AP Biology

## Learning Outcomes

- Define population genetics in terms of allele frequencies and genetic variation.
- Explain how gene flow changes allele frequencies through immigration, emigration, or pollen transfer.
- Describe why genetic drift has stronger effects in small populations than in large populations.
- Distinguish between the bottleneck effect and the founder effect using examples.
- Explain how mutation introduces new alleles and supplies variation for evolution.
- Predict how gene flow, genetic drift, and mutation affect Hardy-Weinberg equilibrium.
- Avoid common AP Biology mistakes about randomness, fitness, and population-level evolution.

## AP Biology Syllabus

In AP Biology Unit 7, you are expected to explain how populations evolve through changes in allele frequencies. Gene flow, genetic drift, and mutation are mechanisms that can change the gene pool of a population, either by moving alleles, randomly losing alleles, or creating new alleles. These mechanisms are often tested through data sets, scenarios, Hardy-Weinberg conditions, and short free-response explanations.

- Evolution is defined as a change in allele frequencies in a population over time.
- Gene flow occurs when alleles move into or out of a population.
- Genetic drift is random change in allele frequencies, with the strongest effects in small populations.
- The bottleneck effect occurs after a sharp reduction in population size.
- The founder effect occurs when a small group starts a new population with a nonrepresentative sample of alleles.
- Mutation can create new alleles and introduce genetic variation.
- Hardy-Weinberg equilibrium requires no gene flow, no mutations, and a very large population to reduce drift.
- Natural selection is not the same as drift, gene flow, or mutation, although all can interact during evolution.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why is genetic drift more likely to remove an allele from a population of 10 individuals than from a population of 1,000 individuals?
2. A few birds from a mainland population colonize an island. Their descendants have allele frequencies very different from the mainland population. Which mechanism best explains this?
3. How can pollen transfer between plant populations cause gene flow?
4. Why does mutation introduce genetic variation even though most mutations are rare?
5. Which Hardy-Weinberg conditions are directly violated by gene flow, genetic drift, and mutation?

## Introduction

Population genetics is the study of genetic variation within populations and the processes that change allele frequencies. In AP Biology, this topic connects natural selection, heredity, evolution, and Hardy-Weinberg equilibrium. You should be able to read a scenario and identify whether allele frequencies are changing because of gene flow, genetic drift, mutation, or selection.

Gene flow, genetic drift, and mutation are not the same process. Gene flow moves existing alleles between populations. Genetic drift changes allele frequencies by chance, especially in small populations. Mutation changes DNA and can create new alleles. Together, these mechanisms help explain why populations of the same species may become genetically different over time.

> **Key Term: population genetics**
> The study of genetic variation within populations and the processes that cause allele frequencies to change.
>
> **Key Term: allele frequency**
> The proportion of a specific allele among all copies of that gene in a population.

### Allele frequencies and evolutionary change

A population is a group of individuals of the same species living in the same area and able to interbreed. A gene pool includes all alleles present in that population. When the frequency of alleles in the gene pool changes over generations, evolution has occurred.

For example, imagine a prairie dog population with two alleles for a gene, $$A_1$$ and $$A_2$$. If $$A_1$$ is absent in years 1 and 2, then appears at a frequency of 0.10 in year 3 and rises to 0.15 by year 5, the population’s allele frequencies have changed. The question is: what caused the change? If prairie dogs carrying $$A_1$$ migrated into the population, gene flow is likely. If the population was very small and allele frequencies changed by chance, genetic drift may be responsible. If a DNA change produced a new $$A_1$$ allele, mutation may be involved.

Population genetics is also tested through Hardy-Weinberg equilibrium. A population in Hardy-Weinberg equilibrium has stable allele and genotype frequencies, meaning it is not evolving at the gene being studied. For equilibrium to hold, several conditions must be met: large population size, random mating, no gene flow, no natural selection, and no mutations. Gene flow, drift, and mutation each violate one or more of these assumptions.

