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Teaching Evolution through the Hardy-Weinberg Principle: A Real-Time, ActiveLearning Exercise using Classroom Response Devices
Author(s): Michael S. Brewer and Grant E. Gardner
Source: The American Biology Teacher, 75(7):476-479. 2013.
Published By: National Association of Biology Teachers
URL: http://www.bioone.org/doi/full/10.1525/abt.2013.75.7.6
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INQUIRY &
I N V E S T I G AT I O N
Teaching Evolution through
the Hardy-Weinberg Principle:
A Real-Time, Active-Learning
Exercise Using Classroom
Response Devices
M I C H A E L S . B R E W E R,
GRANT E. GARDNER
ABSTRACT
undergraduate majors and nonmajors is essential to enhancing the
public’s knowledge and acceptance of this core idea. Instructors at
all levels have a responsibility to portray an accurate representation
of evolution.
Biological evolution is the change in allele frequencies over time,
so teaching principles of population genetics is essential for developing public understanding of basic evolution (Mertens, 1992) and
bridges the gap between genetics and evolution (Ortiz et al., 2000).
Mertens (1992) highlighted four benefits of teaching Hardy-Weinberg
(H-W) principles. Two of these are directly related to learning goals
associated with comparing/contrasting Mendelian with population
Key Words: Evolution; clickers; Hardy-Weinberg; population genetics;
genetics: (1) dynamics of genes within populations differ from the
active learning.
dynamics of inherited genes, and (2) allele frequencies in populations
(as opposed to individuals) are the focus. The other two benefits are
Evolutionary theory is one of five core concepts recommended associated with (3) the importance of population genetics in a higher
by the Vision and Change report (AAAS, 2011) for fostering the dev- cognitive understanding of evolutionary principles, and (4) relevant
elopment of biological literacy in undergraduate education. Despite examples harnessed to demonstrate the principles of H-W equilibthis focus, there are persistent misconceptions associated with stu- rium. Despite its utility in teaching evolutionary concepts, H-W is
dent understanding of the theory (Alters & Nelson, 2002; Andrews often presented as a mathematical model, leading to high math anxet al., 2012; Foster, 2012). Lessons on evolution are often complicated iety in many students. Consequently, students may have difficulty
grasping concepts while negotiating their own
and may clash with students’ long-standing
fears of math.
beliefs, resulting in difficulties grasping the
Active learning has been shown to be an
concept (Coley & Tanner, 2012). This could
Biological evolution
effective method in teaching evolutionary prinbe explained by the fact that in the United
is the change in allele
ciples (Alters & Nelson, 2002; Abraham et al.,
States, students are unlikely to have received
2012), and activities meant to ease matheadequate instruction in evolutionary biology
frequencies over time.
matical anxiety have demonstrated particular
during high school. Moore (2002) showed that
utility. For example, Fifield and Fall (1992)
state evolution standards meant little in terms
of the quality of instruction received. Additionally, an estimated 16% presented a pen-and-paper example, but doing calculations by hand
of high school science teachers advocate young-Earth creationism, slows student progress and distracts from the key concepts. Several
and another 47% believe that God guided the evolution of humans software packages, such as EVOLVE (Soderberg & Price, 2003) and
(Berkman et al., 2008). Additionally, 10% of creationism advocates GENALEX 6 (Peakall & Smouse, 2006), have also been developed
rate themselves as exceptional teachers (Berkman & Plutzer, 2011). for the classroom. While these tools are useful for quick yet in-depth
Rutledge and Mitchell (2002) found that high school biology teachers demonstrations, they often intimidate students and may not promote
were more likely to accept evolution and spend more classroom time engagement with the material.
