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BIO312: Evolutionary Biology

Unit 2: Hardy-Weinberg Equilibrium   *At this point in your biology studies, you should be familiar with the Hardy-Weinberg Equilibrium. You might even know how it’s calculated. But how is it really used in the field? What does it measure?

In this unit, we will learn the practical uses for Hardy-Weinberg Equilibrium calculations. We will then practice using these calculations in hypothetical situations, as well as in order to determine whether the population is in Hardy-Weinberg equilibrium. Your activity in this unit will involve an assignment or simulation in which you will be required to calculate the Hardy-Weinberg Equilibrium and use it to make predictions or draw conclusions.*

Unit 2 Time Advisory
Completing this unit should take you approximately 5.5 hours.

☐    Subunit 2.1: 0.5 hours

☐    Subunit 2.2: 3.5 hours

☐    Subunit 2.3: 1.5 hours

Unit2 Learning Outcomes
Upon successful completion of this unit, you will be able to:
- calculate the allele and genotypic frequencies based on the concepts of Hardy-Weinberg  Equilibrium; and - use Hardy-Weinberg Equilibrium to determine whether a population is evolving.

2.1 How Are Hardy-Weinberg Equilibrium Calculations Used?   - Reading: Massachusetts Institute of Technology’s OpenCourseWare: “Population Genetics: Hardy-Weinberg” Link: Massachusetts Institute of Technology’s OpenCourseWare: Population Genetics: Hardy-Weinberg (PDF)
 
Instructions: At the course page, download and read the PDF file for item  25, Population Genetics: Hardy-Weinberg equilibrium. The reading covers sections 2.1.1–2.1.2 along with evolutionary forces associated with changes in allele frequencies.
 
Terms of Use: This resource is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States. It is attributed to Massachusetts Institute of Technology’s OpenCourseWare.

2.1.1 Baseline Measure for Genetic Change   Note: Before we can predict whether a selective pressure can cause a change in the allele frequency in a population, we first need to understand how the allele will behave in the absence of the  selective pressure. This subunit is covered by the Massachusetts Institute of Technology reading on Population Genetics assigned just beneath subunit 2.1.

2.1.2 Predict Allele or Genotype Frequencies   Note: Within a population, it is important to know whether a particular allele will become more or less common over time. This subunit is covered by the Massachusetts Institute of Technology reading on Population Genetics assigned just beneath subunit 2.1.

2.2 Examples   - Lecture: Khan Academy: Salman Khan’s “Hardy-Weinberg Principle” (YouTube) Link: Khan Academy: Salman Khan’s “Hardy-Weinberg Principle” (YouTube)
 
Also available in:
iTunes U
 
Instructions: This 15-minute video walks you through an example of Hardy-Weinberg along with appropriate terminology.
 
Terms of Use: Please respect the copyright and terms of use displayed on the webpage above.

2.2.1 Predicting Allele Frequencies in a Population   Note: For a population with random mating, the frequency of alleles in the gene pool can be calculated from the genotypic frequencies. If there are two alleles at a specific locus, allele one (A1) and  allele two (A2), and the frequency of A1 is p, while the frequency of  A2 in the gene pool is q. If A1 and A2 are the only alleles of the gene A in that population, then (p + q = 1). This is covered in the reading in Section 2.2.

2.2.2 Predicting Genotype Frequencies in a Population   Note: With random mating in a population, we can predict the frequency of zygotes produced in a hypothetical life cycle  (generation) and calculate the frequencies of the genotypes in the next generation. The frequency of the homozygous dominant genotype (A1A1) in the population can be calculated as p2. The frequency of the heterozygous genotype (A1A2) in the population can be calculated as 2pq. The frequency of the homozygous recessive genotype (A2A2) in the population can be calculated as q2. Thus, for the entire population under random mating (p + q)2 =1, the genotypic frequencies are calculated as: p2 + 2pq + q2=1. This is covered in the reading in Section 2.2.

