Learning Objectives
In this section, you will explore the following questions:
- What are the different types of variation in a population?
- Why can only heritable variation be acted upon by natural selection?
- How can genetic drift, the bottleneck effect, and the founder effect influence allele frequencies in a population?
- How can gene flow, mutation, nonrandom mating, and environmental variation affect allele frequencies in a population?
Connection for AP® Courses
Take a look at your classmates. Individuals of a population often display different phenotypes, or express different alleles of a particular gene. These differences are called polymorphisms. The distribution of phenotypes among individuals, known as population variation, is influenced by several factors, including the population’s genetic structure and the environment (Figure 19.3). Understanding the sources of phenotypic variation is important for determining how a population will evolve in response to different evolutionary pressures. Only those variations that are encoded in an individual’s genes can be passed to its offspring and be a target of natural selection.
As you learn in the chapter that discusses the evolution and origin of species, natural selection works by selecting for phenotypes—and the alleles that determine them—that confer beneficial traits or behaviors. Deleterious qualities are selected against. Genetic drift stems from the chance occurrence that some individuals have more offspring than others and, thus, will pass on more of their genes to the next generation. Small and isolated populations are more susceptible to genetic drift. Natural events, such as wildfires or hurricanes, can magnify genetic drift when a large portion of the population is killed. Because a fire does not distinguish between the genotypes of various organisms, no particular genotype survives the fire better than another. Therefore, the genetic structure of the surviving population may be very different from the genetic structure of the original population. This is called the bottleneck effect. Another scenario in which populations might experience a strong influence of genetic drift occurs when some portion of the population leaves to start a new population in a new location or gets separated by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, a phenomenon called the founder effect. Both the bottleneck effect and the founder effect reduce genetic variation within a population—and genetic variation is the basis for natural selection. When individuals leave or join a population, they carry their alleles with them, resulting in changes in the population’s allele frequencies. Allele frequencies also can change due to mutation in DNA and when individuals do not randomly mate with others; when an individual selects a mate based on phenotype, the genotype is also selected. In summary, any of these conditions can result in deviations from the Hardy–Weinberg equilibrium—and lead to the microevolution of a population.
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 | 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.2 Natural selection acts on phenotypic variations in populations. |
| Science Practice | 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. |
| 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. |
| Essential Knowledge | 1.A.2 Natural selection acts on phenotypic variations in populations. |
| Science Practice | 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. |
| Learning Objective | 1.4 The student is able to evaluate data-based evidence that describes evolutionary changes in the genetic makeup of a population over time. |
| Essential Knowledge | 1.A.3 Evolutionary change is also driven by random processes. |
| Science Practice | 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. |
| Learning Objective | 1.8 The student is able to make predictions about the effects of genetic drift, migration, and artificial selection on the genetic makeup of a population. |
| Essential Knowledge | 1.A.3 Evolutionary change is also driven by random processes. |
| Science Practice | 1.4 The student can use representatives and models to analyze situations or solve problems qualitatively and quantitatively. |
| Science Practice | 2.1 The student can justify the selection of a mathematical routine to solve problems. |
| Learning Objective | 1.6 The student is able to use data from mathematical models based on the Hardy–Weinberg equilibrium to analyze genetic drift and the effects of selection in the evolution of specific populations. |
| Essential Knowledge | 1.A.3 Evolutionary change is also driven by random processes. |
| Science Practice | 2.1 The student can justify the selection of a mathematical routine to solve problems. |
| Learning Objective | 1.7 The student is able to justify data from mathematical models based on the Hardy–Weinberg equilibrium to analyze genetic drift and the effects of selection in the evolution of specific populations. |
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][APLO 1.3][APLO 1.4][APLO 1.8][APLO 1.23][APLO 1.24][APLO 1.25][APLO 1.6][APLO 1.7][APLO 1.22]
Genetic Variability
Natural selection and some of the other evolutionary forces can only act on heritable traits, namely an organism’s genetic code. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors may be selected for, while deleterious alleles may be selected against. Acquired traits, for the most part, are not heritable. For example, if an athlete works out in the gym every day, building up muscle strength, the athlete’s offspring will not necessarily grow up to be a body builder. If there is a genetic basis for the ability to run fast, on the other hand, this may be passed to a child.
Link to Learning
Before Darwinian evolution became the prevailing theory of the field, French naturalist Jean-Baptiste Lamarck theorized that acquired traits could, in fact, be inherited; while this hypothesis has largely been unsupported, scientists have recently begun to realize that Lamarck was not completely wrong. Visit this site to learn more.
Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variability, among individuals in a population. The greater the hereditability of a population’s phenotypic variation, the more susceptible it is to the evolutionary forces that act on heritable variation.