In an attempt to keep the focus on the material and keep stucovering it if they took more college biology, took a course on the
nature of science, or completed an evolutionary biology course. dents engaged, we present an approach to exploring population
These results indicate that teaching evolution effectively to both genetics using handheld response systems (i.e., “clickers”). Similar
Teaching population genetics provides a bridge between genetics and evolution
by using examples of the mechanisms that underlie changes in allele frequencies
over time. Existing methods of teaching these concepts often rely on computer
simulations or hand calculations, which distract students from the material and
are problematic for those with high math anxiety. We outline an exercise that
engages students and provides real-time feedback through the use of classroom
response devices. This exercise has been used with success and employs a conceptual-change approach to teach the fundamental, yet often misunderstood,
concept of biological evolution.
The American Biology Teacher, Vol. 75, No. 7, pages 476–479. ISSN 0002-7685, electronic ISSN 1938-4211. ©2013 by National Association of Biology Teachers. All rights reserved.
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DOI: 10.1525/abt.2013.75.7.6
476
THE AMERICAN BIOLOGY TEACHER
VOLUME 75, NO. 7, SEPTEMBER 2013
to an exercise in the 2001 AP Biology Lab Manual for Students, our
method involves students’ participation as members of a “population”
and explores major violations of H-W. Where our method differs is
in the use of clickers to give immediate, visual feedback. Additionally,
our exercise is applicable beyond high school classrooms, where class
sizes are typically much smaller and learning objectives may differ.
By engaging students before, during, and after the activity, we are
able to reinforce concepts while uncovering and addressing misconceptions in real time (Cotner et al., 2008). Real-time graphing
also provides a powerful means to increase student learning.
The following exercise was piloted in a nonmajors undergraduate biology classroom comprising 120 students. To keep students engaged, participants were swapped into and out of the exercise
between steps. The activity can be scaled up or down to accommodate classes of various sizes.
Previous Knowledge
J Prior to completing this exercise, students should be able to
1.
2.
3.
4.
Describe basic concepts of genetics.
Define gene and allele.
Understand dominant/recessive inheritance.
Understand the origin of gametes, meiosis, and independent
assortment.
Learning Objectives
J Materials Needed
J UÊ Green, orange, and blue paper: 100 sheets each
UÊ 40 coins
UÊ A classroom with a projector
UÊ “Clicker” classroom response devices
UÊ Access to a computer with Powerpoint and clicker software
installed
Preclass Setup
J Prepare randomized lists of numbers corresponding to the total
number of participants. For example, for a 40-student activity,
generate random number lists from 1 to 40. Free online randomnumber generators and Microsoft Excel can be used. These lists are
used periodically to eliminate a set number of random individuals
from the population to simulate genetic drift, or changes in allele
frequency due to random sampling. Clicker response slides for each
successive generation of the trial must be prepared (Figure 1). Three
to five generations per H-W violation has worked in the past, but
more can be used to generate more dramatic results. Past experiences
indicate that 40 students works best, depending on classroom space
and time constraints, and this is assumed below. The activity is presented as an interrupted demonstration, collecting data and asking
students to answer throughout. Instructor answers for each question
are provided in italics.
Before attempting this exercise, students should have an understanding of the “previous knowledge” objectives above.
Following this exercise, students should be able to
1. List the violations of H-W equilibrium.
2. Compare/contrast all violations to each other and the null
hypothesis in terms of allele frequency changes.
3. Discuss how dominance affects the rate of allele frequency
change.
4. Relate genotype to phenotype and both to fitness.
5. Transition into evolutionary concepts and recognize that
evolution is changes in allele frequencies in a population.
Conducting the Exercise
J Assign each participant a number from 1 to 40. Tell the class to
assume that participating students represent individuals of a population. Assign genotypes (i.e., two sheets of paper with colors corresponding to alleles) at the following frequencies, where AG and
AO are alleles for the same gene and AG is dominant to AO: 25%
AG AG, 50% AG AO, and 25% AO AO. Say that this gene controls
expression of a trait (e.g., a beetle’s horn), where AG confers a green
Figure 1. Histograms showing the “genotypes” of students under complete assortative mating. As expected, the frequencies of
homozygotes increase from generation to generation.
THE AMERICAN BIOLOGY TEACHER
HARDY-WEINBERG PRINCIPLE
477
phenotype and AO creates an orange phenotype. These organisms
mate once per generation, produce four offspring, and immediately
die. Genetic drift removes half the population in each generation.