The Hardy-Weinberg allele frequency equation is:

$$
p + q = 1
$$

Here, $$p$$ is the frequency of one allele, often the dominant allele, and $$q$$ is the frequency of the other allele, often the recessive allele. The genotype frequency equation is:

$$
p^2 + 2pq + q^2 = 1
$$

Here, $$p^2$$ represents the homozygous dominant genotype frequency, $$2pq$$ represents the heterozygous genotype frequency, and $$q^2$$ represents the homozygous recessive genotype frequency.

> **Test Tip:**
> If a question asks for an allele frequency, look for $$p$$ or $$q$$. If it asks for a genotype frequency, look for $$p^2$$, $$2pq$$, or $$q^2$$. Do not confuse alleles with genotypes.

A key AP Biology idea is that evolution occurs in populations, not individuals. An individual organism does not change its alleles because it “needs” to adapt. Instead, individuals with certain inherited traits may reproduce more, alleles may move between populations, alleles may be lost by chance, or mutations may introduce new variation.

### Gene flow moves alleles between populations

Gene flow is the transfer of alleles from one population to another. It commonly occurs when individuals migrate into or out of a population. If migrants carry alleles at different frequencies from the receiving population, they can change allele frequencies in that population.

> **Key Term: gene flow**
> The movement of alleles into or out of a population, often caused by migration of individuals or gametes.

For example, suppose a mainland population of beetles has a high frequency of an allele for dark coloration. A nearby island population has mostly light-colored beetles and a very low frequency of the dark-color allele. If dark beetles migrate from the mainland to the island and reproduce, the dark-color allele may increase in the island population. That is gene flow.

Gene flow can also occur without whole organisms moving. In plants, pollen can carry alleles from one population to another. Wind, insects, birds, or other pollinators may move pollen between separated plant populations. If the pollen fertilizes ovules in the receiving population, new allele combinations can appear in the next generation.

Gene flow can have several effects:

- It can introduce alleles into a population.
- It can increase genetic variation within the receiving population.
- It can make two populations more genetically similar if alleles move between them regularly.
- It can oppose the effects of local natural selection if migrants continually introduce alleles that are less favorable in the local environment.

Consider two populations of wildflowers. One grows in dry soil and has a high frequency of alleles associated with drought tolerance. The other grows near a stream and has a lower frequency of those alleles. If pollinators regularly move pollen from the dry-soil population to the streamside population, drought-tolerance alleles may enter the streamside population. If the environment later becomes drier, those alleles might become useful for survival and reproduction.

Gene flow can also reduce differences between populations. If two populations exchange many individuals every generation, their gene pools may remain similar. If gene flow stops because of geographic isolation, behavioral isolation, or habitat separation, allele frequencies may diverge. Over long periods, reduced gene flow can contribute to speciation when populations become genetically and reproductively distinct.

> **Exam Warning:**
> Do not describe gene flow as the creation of new alleles. Gene flow moves alleles that already exist in one population into another population. Mutation is the mechanism that creates new alleles.

Gene flow can involve immigration or emigration. Immigration adds individuals and their alleles to a population. Emigration removes individuals and their alleles from a population. Either process can alter allele frequencies, especially when the moving individuals differ genetically from the population they enter or leave.

### Genetic drift changes small populations by chance

Genetic drift is random change in allele frequencies from one generation to the next. It is especially likely to affect small populations because chance events can remove a large fraction of the gene pool.

> **Key Term: genetic drift**
> Random change in allele frequencies in a population, often causing alleles to be lost by chance.

The basic logic is simple. Suppose an allele is found in 10% of a population. In a population of 1,000 individuals, about 100 individuals may carry that allele. It is likely that at least some of those individuals will survive and reproduce. In a population of 10 individuals, however, only about one individual may carry the allele. If that individual dies before reproducing, fails to find a mate, or leaves no offspring by chance, the allele may disappear from the population.