The diversity of alleles and genotypes within a population is called genetic variability. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variability to preserve as much of the phenotypic diversity as they can. This also helps reduce the risks associated with inbreeding, the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. For example, a disease that is caused by a rare, recessive allele might exist in a population, but it will only manifest itself when an individual carries two copies of the allele. Because the allele is rare in a normal, healthy population with unrestricted habitat, the chance that two carriers will mate is low, and even then, only 25 percent of their offspring will inherit the disease allele from both parents. While it is likely to happen at some point, it will not happen frequently enough for natural selection to be able to swiftly eliminate the allele from the population, and as a result, the allele will be maintained at low levels in the gene pool. However, if a family of carriers begins to interbreed with each other, this will dramatically increase the likelihood of two carriers mating and eventually producing diseased offspring, a phenomenon known as inbreeding depression.
Changes in allele frequencies that are identified in a population can shed light on how it is evolving. In addition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation, nonrandom mating, and environmental variation.
Genetic Drift
The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others; thus, they will pass on more of their genes to the next generation. A big, powerful male gorilla, for example, is much more likely than a smaller, weaker one to become the population’s silverback, the pack’s leader who mates far more than the other males of the group. The pack leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average. That is, this would occur if this particular selection pressure, or driving selective force, were the only one acting on the population. In other examples, better camouflage or a stronger resistance to drought might pose a selection pressure.
Another way a population’s allele and genotype frequencies can change is genetic drift (Figure 19.4), which is simply the effect of chance. By chance, some individuals will have more offspring than others—not due to an advantage conferred by some genetically-encoded trait, but just because one male happened to be in the right place at the right time (when the receptive female walked by) or because the other one happened to be in the wrong place at the wrong time (when a fox was hunting).
Visual Connection
As seen from the table, the frequency of the b allele is reduced as a percentage of the population due to genetic drift.
| Generation | Individuals with genotype BB | Individuals with genotype Bb | Individuals with genotype bb |
|---|---|---|---|
| 1 | 22 | 53 | 25 |
| 2 | 108 | 118 | 25 |
| 3 | 633 | 0 | 0 |
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Inbreeding depression.
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Genetic drift.
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Bottleneck effect.
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Founder effect
Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance. If one individual of a population of 10 individuals happens to die at a young age before it leaves any offspring to the next generation, all of its genes—1/10 of the population’s gene pool—will be suddenly lost. In a population of 100, that’s only 1 percent of the overall gene pool; therefore, it is much less impactful on the population’s genetic structure.
Link to Learning
Go to this site to watch an animation of random sampling and genetic drift in action.
Genetic drift can also be magnified by natural events, such as a natural disaster that kills—at random—a large portion of the population. Known as the bottleneck effect, it results in a large portion of the gene pool suddenly being wiped out (Figure 19.5). In one fell swoop, the genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population.
Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location or if a population gets divided by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, which results in the founder effect. The founder effect occurs when the genetic structure changes to match that of the new population’s founding fathers and mothers. The founder effect is believed to have been a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners but rare in most other populations. This is likely due to the fact that a higher-than-normal proportion of the founding colonists carried these mutations. As a result, the population expresses unusually high incidences of Huntington’s disease (HD) and Fanconi anemia (FA), a genetic disorder known to cause blood marrow and congenital abnormalities.
Link to Learning
Watch this short video to learn more about the founder and bottleneck effects.
Scientific Method Connection
Testing the Bottleneck Effect
Question: How do natural disasters affect the genetic structure of a population?
Background: When much of a population is suddenly wiped out by an earthquake or hurricane, the individuals that survive the event are usually a random sampling of the original group. As a result, the genetic makeup of the population can change dramatically. This phenomenon is known as the bottleneck effect.
Hypothesis: Repeated natural disasters will yield different population genetic structures; therefore, each time this experiment is run, the results will vary.
Test the hypothesis: Count out the original population using different colored beads. For example, red, blue, and yellow beads might represent red, blue, and yellow individuals. After recording the number of each individual in the original population, place them all in a bottle with a narrow neck that will only allow a few beads out at a time. Then, pour 1/3 of the bottle’s contents into a bowl. This represents the surviving individuals after a natural disaster kills a majority of the population. Count the number of the different colored beads in the bowl, and record it. Then, place all of the beads back in the bottle and repeat the experiment four more times.
Analyze the data: Compare the five populations that resulted from the experiment. Do the populations all contain the same number of different colored beads, or do they vary? Remember, these populations all came from the same exact parent population.
Form a conclusion: Most likely, the five resulting populations will differ quite dramatically. This is because natural disasters are not selective—they kill and spare individuals at random. Now think about how this might affect a real population. What happens when a hurricane hits the Mississippi Gulf Coast? How do the seabirds that live on the beach fare?
Gene Flow
Another important evolutionary force is gene flow: the flow of alleles in and out of a population due to the migration of individuals or gametes (Figure 19.6). While some populations are fairly stable, others experience more flux. Many plants, for example, send their pollen far and wide, by wind or by bird, to pollinate other populations of the same species some distance away. Even a population that may initially appear to be stable, such as a pride of lions, can experience its fair share of immigration and emigration as developing males leave their mothers to seek out a new pride with genetically unrelated females. This variable flow of individuals in and out of the group not only changes the gene structure of the population, but it can also introduce new genetic variation to populations in different geological locations and habitats.
