ very small 6 Mutation Rates are Usually Very Low 7 Topic 2 Genetic Drift: Random Change in Allelic Frequency due to Chance 8 Genetic Drift is the Result of Sampling Error Sampling Error can be measured by standard deviation Sp as an indication of magnitude of genetic drift Sp = pq 2Ne = Effective population size Ne 4 X Nf X Nm Nf + Nm Nf = number of breading female Nm = number of breading male The Smaller the Population, the Bigger the Drift 9 Conditions Lead to Large Genetic Drift Population was Constantly Small Founder Effect: Population was Initially Small Bottleneck Effect: Population was Small at One Point 10 Genetic Drift in 107 Populations of 16 Drosophila N=16 Ne<16 11 Genetic Drift Could Lead to Fixation: Lost of Hetrozygosity Allelic Frequency Small Population 1 Fixed Small Population 2 Mean Frequency Small Population 3 Fixed Small Population 4 Ne=20 12 Population Size and Initial Allelic Frequency Affect Time to Fixation T N=Effective Population Size 13 Consequences of Genetic Drift Decreased genetic variation within the population Increased genetic divergence between the population Fluctuation of allelic frequencies within the population 14 Topic 3 Gene Flow due to Migration 15 Effect of Migration on the Gene Pool of a Population in population II m: migrants in Allelic Frequency Change p = m ( px py ) 16 Consequences of Gene Flow Increased genetic variation within the population Decreased genetic divergence between the population Increased effective population size of the recipient population 17 Cheetahs 18 Habitats Shrinkage and Fragmentation Cheetah range in 1900 African cheetah Asiatic cheetah Cheetah range today 19 Lack of Allelic Genetic Diversity Lack of Diversity in Cheetahs 1. Take blood samples from many individuals and isolate proteins. 2. Load protein samples from different individuals into wells in gel. 3. Put gel into an electric field. Proteins separate according to charge and mass: electrophoresis. 4. Treat gel with a solution that stains a specific enzyme. One band implies that the individual is homozygous at the locus for the enzyme. Two bands imply that the individual is heterozygous at this locus. lack of diversity in cheetahs: 20 Topic 4 Natural Selection: Differential Reproduction of Genotypes 21 Measurement of Nature Selection Darwinian Fitness W = relative reproductive success of a genotype For the most successful genotype W=1 All other W are between 0 and 1 Selection Coefficient s=1-W measures intensity of selection 22 An Example of Calculating Fitness and Selection Coefficient 23 Effect of Selection on Genetic Structure (average fitness) p2 + 2pd +q2 = 1 24 P' + H' + Q' = 1 Selection Against a Recessive Lethal Genotype (Waa=0) Initial Frequency=0.9 Initial Frequency=0.5 Initial Frequency=0.1 Recessive lethal allele will be reduced but never eliminated 25 Directional Selection Decreases Diversity A2 2 A A1 against A2 additively (A1 A2 Incomplete dominant) Directional Change - Nature Selection Decreases Diversity 26 Directional Selection Against Recessive, Additive or Dominant Allele 2 of allele 2 W11=1 in all cases W12=0.5 W22=0.5 W12=1 W22=0.5 W12=0.75 W22=0.5 27 Balancing Selection Increases or Maintains Diversity A2 2 A A1 against A2 additively (incomplete dominant) Heterozygote Superiority- Nature Selection Increases Diversity 28 Heterozygous for Hb-S Results in Malaria Resistance 29 Heterozygote Superiority: Allelic Diversity Increases Resistance to HIV 100 Percentage of HIV-infected people still alive 80 60 Heterozygous for MHC 40 Homozygous for MHC 20 0 Years since infected with HIV 0 2 4 6 8 10 30 12 14 16 18 Animation: Nature Selection 31 Topic 5 Non-random Mating: Positive or Negative Assortative Mating Changes Genetic Structure 32 None- Random Mating Decreases Effective Population Size Negative Assortative Mating: mate preferentially with dissimilar ones -- heterozygocity increase Positive Assortative Mating: mate preferentially with similar ones -- homozygocity increase Positive Assortative Mating Between Non-relatives Inbreeding: Positive Assortative Mating Between Genetic Relatives Inbreeding Between 2 Individuals Self Fertilization 33 Extreme Inbreeding: Self Fertilization Decreases Heterozygosity A1A1 Generation 1 100% Generation 2 100% Generation 3 100% 25% 50% 25% 100% A1A2 A2A2 25% 50% 25% 100% Generation 4 100% 0 25 50 34 75 100 Inbreeding Reduces Fitness In Human: Inbreeding Depression 35 Summary of Evolutionary Mechanisms Increase variation by introducing new alleles. This Mutation occurs too infrequent to be the sole cause of allele frequency change, however. Gene Reduces differences among the populations. Increases genetic variability of a population Flow Genetic Causes random changes in allele frequencies; Drift tends to lower genetic variation by loss and fixation of alleles within a population Change genotype frequencies by increasing homozygosity; if inbreeding depression occurs, Inbreeding genetic variability is reduced Increases adaptation; can leads to maintenance, increase, or reduction of genetic variation Selection depends on pattern of selection 36 An Example of Evolution 2 Genetically Identical Populations Eye Color Mutation Abdomen Color Mutation Genetically Different Populations 37 Summary Mutation is the source of all variation but contribute little to the changes in allelic frequencies. Genetic drift is the allelic frequency change due to the small effective population size. It leads to loss of genetic variation within a population and to divergence among populations. Gene flow due to migration has the opposite effects. Selection is measured by the relative reproductive success of the genotypes. It results in different genetic changes depending on pattern of selection. Nonrandom mating reduces effective population size and change genotypic and allelic or only allelic frequencies. Inbreeding increases homozygosity. 38 ...
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This note was uploaded on 04/29/2008 for the course BIO SCI 325 taught by Professor Zang during the Spring '08 term at Wisconsin Milwaukee.

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