Hardy-Weinberg and Genetic Drift
Evolutionary change is interesting and important to study, but changes in populations occur over long periods of time and in huge physical spaces and…
Scientists once wondered why dominant traits like tan-colored giraffe spots do not become more frequent with each generation and replace recessive traits like dark brown spots. In thinking about this conundrum, in 1908, independent from each other, Godfrey H. Hardy and Wilhelm Weinberg independently derived a theory, today known as the Hardy-Weinberg Principle and represented by this equation.
The principle states that in the absence of evolution, i.e. at equilibrium, the allele and genotype frequencies of a population will remain constant from one generation to the next. To understand this equation, let's go back to the giraffe example. Uppercase A represents the tan allele since it is dominant, and lowercase a is for the brown allele because it is recessive. The frequency of these two alleles in the population are designated as p and q respectively. So how do we know the allele frequency? Well, each individual has two alleles. In this example, 40% of the alleles in the gene pool are tan. Thus, the frequency of the tan allele, p, is 0.4, and the frequency of the brown allele, q, is 0.6. Note that p plus q is always equal to one.
Now let's go back to the Hardy-Weinberg equation. Each term in the equation represents one genotype frequency. The frequency of the homozygous dominant genotype is p squared, and the homozygous recessive is represented by q squared. The heterozygous genotype is two pq. The reason we multiply by two here is that there are two different ways of generating a heterozygous genotype. Combined, these all represent 100% of genotypes. Thus, a total frequency of one. Using the values for p and q from our giraffe example, we can determine the genotype distribution of the color gene alleles in our giraffe population. Therefore, as per the Hardy-Weinberg Principle, at equilibrium, 16% of the giraffe population will be homozygous dominant, 48% will be heterozygous, and 36% are homozygous recessive.
To maintain this balance, the Hardy-Weinberg Equilibrium Principle states that a population should meet five main assumptions. There should be random mating, large population size, no mutation, no selection on the gene in question, and no gene flow in or out of the population. Most natural populations violate at least one of these assumptions and so equilibrium is rare…but in spite of this, the principle is used as a null model for population genetics. By comparing these expected values to the actual genotype frequency in a population, it can be determined whether that population is in Hardy-Weinberg Equilibrium. If not, then this means that some form of evolution or change in allele frequency is taking place.
A general misconception about evolution is that it requires natural selection to occur. However, this is not always the case. Genetic drift is one mechanism by which evolution can occur without natural selection. It is defined as a change in the allele frequency of a population due to chance. To envision this, let's go back to the example of a giraffe population and imagine their alleles of tan and brown being represented by marbles of two different colors. We will assume here that each color starts out equally abundant. If we were to start a new generation out of this population, we would need to breed pairs of individuals and thus select from four alleles per pair. If we select a breeding pair at random, then we might end up with two marbles of each color. However, by chance alone, some pairings will have only one color marble, or three of one color and one of the other. These chance deviations from 50-50 over multiple pairings to create a new generation might mean that the next generation no longer has an equal mixture of each allele.
It's this variation of relative allele frequencies over time that defines genetic drift. Therefore, unlike adaptive evolution, where allele frequency changes to select for traits that are fit for the environment, like ladybugs with a greater amount of melanin surviving better in colder climates because of an improved ability to absorb heat, genetic drift represents a type of evolution that is purely due to stochastic change. For example, the random removal of a section of a population through a catastrophic event.
In this lab, you will perform computer and colored bead simulations of Hardy-Weinberg Equilibrium and genetic drift in a population, and then test what happens when assumptions of the equilibrium are violated.
Scientists once wondered why dominant traits like tan-colored giraffe spots do not become more frequent with each generation and replace recessive traits like dark brown spots. In thinking about this conundrum, in 1908, independent from each other, Godfrey H. Hardy and Wilhelm Weinberg independently derived a theory, today known as the Hardy-Weinberg Principle and represented by this equation.
The principle states that in the absence of evolution, i.e. at equilibrium, the allele and genotype frequencies of a population will remain constant from one generation to the next. To understand this equation, let's go back to the giraffe example. Uppercase A represents the tan allele since it is dominant, and lowercase a is for the brown allele because it is recessive. The frequency of these two alleles in the population are designated as p and q respectively. So how do we know the allele frequency? Well, each individual has two alleles. In this example, 40% of the alleles in the gene pool are tan. Thus, the frequency of the tan allele, p, is 0.4, and the frequency of the brown allele, q, is 0.6. Note that p plus q is always equal to one.
