Learning Objectives
In this section, you will explore the following questions:
- What is population genetics and how is population genetics a synthesis of Mendelian inheritance and Darwinian evolution?
- What is the Hardy–Weinberg principle, and how can it be applied to microevolution?
The mechanisms of inheritance, or genetics, were not understood at the time Charles Darwin and Alfred Russel Wallace were developing their idea of natural selection. This lack of understanding was a stumbling block to understanding many aspects of evolution. In fact, the predominant (and incorrect) genetic theory of the time, blending inheritance, made it difficult to understand how natural selection might operate. Darwin and Wallace were unaware of the genetics work by Austrian monk Gregor Mendel, which was published in 1866, not long after publication of Darwin's book, On the Origin of Species. Mendel’s work was rediscovered in the early twentieth century at which time geneticists were rapidly coming to an understanding of the basics of inheritance. Initially, the newly discovered particulate nature of genes made it difficult for biologists to understand how gradual evolution could occur. But over the next few decades genetics and evolution were integrated in what became known as the modern synthesis—the coherent understanding of the relationship between natural selection and genetics that took shape by the 1940s and is generally accepted today. In sum, the modern synthesis describes how evolutionary processes, such as natural selection, can affect a population’s genetic makeup, and, in turn, how this can result in the gradual evolution of populations and species. The theory also connects this change of a population over time, called microevolution, with the processes that gave rise to new species and higher taxonomic groups with widely divergent characters, called macroevolution.
Connection for AP® Courses
Population genetics studies microevolution by measuring changes in a population’s allele frequencies over time. (Remember that we studied genotypes and allele frequencies when we explored inheritance patterns proposed by Mendel.) For example, scientists examining allele frequencies in a pesticide resistance gene in mosquitoes at Equatorial Guinea found that the frequency of one resistance allele was 6.3%, while a second resistance allele’s frequency was 74.6%, and the non-resistance allele’s frequency was 19.0%. These three frequencies add up to 100%.2 A population’s gene pool is the sum of all the alleles. If these frequencies do not change over time, the population is said to be in Hardy–Weinberg principle of equilibrium—a stable, non-evolving state. However, if a phenotype is favored by natural selection, allele frequencies can change. If this is the case, the population is evolving. Sometimes allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies. This phenomenon is called genetic drift. An event that initiates an allele frequency change in an isolated part of the population, which is not typical of the original population, is called the founder effect. In Population Genetics, we will explore how natural selection, random drift, and founder effects can lead to significant changes in the genome of a population.
Hardy–Weinberg equilibrium reflects a state of constancy in a population’s gene pool. In other words, allele frequencies remain stable from generation to generation if certain conditions are met: no mutations, no gene flow, random mating, no genetic drift, and no selection. Because these conditions are rarely met, allele frequencies are typically changing, reflecting evolution. The Hardy–Weinberg principle is represented by the mathematical equation p2 + 2pq + q2 = 1, where p represents the frequency of the dominant allele and q represents the frequency of the recessive allele. Deviations from Hardy–Weinberg equilibrium allow us to measure microevolutionary shifts in a population when one or more of the Hardy–Weinberg parameters change. For example, if we go back to the study of the frequencies of alleles in a pesticide resistance gene, after an area was treated with pesticides for two years, the resistance alleles increased to 11.1% and 83.3%, respectively, while the non-resistance allele decreased to 5.6%. This indicates that microevolution was occurring.
Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.
| Big Idea 1 | The process of evolution drives the diversity and unity of life. |
| Enduring Understanding 1.A | Change in the genetic makeup of a population over time is evolution. |
| Essential Knowledge | 1.A.1 Natural selection is a major mechanism of evolution. |
| Science Practice | 1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain. |
| Science Practice | 1.1 The student is able to convert a data set from a table of numbers that reflect a change in the genetic makeup of a population over time and to apply mathematical methods and conceptual understandings to investigate the cause(s) and effect(s) of this change. |
| Learning Objective | 1.1 The student is able to convert a data set from a table of numbers that reflect a change in the genetic makeup of a population over time and to apply mathematical methods and conceptual understandings to investigate the cause(s) and effect(s) of this change. |
| Essential Knowledge | 1.A.1 Natural selection is a major mechanism of evolution. |
| Science Practice | 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. |
| Learning Objective | 1.2 The student is able to evaluate evidence provided by data to qualitatively and quantitatively investigate the role of natural selection in evolution. |
| Essential Knowledge | 1.A.1 Natural selection is a major mechanism of evolution. |
| Science Practice | 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. |
| Learning Objective | 1.3 The student is able to apply mathematical methods to data from a real or simulated population to predict what will happen to the population in the future. |
The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 1.1]
Teacher Support
A good video tutorial by Bozeman on Hardy–Weinberg can be found here.