2.2.3 How to Determine When Evolution Is Occurring   2.2.3.1 Driving Forces   Note: Changes in the allele frequencies in a population signal that evolution is occurring. This is covered in the reading in Section 2.2.

2.2.3.2 Determination of Specific Driving Force   - Reading: Stanford Encyclopedia of Philosophy: Samir Okasha: “Population-Genetic Models of Evolution” Link: Stanford Encyclopedia of Philosophy: Samir Okasha: Population-Genetic Models of Evolution (HTML)
 
Instructions: As you read this article, take a note of the different factors that can affect allele frequencies, and the equations you can use to test if the population is evolving and which factor(s) of evolution might be affecting the population. Keep in mind when you are looking for evolutionary change in a population that it may not be present at every (or any) locus you test. Some loci may show change while others do not.
 
Terms of Use: Please respect the copyright and terms of use displayed on the webpage above.

  • Assessment: The Saylor Foundation’s “Assessment 1: Hardy-Weinberg” Link: The Saylor Foundation’s Assessment 1: Hardy-Weinberg” (PDF)
     
    Instructions: Using the population data provided, you will calculate allele frequencies (at a specific locus, for a genetic characteristic) and determine whether the population is in Hardy-Weinberg Equilibrium. Based on your calculations, you should be able to discern whether or not the population is evolving. You should also be able to make some conjectures about the driving force behind the evolutionary change. After completing the assessment, you should be able to address each of the points below and come up with some ideas concerning effective management techniques, based upon the specified goals below:

    • Discovering evolutionary changes
    • Managing directional selection in a population
    • Managing genetic diversity in a population

    When you are finished, you can check your answers against this Answer Key. (PDF)

2.3 Summary of Population Genetics and Mathematical Treatments of the Mechanisms of Evolution   - Interactive Lab: Massachusetts Institute of Technology’s OpenCourseWare: “Laboratory 1: Forces of Evolution” Link: Massachusetts Institute of Technology’s OpenCourseWare: “Laboratory 1: Forces of Evolution” (PDF)

 Instructions: Population genetics & evolution simulation -
*Populus* java computer program, version 5.3 & PopG computer
program  

 Populus: Download site and installation instructions at the
University of Minnesota located at
http://www.cbs.umn.edu/populus/Download/download.html. (.jar files
available for PC, Mac, Linux-Unix)  

 Make sure you have checked for java compatibility according to the
instructions on that page. A PDF file of instructions for all
modules is available in the labs section. Check with us if you need
to run the program on the MIT server workstations rather than your
own computer.  

 PopG: Download site and installation instructions at the University
of Washington located here.  

 This lab will allow you to explore the impact of various *forces*
on allele dynamics. Here we will present an overview of the
laboratory, along with some introductory questions. This is followed
by a link to a pdf file for the full laboratory itself. We also
include a 'review ‘section on the Hardy-Weinberg equilibrium theory,
just to make sure that is understood before you tackle evolutionary
forces.  

 Terms of Use: This resource is licensed under a [Creative Commons
Attribution-NonCommercial-ShareAlike 3.0 United
States](http://creativecommons.org/licenses/by-nc-sa/3.0/us/). It is
attributed to Massachusetts Institute of Technology’s
OpenCourseWare.
  • Interactive Lab: Massachusetts Institute of Technology’s OpenCourseWare: “Laboratory 1, Part 2: Forces of Evolution” Link: Massachusetts Institute of Technology’s OpenCourseWare: “Laboratory 1, Part 2: Forces of Evolution” (PDF)

    Instructions: Read the introduction and work on the Lab 1 Part 1, and Lab 1 Part 2 problems. These modules teach you how population geneticists study frequencies of genotypes and alleles within populations. You will compare the observed frequencies with those predicted by the Hardy-Weinberg Equilibrium, as a null model that assumes no evolutionary change. Selection, Mutation, Migration and Genetic Drift are mathematical violations of the H-W equilibrium and thus, mechanisms of evolution.

    Terms of Use: This resource is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States. It is attributed to Massachusetts Institute of Technology’s OpenCourseWare.