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Modern Evolutionary Biology I. Population Genetics A. Overview

This article provides an overview of the genetic structure of populations in modern evolutionary biology, with a focus on the Hardy-Weinberg equilibrium model. It explains the definitions and basic computations related to gene and genotypic frequencies, and discusses the conditions required for a population to be in equilibrium. A case example is provided to illustrate the concept, and the utility of the Hardy-Weinberg equilibrium model in detecting deviations from equilibrium is discussed. This article also highlights the sources of variation and agents of change, including mutation, recombination, migration, drift, and non-random mating.

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Modern Evolutionary Biology I. Population Genetics A. Overview

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  1. Modern Evolutionary Biology I. Population Genetics A. Overview Sources of Variation Agents of Change Mutation N.S. Recombination mutation (polyploidy) - crossing over - independent assortment VARIATION

  2. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population G. Hardy and W. Weinberg 1. Definitions - Evolution: a change in the genetic structure of a population - Population: a group of interbreeding organisms that share a common gene pool; spatiotemporally and genetically defined - Gene Pool: sum total of alleles held by individuals in a population - Genetic structure: Gene array and Genotypic array - Gene/Allele Frequency: % of alleles at a locus of a particular type - Gene Array: % of all alleles at a locus: must sum to 1. - Genotypic Frequency: % of individuals with a particular genotype - Genotypic Array: % of all genotypes for loci considered; must = 1.

  3. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population 1. Definitions 2. Basic Computations

  4. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population 1. Definitions 2. Basic Computations

  5. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population 1. Definitions 2. Basic Computations

  6. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population 1. Definitions 2. Basic Computations

  7. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population 1. Definitions 2. Basic Computations - Determining the Gene Array from the Genotypic Array a. f(A) = f(AA) + f(Aa)/2 = .35 + .4/2 = .35 + .2 = .55 b. f(a) = f(aa) + f(Aa)/2 = .25 + .4/2 = .25 + .2 = .45 KEY: The Gene Array CAN ALWAYS be computed from the genotypic array; the process just counts alleles instead of genotypes. No assumptions are made when you do this.

  8. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population C. The Hardy-Weinberg Equilibrium Model 1. Goal: Describe what the genetic structure of the population would be if there were NO evolutionary change – if the population was in equilibrium.

  9. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population C. The Hardy-Weinberg Equilibrium Model 1. Goal: Describe what the genetic structure of the population would be if there were NO evolutionary change – if the population was in equilibrium. For a population’s genetic structure to remain static, the following must be true: - random mating - no selection - no mutation - no migration - the population must be infinitely large

  10. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population C. The Hardy-Weinberg Equilibrium Model 2.Example:

  11. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population C. The Hardy-Weinberg Equilibrium Model 2.Example:

  12. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population C. The Hardy-Weinberg Equilibrium Model 2.Example:

  13. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population C. The Hardy-Weinberg Equilibrium Model 2.Example: After one generation with these conditions, the population equilibrates

  14. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population C. The Hardy-Weinberg Equilibrium Model 2.Example 3. Utility: If no populations meets these conditions explicitly, how can it be useful?

  15. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population C. The Hardy-Weinberg Equilibrium Model 2.Example 3. Utility: If no populations meets these conditions explicitly, how can it be useful? For comparison, like a “perfectly balanced coin”

  16. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population C. The Hardy-Weinberg Equilibrium Model 3. Utility: - if a population is NOT in HWE, then one of the assumptions must be violated. Sources of Variation Agents of Change Mutation N.S. Recombination Drift - crossing over Migration - independent assortment Mutation Non-random Mating VARIATION So, if NO AGENTS are acting on a population, then it will be in equilibrium and WON'T change.

  17. Modern Evolutionary Biology I. Population Genetics A. Overview B. The Genetic Structure of a Population C. The Hardy-Weinberg Equilibrium Model 3. Utility: - if a population is NOT in HWE, then one of the assumptions must be violated. -Also, If HWCE is assumed and the frequency of homozygous recessives can be measured, then the number of heterozygous carriers can be estimated. For example: If f(aa) = .01, then estimate f(a) = .1 and f(A) must be .9. f(Aa) = 2(.1)(.9) = 0.18.

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