Everyday Connection for AP® Courses
Evolution and Flu Vaccines
Every fall, the media starts reporting on flu vaccinations and potential outbreaks. Scientists, health experts, and institutions determine recommendations for different parts of the population, predict optimal production and inoculation schedules, create vaccines, and set up clinics to provide inoculations. You may think of the annual flu shot as a lot of media hype, an important health protection, or just a briefly uncomfortable prick in your arm. But do you think of it in terms of evolution?
The media hype of annual flu shots is scientifically grounded in our understanding of evolution. Each year, scientists across the globe strive to predict the flu strains that they anticipate being most widespread and harmful in the coming year. This knowledge is based in how flu strains have evolved over time and over the past few flu seasons. Scientists then work to create the most effective vaccine to combat those selected strains. Hundreds of millions of doses are produced in a short period in order to provide vaccinations to key populations at the optimal time.
Because viruses, like the flu, evolve very quickly (especially in evolutionary time), this poses quite a challenge. Viruses mutate and replicate at a fast rate, so the vaccine developed to protect against last year’s flu strain may not provide the protection needed against the coming year’s strain. Evolution of these viruses means continued adaptations to ensure survival, including adaptations to survive previous vaccines.
Population Genetics
Recall that a gene for a particular character may have several alleles, or variants, that code for different traits associated with that character. For example, in the ABO blood type system in humans, three alleles determine the particular blood-type carbohydrate on the surface of red blood cells. Each individual in a population of diploid organisms can only carry two alleles for a particular gene, but more than two may be present in the individuals that make up the population. Mendel followed alleles as they were inherited from parent to offspring. In the early twentieth century, biologists in a field of study known as population genetics began to study how selective forces change a population through changes in allele and genotypic frequencies.
The allele frequency (or gene frequency) is the proportion of a specific allele within a population, relative to all other alleles of that gene that are present in the population. Until now we have discussed evolution as a change in the characteristics of a population of organisms, but behind that phenotypic change is genetic change. In population genetics, the term evolution is defined as a change in the frequency of an allele in a population. Using the ABO blood type system as an example, the frequency of one of the alleles, IA, is the number of copies of that allele divided by all the copies of the ABO gene in the population. For example, a study in Jordan3 found a frequency of IA to be 26.1 percent. The IB and I0 alleles made up 13.4 percent and 60.5 percent of the alleles respectively, and all of the frequencies added up to 100 percent. A change in this frequency over time would constitute evolution in the population.
The allele frequency within a given population can change depending on environmental factors; therefore, certain alleles become more widespread than others during the process of natural selection. Natural selection can alter the population’s genetic makeup; for example, if a given allele confers a phenotype that allows an individual to better survive or have more offspring. Because many of those offspring will also carry the beneficial allele, and often the corresponding phenotype, they will have more offspring of their own that also carry the allele, thus, perpetuating the cycle. Over time, the allele will spread throughout the population. Some alleles will quickly become fixed in this way, meaning that every individual of the population will carry the allele, while detrimental mutations may be swiftly eliminated if derived from a dominant allele from the gene pool. The gene pool is the sum of all the alleles in a population.
Sometimes, allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies. This phenomenon is called genetic drift. Natural selection and genetic drift usually occur simultaneously in populations and are not isolated events. It is hard to determine which process dominates because it is often nearly impossible to determine the cause of change in allele frequencies at each occurrence. An event that initiates an allele frequency change in an isolated part of the population, which is not typical of the original population, is called the founder effect. Natural selection, random drift, and founder effects can lead to significant changes in the genome of a population.
Hardy–Weinberg Principle of Equilibrium
In the early twentieth century, English mathematician Godfrey Hardy and German physician Wilhelm Weinberg stated the principle of equilibrium to describe the genetic makeup of a population. The theory, which later became known as the Hardy–Weinberg principle of equilibrium, states that a population’s allele and genotype frequencies are inherently stable—unless some kind of evolutionary force is acting upon the population, neither the allele nor the genotypic frequencies would change. The Hardy–Weinberg principle assumes conditions with no mutations, migration, emigration, or selective pressure for or against genotype, plus an infinite population; while no population can satisfy those conditions, the principle offers a useful model against which to compare real population changes.
Working under this theory, population geneticists represent different alleles as different variables in their mathematical models. The variable p, for example, typically represents the frequency of the dominant allele, say Y for the trait of yellow in Mendel's peas. The variable q represents the frequency of the recessive allele, in this case y, that confers the color green. If these are the only two possible alleles for a given locus in the population, p + q = 1. In other words, all the p alleles and all the q alleles make up all of the alleles for that locus that are found in the population.