Mutation
Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Species evolve because of the accumulation of mutations that occur over time. The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variability. Some mutations are unfavorable or harmful and are quickly eliminated from the population by natural selection. Others are beneficial and will spread through the population. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce. Some mutations do not do anything and can linger, unaffected by natural selection, in the genome. Some can have a dramatic effect on a gene and the resulting phenotype.
Nonrandom Mating
If individuals nonrandomly mate with their peers, the result can be a changing population. There are many reasons nonrandom mating occurs. One reason is simple mate choice; for example, female peahens may prefer peacocks with bigger, brighter tails. Traits that lead to more matings for an individual become selected for by natural selection. One common form of mate choice, called assortative mating, is an individual’s preference to mate with partners who are phenotypically similar to themselves.
Another cause of nonrandom mating is physical location. This is especially true in large populations spread over large geographic distances where not all individuals will have equal access to one another. Some might be miles apart through woods or over rough terrain, while others might live immediately nearby.
Environmental Variation
Genes are not the only players involved in determining population variation. Phenotypes are also influenced by other factors, such as the environment (Figure 19.7). For example, sun exposure is an environmental factor, as a person who spends more time in the sun will likely have darker skin than a person who spends most of their time indoors (assuming both people had similarly-colored skin to start with). Some major characteristics, such as sex, are determined by the environment for some species. For example, some turtles and other reptiles have temperature-dependent sex determination (TSD). TSD means that individuals develop into males if their eggs are incubated within a certain temperature range, or females at a different temperature range.
Geographic separation between populations can lead to differences in the phenotypic variation between those populations. Such geographical variation is seen between most populations and can be significant. One type of geographic variation, called a cline, can be seen as populations of a given species vary gradually across an ecological gradient. Species of warm-blooded animals, for example, tend to have larger bodies in the cooler climates closer to the earth’s poles, allowing them to better conserve heat. This is considered a latitudinal cline. Alternatively, flowering plants tend to bloom at different times depending on where they are along the slope of a mountain, known as an altitudinal cline.
If there is gene flow between the populations, the individuals will likely show gradual differences in phenotype along the cline. Restricted gene flow, on the other hand, can lead to abrupt differences, even speciation.
Science Practice Connection for AP® Courses
AP® Biology Investigative Labs: Inquiry-Based Approach, Investigation 1: Artificial Selection. Using Wisconsin Fast Plants, you explore evolution by conducting an artificial selection investigation to increase or decrease genetic variation in a population and then determine if extreme selection can change the expression of a quantitative trait.
Think About It
- Do you think genetic drift would happen more quickly on an island or on the mainland? Provide reasoning for your answer.
- Consider the population of red and blue flowers you analyzed in Section 1 to determine if they were undergoing microevolution. Recall that you counted 600 blue flowers and 200 red flowers.
- Imagine that you return four years after your initial visit, and the flowers at the site have been split into two different populations by a newly formed river, which isolates the two populations. In the population 1, you counted 125 blue flowers and 10 red flowers. In the population 2, you counted 450 blue flowers and 300 red flowers. Did genetic drift or natural selection likely cause these change in allele frequencies in population 1? What about population 2? Explain how you know for each population.
Teacher Support
- The lab investigation is an application of AP® Learning Objective 1.8 and Science Practice 6.4 because students are investigating the effect(s) of artificial selection on the genetic makeup of a population.
- The first Think About It question is an application AP® Learning Objective 1.8 and Science Practice 6.4 because students are making a prediction about the effect of genetic drift on the genetic makeup of a population, and the factors that influence the effects of genetic drift.
- First think about it question answer: Genetic drift is more likely on an island because of the founder effect. New island populations are often started by specific individuals of an original population, carrying gene frequencies different from those of the parent population. This causes genetic drift.
- Second Think About It Answers: In the first visit, it was determined that 200 out of 800 flowers had the recessive homozygous phenotype. Therefore, q2=0.25 and q = 0.5. For the first population, 10 out of 135 flowers were red. Therefore, q = 0.27. If bees were selecting for red flowers, the frequency of red flowers should increase, not decrease. Therefore, this evolutionary change in population 1 was likely caused by genetic drift. In the second population, 300 out of 750 flowers were red. Therefore, q=0.63 and p=0.37. This indicates that the frequency of the recessive allele q, which causes red coloration, is increasing, and is consistent with the hypothesis that red flowers are being selected for by bees.
- The second Think About It questions are applications AP® Learning Objective 1.6 and Science Practices 1.4 and 2.1, and Learning Objective 1.7 and Science Practice 2.1, because students are using data sets that require the use of the Hardy–Weinberg equation to justify if genetic drift or selection is involved in the evolution of specific populations.