Genetic drift can reduce genetic diversity because alleles may be lost randomly. This matters in conservation biology. Endangered species often have small population sizes, so drift can remove alleles that might have helped the population survive future environmental change. Low genetic diversity can also increase the chance of inbreeding and reduce the range of phenotypes available for selection.

Genetic drift is not natural selection. Natural selection depends on differential reproductive success related to phenotype. Genetic drift depends on chance. An allele can increase because its carriers happened to survive, not because the allele improved survival or reproduction.

> **Test Tip:**
> If a scenario says allele frequencies changed after a random disaster, small population size, or chance survival, think genetic drift. If the scenario says a phenotype improved survival or reproduction, think natural selection.

A major AP Biology pitfall is assuming that alleles left after drift are always the “best” alleles. They may not be. A hurricane might kill birds randomly, leaving only a few survivors. The alleles in the next generation come from those survivors, even if their traits were not better adapted before the storm.

Two named forms of genetic drift are especially common on exams: the bottleneck effect and the founder effect.

> **Key Term: bottleneck effect**
> A form of genetic drift that occurs when a population is sharply reduced in size, leaving a surviving population with less genetic diversity than the original population.

A bottleneck occurs when population size is greatly reduced for one or more generations. Natural disasters such as fires, floods, volcanic eruptions, droughts, or severe storms can cause bottlenecks. Human actions such as overhunting and rapid habitat destruction can also cause them. Because only a small subset of individuals survives, the remaining population may not contain all the alleles that were present before the event.

A classic example is the northern elephant seal. Hunting in the 1800s reduced the population to fewer than 30 individuals. Protection later allowed the population size to rise, but the species still has low genetic diversity compared with southern elephant seals, which did not experience the same severe hunting pressure. This shows that population size can recover numerically while genetic diversity remains reduced.

The bottleneck effect is often modeled with colored objects representing alleles. Imagine a large population containing black, gray, and white allele types. A disaster randomly kills most individuals. If no gray-allele carriers survive, the gray allele is absent from the future population, even if it was useful or neutral before the disaster.

> **Key Term: founder effect**
> A form of genetic drift that occurs when a few individuals start a new population with allele frequencies that differ from the original population.

The founder effect occurs when a small group leaves a larger population and establishes a new population. The founders may carry only a small portion of the genetic diversity found in the original population. Their allele frequencies may also be a nonrandom sample of the original population.

For example, suppose the original population has two alleles, black and gray, in roughly equal frequencies. If a few individuals carrying mostly gray alleles colonize a new island, the island population may have a much higher gray allele frequency than the original population. Another island founded by individuals carrying mostly black alleles may have a very different gene pool. The difference is due to chance sampling of founders, not necessarily to selection.

A well-known human example involves the Amish population in Pennsylvania. The founding group included about 200 immigrants from Europe in the 1700s. Some founders carried an allele associated with Ellis-van Creveld syndrome, a rare skeletal condition. Because the population began from a small group with a higher frequency of that allele, the allele occurs at a higher frequency among descendants than in the wider United States population. This is the founder effect.

To distinguish bottleneck and founder effect on the exam, ask what happened first. If a large population was suddenly reduced, it is a bottleneck. If a small group colonized or started a new population, it is the founder effect.

### Mutation introduces new alleles

Mutation is a change in genetic material. Mutations can occur through errors in DNA replication, exposure to mutagens, or changes in chromosome structure. In population genetics, mutation matters because it can create new alleles.

> **Key Term: mutation**
> A change in DNA sequence or chromosome structure that can create new genetic variation.

Mutations are random with respect to need. This means organisms do not mutate because they “want” or “need” a trait. For example, bacteria exposed to an antibiotic do not individually decide to become resistant. In a bacterial population, some cells may already have mutations that provide resistance, or new mutations may occur randomly. When antibiotic is present, resistant bacteria are more likely to survive and reproduce, so resistance alleles become more common.

