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.
Evolutionary theory is one of five core concepts recommended by the Vision and Change report (AAAS, 2011) for fostering the development of biological literacy in undergraduate education. Despite this focus, there are persistent misconceptions associated with student understanding of the theory (Alters & Nelson, 2002; Andrews et al., 2012; Foster, 2012). Lessons on evolution are often complicated and may clash with students’ long-standing beliefs, resulting in difficulties grasping the concept (Coley & Tanner, 2012). This could be explained by the fact that in the United States, students are unlikely to have received adequate instruction in evolutionary biology during high school. Moore (2002) showed that state evolution standards meant little in terms of the quality of instruction received. Additionally, an estimated 16% of high school science teachers advocate young-Earth creationism, and another 47% believe that God guided the evolution of humans (Berkman et al., 2008). Additionally, 10% of creationism advocates rate themselves as exceptional teachers (Berkman & Plutzer, 2011). Rutledge and Mitchell (2002) found that high school biology teachers were more likely to accept evolution and spend more classroom time covering it if they took more college biology, took a course on the nature of science, or completed an evolutionary biology course. These results indicate that teaching evolution effectively to both 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 genetics: (1) dynamics of genes within populations differ from the dynamics of inherited genes, and (2) allele frequencies in populations (as opposed to individuals) are the focus. The other two benefits are associated with (3) the importance of population genetics in a higher cognitive understanding of evolutionary principles, and (4) relevant examples harnessed to demonstrate the principles of H-W equilibrium. Despite its utility in teaching evolutionary concepts, H-W is often presented as a mathematical model, leading to high math anxiety in many students. Consequently, students may have difficulty grasping concepts while negotiating their own fears of math.
Active learning has been shown to be an effective method in teaching evolutionary principles (Alters & Nelson, 2002; Abraham et al., 2012), and activities meant to ease mathematical anxiety have demonstrated particular utility. For example, Fifield and Fall (1992) presented a pen-and-paper example, but doing calculations by hand slows student progress and distracts from the key concepts. Several software packages, such as EVOLVE (Soderberg & Price, 2003) and GENALEX 6 (Peakall & Smouse, 2006), have also been developed for the classroom. While these tools are useful for quick yet in-depth demonstrations, they often intimidate students and may not promote engagement with the material.
In an attempt to keep the focus on the material and keep students engaged, we present an approach to exploring population genetics using handheld response systems (i.e., “clickers”). Similar 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.
Prior to completing this exercise, students should be able to
Describe basic concepts of genetics.
Define gene and allele.
Understand dominant/recessive inheritance.
Understand the origin of gametes, meiosis, and independent assortment.
Following this exercise, students should be able to
List the violations of H-W equilibrium.
Compare/contrast all violations to each other and the null hypothesis in terms of allele frequency changes.
Discuss how dominance affects the rate of allele frequency change.
Relate genotype to phenotype and both to fitness.
Transition into evolutionary concepts and recognize that evolution is changes in allele frequencies in a population.
Green, orange, and blue paper: 100 sheets each
A classroom with a projector
“Clicker” classroom response devices
Access to a computer with Powerpoint and clicker software installed
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 random-number 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.
Conducting the Exercise
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% AGAG, 50% AGAO, and 25% AOAO. Say that this gene controls expression of a trait (e.g., a beetle’s horn), where AG confers a green 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
List the violations of H-W equilibrium.
Finite population size, natural selection, nonrandom mating, mutation, and gene flow between populations.
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
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.
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
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.
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.
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
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.
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 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 AOAO 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 AOAO individuals will express the orange phenotype. To reflect this disadvantage, remove all AOAO individuals from each generation and then allow genetic drift to randomly remove individuals until the population size is 20.
Potential Natural Selection Questions
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.
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
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.