Now let's go back to the Hardy-Weinberg equation. Each term in the equation represents one genotype frequency. The frequency of the homozygous dominant genotype is p squared, and the homozygous recessive is represented by q squared. The heterozygous genotype is two pq. The reason we multiply by two here is that there are two different ways of generating a heterozygous genotype. Combined, these all represent 100% of genotypes. Thus, a total frequency of one. Using the values for p and q from our giraffe example, we can determine the genotype distribution of the color gene alleles in our giraffe population. Therefore, as per the Hardy-Weinberg Principle, at equilibrium, 16% of the giraffe population will be homozygous dominant, 48% will be heterozygous, and 36% are homozygous recessive.
To maintain this balance, the Hardy-Weinberg Equilibrium Principle states that a population should meet five main assumptions. There should be random mating, large population size, no mutation, no selection on the gene in question, and no gene flow in or out of the population. Most natural populations violate at least one of these assumptions and so equilibrium is rare…but in spite of this, the principle is used as a null model for population genetics. By comparing these expected values to the actual genotype frequency in a population, it can be determined whether that population is in Hardy-Weinberg Equilibrium. If not, then this means that some form of evolution or change in allele frequency is taking place.
A general misconception about evolution is that it requires natural selection to occur. However, this is not always the case. Genetic drift is one mechanism by which evolution can occur without natural selection. It is defined as a change in the allele frequency of a population due to chance. To envision this, let's go back to the example of a giraffe population and imagine their alleles of tan and brown being represented by marbles of two different colors. We will assume here that each color starts out equally abundant. If we were to start a new generation out of this population, we would need to breed pairs of individuals and thus select from four alleles per pair. If we select a breeding pair at random, then we might end up with two marbles of each color. However, by chance alone, some pairings will have only one color marble, or three of one color and one of the other. These chance deviations from 50-50 over multiple pairings to create a new generation might mean that the next generation no longer has an equal mixture of each allele.
It's this variation of relative allele frequencies over time that defines genetic drift. Therefore, unlike adaptive evolution, where allele frequency changes to select for traits that are fit for the environment, like ladybugs with a greater amount of melanin surviving better in colder climates because of an improved ability to absorb heat, genetic drift represents a type of evolution that is purely due to stochastic change. For example, the random removal of a section of a population through a catastrophic event.
In this lab, you will perform computer and colored bead simulations of Hardy-Weinberg Equilibrium and genetic drift in a population, and then test what happens when assumptions of the equilibrium are violated.
View the full transcript and gain access to JoVE Lab Manual videos
Q1: What does the Hardy-Weinberg equation tell us about allele frequencies in a population?
The Hardy-Weinberg equation (p² + 2pq + q² = 1) predicts that allele and genotype frequencies remain constant across generations when a population is at equilibrium. Each term represents a genotype frequency: p² for homozygous dominant, q² for homozygous recessive, and 2pq for heterozygous individuals. This serves as a null model to detect whether evolution is occurring.
Q2: What five conditions must a population meet to maintain Hardy-Weinberg equilibrium?
Hardy-Weinberg equilibrium requires random mating, large population size, no mutation, no selection on the gene in question, and no gene flow into or out of the population. Most natural populations violate at least one assumption, making true equilibrium rare. When observed frequencies deviate from predictions, it indicates that evolution or allele frequency change is occurring.
Q3: How do you calculate allele frequencies from a population's gene pool?
Allele frequency is determined by counting the proportion of each allele in the total gene pool. Since each individual carries two alleles, if 40% of alleles are dominant (p = 0.4), then 60% are recessive (q = 0.6). These frequencies must sum to one (p + q = 1), allowing you to predict genotype distributions using the Hardy-Weinberg equation.
Q4: What is genetic drift and how does it differ from natural selection?
Genetic drift is a change in allele frequency due to chance alone, not environmental fitness. Unlike natural selection, which favors adaptive traits, genetic drift represents stochastic evolution occurring randomly in populations. It occurs when breeding pairs are selected randomly, causing some alleles to become more or less frequent by chance rather than because they improve survival or reproduction.
Q5: What is the founder effect and how does it relate to genetic drift?
The founder effect occurs when a small group of individuals establishes a new population in an isolated location, carrying only a subset of the original population's alleles. This new gene pool may have different allele frequencies than the original population purely by chance. The founder effect is a type of genetic drift that shapes future generations based on the random genetics of the founding individuals.
Q6: How can the Hardy-Weinberg principle be used to detect evolution in a population?
By comparing observed genotype frequencies to those predicted by the Hardy-Weinberg equation, scientists can determine if a population is evolving. If measured frequencies differ significantly from predictions, it indicates that one or more equilibrium assumptions have been violated. This deviation reveals that evolution through natural selection, genetic drift, or other mechanisms is occurring.
Q7: Why does the Hardy-Weinberg equation multiply the heterozygous term by two?
The heterozygous term (2pq) is multiplied by two because there are two different ways to produce a heterozygous genotype: an individual can inherit the dominant allele from one parent and the recessive from the other, or vice versa. This accounts for both possible combinations, ensuring the equation accurately represents all genotype frequencies in the population.