Mutation is the original source of all new alleles. Without mutation, gene flow could only move existing alleles, and natural selection could only sort existing variation. Mutation supplies the raw genetic variation that other evolutionary mechanisms can act on.

Mutations vary in scale. A small mutation may change one nucleotide. Insertions or deletions may add or remove nucleotides. Larger mutations may duplicate, delete, or rearrange sections of chromosomes. The effect of a mutation depends on where it occurs and how it changes gene expression or protein function.

In eukaryotes, only mutations in cells that produce gametes can be passed to offspring through sexual reproduction. A mutation in a skin cell, for example, may affect that individual cell line but will not usually enter the next generation. In bacteria and other organisms that reproduce asexually, mutations can be passed to daughter cells during cell division.

Most mutations are neutral or harmful in a given environment, but some can be beneficial. Whether a mutation is beneficial depends on environmental context. An allele that helps bacteria survive in the presence of an antibiotic may provide no advantage when the antibiotic is absent. A coloration allele that improves camouflage in one habitat may make an organism more visible in another.

> **Exam Warning:**
> Avoid saying that mutations happen “so that” organisms can adapt. A better AP Biology explanation is: mutations occur randomly, and natural selection can increase the frequency of beneficial phenotypes produced by some mutations.

Mutation also violates Hardy-Weinberg equilibrium. One condition of equilibrium is no mutations. If mutation changes one allele into another, allele frequencies may shift. In many large populations, mutation alone changes allele frequencies slowly because mutation rates are usually low. However, over many generations, mutation is essential because it creates new variation.

Mutation, gene flow, and drift can interact. Imagine an island lizard population. A mutation creates a new scale-color allele. If the population is small, genetic drift might eliminate it by chance before selection can act. If lizards carrying the allele migrate to another island, gene flow can spread it. If the allele improves camouflage, natural selection may increase its frequency. AP questions often ask you to separate these mechanisms even when they occur together.

A strong free-response answer should identify the mechanism, explain how allele frequencies change, and connect the process to the scenario. For example:

> **Question:**
> A wildfire randomly kills most individuals in a beetle population. The survivors lack an allele that was common before the fire. Explain the evolutionary mechanism involved.
>
> **Answer:**
> This is genetic drift through the bottleneck effect. The wildfire sharply reduced population size, and the surviving beetles were not fully representative of the original gene pool. Because no survivors carried the allele, it was lost from the population, reducing genetic diversity.

That answer earns credit because it names the mechanism, describes the population reduction, explains random allele loss, and states the effect on genetic diversity.

## Key Point Checklist

This article has covered the following key knowledge points:

- Population genetics studies allele frequencies and genetic variation within populations.
- Evolution occurs when allele frequencies in a population change over generations.
- Gene flow transfers alleles between populations through migration or gamete movement, such as pollen transfer.
- Gene flow can increase genetic variation within a population and reduce differences between populations.
- Genetic drift is random change in allele frequencies and has the strongest effect in small populations.
- Genetic drift can remove alleles from a population even if those alleles are useful or neutral.
- The bottleneck effect occurs when population size is sharply reduced, often lowering genetic diversity.
- The founder effect occurs when a few individuals start a new population with allele frequencies unlike the original population.
- Mutation changes DNA and can create new alleles.
- Mutations are random with respect to an organism’s needs.
- Hardy-Weinberg equilibrium requires no gene flow, no mutations, and a large population size to reduce drift.
- Natural selection acts on phenotypes, while evolution is measured as changes in allele frequencies in populations.

## Key Terms and Concepts

- population genetics
- allele frequency
- gene flow
- genetic drift
- bottleneck effect
- founder effect
- mutation
- gene pool
- Hardy-Weinberg equilibrium
- immigration
- emigration
- genetic diversity
- natural selection
- differential reproductive success


Population genetics - Gene flow, genetic drift, mutation