Use a different randomized number list to select 20 individuals
for elimination prior to the first round of mating and at the end
of each generation. This maintains a constant population size of
20 individuals after 40 individuals are born and half die. Therefore,
all 40 students participate in each generation. A round of mating
(i.e., one generation of the population) in which the assumptions of
H-W are satisfied is as follows: (1) students randomly choose a mate
(a “musical chairs” approach whereby students mate with the nearest
individual after moving about for a certain time until stopped by
the instructor); (2) each pair of students produces four offspring,
randomly pairing gametes (i.e., alleles) by flipping a coin to decide
which of their alleles is passed on to each offspring; (3) the “parents”
die and then obtain the proper alleles in order to represent the next
generation, along with two of those eliminated as a result of genetic
drift; (4) genotype frequencies of the population are recorded via a
clicker question (see Figure 1); and (5) genetic drift removes half of
the population. These instructions will be modified under each of
the violations of H-W equilibrium.
Potential Hardy-Weinberg Equilibrium
Questions
2. How would the amount of time expected for an allele to
become fixed change if the population were larger? smaller?
Larger = longer; smaller = sooner.
Sexual Selection via Assortative Mating. Under this scenario,
the first step in the mating process (“students randomly choose a
mate”) is modified. Students randomly choose a mate, but only if
he or she has the same phenotype; essentially, students mate with
the closest individual with the same phenotype. The rest of the
steps are unaltered. Conduct three to five rounds of mating. The
resulting genotype frequency distributions should show a decrease
in heterozygotes, whereas both homozygous genotype frequencies
increase (Figure 1). DO NOT CARRY THIS SCENARIO FORWARD
INTO THE SUBSEQUENT PARTS OF THE ACTIVITY.
Potential Assortative Mating
(Nonrandom Mating) Questions
J 1. How would the rate of allele frequency change if a small percentage of individuals mated with unlike partners (i.e., 90%
green–green and orange–orange with 10% green–orange)?
The results would be similar, but homozygotes would increase at
a slower rate. Additionally, more heterozygotes would remain in
the population over time.
J 1. List the violations of H-W equilibrium.
2. Under assortative mating, would you ever see the heterozygotes completely disappear? Explain why or why not.
Not for a very long time, because heterozygotes display the dominant phenotype. Heterozygotes can be produced following the
mating of two individuals with the dominant phenotype if at least
one is heterozygous.
Finite population size, natural selection, nonrandom mating,
mutation, and gene flow between populations.
2. If a population has two alleles for a gene and the assumptions
of H-W are satisfied, do the proportions of homozygous dominant, heterozygous, and homozygous recessive individuals
have to be 25%, 50%, and 25%, respectively? Explain why or
why not.
No. If alleles are at equilibrium and all assumptions are met,
they can deviate from the above ratios.
Genetic Drift due to Small Population Size. Explain that the environment has a carrying capacity of 20 individuals. Conduct three
to five rounds of mating, satisfying H-W assumptions (except for
infinite population size, which cannot be satisfied with a limited
number of students). When the genotype frequencies are recorded,
there should not be much change in the overall genotype or allele
frequencies from generation to generation. This will be a point of
comparison for the remainder of the exercise when the assumptions
of H-W are violated.
Potential Genetic Drift
(Small Population Size) Questions
J 1. Given the small size of our population, would you expect one
of the alleles to go extinct given enough time? Why or why
not?
Yes. Given enough time, alleles will go extinct in small to very
large populations as a result of genetic drift. However, smaller
populations can experience changes in allele frequency at high
rates. This could lead to a rapid extinction of one allele and the
fixation of another.
478
THE AMERICAN BIOLOGY TEACHER
Introduction of New Alleles via Mutation or Gene Flow. Introduce
a single blue allele (e.g., blue beetle horn phenotype), AB, to replace
one of the existing alleles at random. This can be thought of as a
novel mutation and should be explained to the class as such. Assume
that the AB allele is dominant to both AG and AO. Conduct three to five
rounds of mating, assuming that the assumptions of H-W are satisfied, or until the AB allele is removed from the population as a result
of genetic drift.