But what ultimately interests most biologists is not the frequencies of different alleles, but the frequencies of the resulting genotypes, known as the population’s genetic structure, from which scientists can surmise the distribution of phenotypes. If the phenotype is observed, only the genotype of the homozygous recessive alleles can be known; the calculations provide an estimate of the remaining genotypes. Since each individual carries two alleles per gene, if the allele frequencies (p and q) are known, predicting the frequencies of these genotypes is a simple mathematical calculation to determine the probability of getting these genotypes if two alleles are drawn at random from the gene pool. So in the above scenario, an individual pea plant could be pp (YY), and thus produce yellow peas; pq (Yy), also yellow; or qq (yy), and thus producing green peas (Figure 19.2). In other words, the frequency of pp individuals is simply p2; the frequency of pq individuals is 2pq; and the frequency of qq individuals is q2. And, again, if p and q are the only two possible alleles for a given trait in the population, these genotypes frequencies will sum to one: p2 + 2pq + q2 = 1.
Visual Connection
In theory, if a population is at equilibrium—that is, there are no evolutionary forces acting upon it—generation after generation would have the same gene pool and genetic structure, and these equations would all hold true all of the time. Of course, even Hardy and Weinberg recognized that no natural population is immune to evolution. Populations in nature are constantly changing in genetic makeup due to drift, mutation, possibly migration, and selection. As a result, the only way to determine the exact distribution of phenotypes in a population is to go out and count them. But the Hardy–Weinberg principle gives scientists a mathematical baseline of a non-evolving population to which they can compare evolving populations and thereby infer what evolutionary forces might be at play. If the frequencies of alleles or genotypes deviate from the value expected from the Hardy–Weinberg equation, then the population is evolving.
Link to Learning
Use this online calculator to determine the genetic structure of a population.
Science Practice Connection for AP® Courses
AP® Biology Investigative Labs: Inquiry-Based Approach, Investigation 2: Mathematical Modeling: Hardy–Weinberg. In this lab investigation, you apply the Hardy–Weinberg equation and create a spreadsheet to study changes in allele frequencies in a population and to examine possible causes for these changes.
Think About It
Imagine you are trying to determine if a population of flowers is undergoing microevolution. You suspect there is selection pressure on the color of the flower because bees seem to cluster around red flowers more often than blue flowers. In a separate experiment, you discover that blue flower color is dominant to red flower color. In a field, you count 600 blue flowers and 200 red flowers. Based on the H-W equation, what are the expected allele frequencies for flower color?
Two years later, you revisit the same field and discover that out of 1,000 flowers, 650 are blue. Use the H–W equation to determine if the population of flowers is undergoing evolution.
Teacher Support
- This lab investigation is an application of AP® Learning Objective 1.1 and Science Practices 1.5 and 2.2, Learning Objective 1.2 and Science Practices 2.2 and 5.3, and Learning Objective 1.3 and Science Practice 2.2 because students are analyzing data sets and applying the Hardy–Weinberg equation to calculate allele frequencies and determine if a population if evolving based on changes in allele frequencies.
- For additional interactive programs and tutorials to aid students in understanding allelles, go to this website.
- Think About It Answers: In the first example, 200 out of 800 flowers had the recessive homozygous phenotype. The q2=0.25 and thus the frequency of q = 0.5. Since p + q = 1, the frequency of p is also 0.5. In the second example, using the same math, the frequency of p is 0.57 and q is 0.43, so it is clear that the allelic frequencies are changing and that the populations is indeed undergoing natural selection.
- The Think About It questions are applications of AP® Learning Objective 1.1 and Science Practices 1.5 and 2.2, Learning Objective 1.2 and Science Practices 2.2 and 5.3, and Learning Objective 1.3 and Science Practice 2.2 because students are applying the Hardy–Weinberg equation to data sets and using calculated allele frequencies to determine if a population is evolving.
Footnotes
- 2Reddy, M. R., Godoy, A., Dion, K., Matias, A., Callender, K., Kiszewski, A. E., Slotman, M. A. (2013). Insecticide Resistance Allele Frequencies in Anopheles gambiae before and after Anti-Vector Interventions in Continental Equatorial Guinea. The American Journal of Tropical Medicine and Hygiene, 88(5), 897–907. doi:10.4269/ajtmh.12-0467
- 3Sahar S. Hanania, Dhia S. Hassawi, and Nidal M. Irshaid, “Allele Frequency and Molecular Genotypes of ABO Blood Group System in a Jordanian Population,” Journal of Medical Sciences 7 (2007): 51-58, doi:10.3923/jms.2007.51.58.