Potential Introduction of New Alleles
(Mutation & Gene Flow) Questions
J 1. Will an allele that is under neutral selection ever be fixed in a
population? Explain why or why not.
If the population size is small and/or the allele is in higher initial
frequency, a neutral allele could be fixed in a population, given
enough time. If the population size is large and/or the allele is in
low starting frequency, it is less likely but still possible.
2. Discuss the differences between the introduction of new alleles
into a population via mutation and gene flow.
New alleles introduced via mutation start out in very low frequencies, usually a single allele (n = 1). Those introduced via
gene flow can be in low starting frequencies or larger frequencies
if more than one individual immigrates.
Positive Natural Selection. If the blue allele was eliminated via
genetic drift in the previous step, introduce a single or a few blue
VOLUME 75, NO. 7, SEPTEMBER 2013
alleles. Explain that the blue phenotype confers an advantage over
the green and orange phenotypes (e.g., a novel predator has entered
the environment and cannot detect individuals with the blue horn).
Individuals with the AB allele will survive and reproduce at higher
rates than those without it. To demonstrate this advantage, do not
allow any individuals with the blue allele to be removed during the
genetic-drift portion of a generation. Conduct three to five rounds
of mating without removing any individuals with the AB allele. If an
individual with the AB allele is in the genetic-drift removal pool, move
further down the random number list until 20 nonblue individuals
are removed. Because the AB allele is dominant to both AG and AO,
the frequency of genotypes with the AB allele will increase quickly.
At first, the AB allele will exist mostly in the heterozygous state.
Homozygous genotypes may begin to appear in the second generation and will increase in frequency moving forward.
Purifying Selection against the Homozygous Recessive Orange
Phenotype. Assume that the AO AO genotype, the orange phenotype,
confers a lethal disadvantage (e.g., cannot survive lower temperatures). Because AO is recessive to both AG and AB, only homozygous
AO AO individuals will express the orange phenotype. To reflect this
disadvantage, remove all AO AO individuals from each generation and
then allow genetic drift to randomly remove individuals until the
population size is 20.
Potential Natural Selection Questions
J 1. How would the rate of allele change have differed if the advantageous blue allele was recessive instead of dominant?
It would have increased in frequency at a slower rate. In fact,
it might have gone extinct as a result of genetic drift because
heterozygotes would not express its phenotype.
2. Will the less advantageous or recessive alleles ever disappear
from the population? Explain why or why not.
Perhaps given enough time, but they will persist for a longer
period in heterozygotes that do not express the less advantageous
phenotype (i.e., carriers).
Summary & Reinforcement
J After the exercise, the graphs can be used to show changes in allele
frequency over the simulated generations (Figure 1). When all students have returned to their seats, a summary of the activity, accompanied by open-ended questions, should be used to reinforce concepts
and identify misconceptions. By visualizing changes in allele frequencies, students gain a better appreciation for the variables that affect
the evolution of populations. This near real-time feedback is possible
through the use of classroom response devices. As a result, students
remain engaged throughout the exercise.
This activity provides a transition from genetics to evolution and
an introduction for students with difficulties accepting evidence for
evolution. By instituting a conceptual-change approach, as defined
in Tanner & Allen (2005), misconceptions regarding the nature of
evolution are actively and visually dispelled. These concepts provide
a foundation upon which the principles of macroevolution can be
taught and provide a better understanding of what evolution is.
THE AMERICAN BIOLOGY TEACHER
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MICHAEL S. BREWER is a Postdoctoral Associate in Department of
Environmental Science, Policy, and Management at the University of
California, Berkeley, Berkeley, CA 94720; e-mail: [email protected].
GRANT E. GARDNER is an Assistant Professor of Biology Education in the
Department of Biology, Middle Tennessee State University, Murfreesboro,
TN 37132; e-mail: [email protected].
HARDY-WEINBERG PRINCIPLE
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