AP Biology

## Learning Outcomes

- Define genetic drift and explain why it has stronger effects in small populations.
- Describe how a bottleneck effect can reduce genetic diversity after a sharp population decline.
- Describe how a founder effect can create a new population with allele frequencies different from the source population.
- Distinguish bottleneck and founder effects from natural selection, gene flow, and mutation.
- Predict how allele frequencies may change after a random event using data or a model.
- Explain why reduced genetic diversity can matter for endangered species and conservation biology.
- Avoid common AP Biology errors, such as saying that individuals evolve or that drift always improves adaptation.

## AP Biology Syllabus

In AP Biology Unit 7, population genetics connects evolution to measurable changes in allele frequencies. Bottleneck and founder effects are specific forms of genetic drift: they change allele frequencies by chance rather than because a phenotype has higher reproductive success. Exam questions may ask you to interpret a model, compare populations before and after a random event, predict genetic diversity, or justify whether a scenario is natural selection or genetic drift.

- Evolution can be described as a change in the genetic makeup of a population over generations.
- Allele frequencies can change through natural selection, gene flow, genetic drift, and mutation.
- Genetic drift is random and has stronger effects in small populations.
- Bottleneck effects occur after a population is greatly reduced for one or more generations.
- Founder effects occur when a few individuals from a larger population establish a new population.
- Both bottleneck and founder effects can reduce genetic diversity.
- Drift can increase, decrease, or eliminate alleles regardless of whether those alleles are beneficial.
- AP free-response answers should connect the event to random sampling, allele frequencies, and population-level change.

## 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 cause large changes in allele frequencies in a population of 10 individuals than in a population of 1,000 individuals?
2. A flood kills most individuals in a beetle population, and the survivors happen to lack one allele that was common before the flood. Is this natural selection or genetic drift? Explain.
3. How does a founder effect differ from a bottleneck effect?
4. Why can a population recover in size after a bottleneck but still have low genetic diversity?
5. What wording should you avoid when explaining evolution in an AP Biology free-response answer?

## Introduction

Population genetics studies genetic variation within populations and the processes that change allele frequencies over time. In AP Biology, this topic links genetics to evolution: if the frequency of alleles changes from one generation to the next, the population has evolved. Natural selection is one major cause of evolutionary change, but it is not the only one. Random events can also alter which alleles are passed on.

Bottleneck and founder effects are two classic forms of genetic drift. In both cases, the alleles in a future population are based on a small sample of the original population. Because small samples may not represent the larger group, some alleles can become more common, less common, or disappear entirely by chance.

> **Key Term: population genetics**
> The study of genetic variation within populations and the processes that cause allele frequencies to change over time.

### Genetic drift changes allele frequencies by chance

Genetic drift is the random loss or change in frequency of alleles in a population. It is not based on whether an allele improves survival or reproduction. Instead, it occurs because not all individuals reproduce, and random events affect which individuals contribute alleles to the next generation.

Imagine a population in which an allele is found in 10% of individuals. In a population of 1,000 individuals, about 100 individuals may carry that allele. Even if many individuals fail to reproduce, there is a good chance at least some carriers will pass the allele on. In a population of 10 individuals, only about one individual may carry that allele. If that individual dies before reproducing, leaves no offspring, or produces offspring that do not inherit the allele, the allele may be lost from the population in a single generation.

> **Key Term: allele frequency**
> The proportion of a particular allele among all copies of a gene in a population.
>
> **Key Term: genetic drift**
> A random change in allele frequencies in a population, especially noticeable in small populations.

This is why population size is central to drift. The smaller the population, the stronger the effect of chance. A large population acts like a large sample: it is more likely to preserve the range of alleles present in the original group. A small population acts like a small sample: it may contain a distorted set of alleles purely by chance.

Genetic drift can affect alleles in several ways:

- An allele may be lost completely.
- An allele may become much more common.
- Genetic diversity may decrease.
- The population may become genetically different from other populations of the same species.
- Harmful, neutral, or beneficial alleles may change in frequency by chance.

The last point is a major exam distinction. Natural selection is nonrandom because individuals with certain phenotypes have higher differential reproductive success in a particular environment. Genetic drift is random. The alleles that increase after drift are not necessarily the alleles that improve survival or reproduction.

> **Test Tip:**
> If the question describes survival because of a useful phenotype, such as camouflage, antibiotic resistance, or drought tolerance, think natural selection. If the question describes a random event, chance survival, or a few individuals starting or rebuilding a population, think genetic drift.

A simple way to describe evolution in population genetics is:

$$
\text{Evolution} = \text{change in allele frequencies in a population over generations}
$$

AP Biology questions may not require heavy calculation for bottleneck and founder effects, but they often ask you to reason from numbers. For example, if a population has 200 copies of an allele before a disaster and 0 copies after, that allele has been lost. If the population later grows from 20 survivors to 2,000 descendants, the lost allele does not automatically return. It can reappear only through mutation or gene flow from another population.

### Bottleneck effects after sharp population size reductions

A bottleneck effect occurs when a population’s size is greatly reduced for one or more generations. The reduction may be caused by a natural disaster, such as a fire, flood, volcanic eruption, drought, or hurricane. It may also be caused by human activity, such as overhunting, rapid habitat destruction, pollution, or the introduction of a new threat.

> **Key Term: bottleneck effect**
> A form of genetic drift that occurs when a population is sharply reduced in size, leaving survivors whose alleles may not represent the original population.

In a bottleneck, the survivors are a small sample of the original population. If survival is mostly due to random luck, the alleles in the surviving group may differ greatly from the alleles in the original group. Some alleles may be missing altogether. The result is often lower genetic diversity in the population after the bottleneck.

> **Key Term: genetic diversity**
> The variety of alleles and genotypes present within a population.

A common model uses colored objects to represent alleles. Suppose a large original population contains black, gray, and white allele types. A fire kills most individuals. By chance, none of the individuals carrying the gray allele survive. The new population is made only from survivors carrying black and white alleles. Even if the population later increases in size, the gray allele is still absent unless mutation or immigration introduces it again.

This point matters in conservation biology. A species can recover numerically but remain genetically limited. The northern elephant seal is a standard example. Hunting in the 1800s reduced the population to fewer than 30 individuals. Protection later allowed the number of seals to rise, but the modern population has lower genetic diversity than related seal populations that did not experience such intense hunting.

Low genetic diversity can create several problems:

- Fewer alleles may mean less ability to respond to future environmental change.
- Harmful recessive alleles may become more common in small populations.
- Closely related individuals may be more likely to mate, increasing inbreeding.
- A disease or environmental change may affect a high proportion of individuals if they are genetically similar.

Do not overstate this. A bottleneck does not always cause immediate extinction, and a population may survive for many generations after one. The AP Biology point is that bottlenecks reduce variation and alter allele frequencies by chance.

> **Exam Warning:**
> Do not write that the disaster “selected the best alleles” unless the scenario clearly says individuals survived because of a phenotype that improved survival. A random flood, fire, or hunting event can leave survivors by chance, so the allele frequency change is genetic drift.

A useful bottleneck explanation for a free-response question might be:

“After the flood, the surviving beetles represented only a small random sample of the original population. Some alleles present before the flood were not carried by the survivors, so those alleles were not passed to the next generation. Therefore, allele frequencies changed and genetic diversity decreased due to genetic drift.”

That answer earns credit because it connects the event to random sampling, allele loss, and population-level change.

### Founder effects when a few individuals start a new population

A founder effect occurs when a small number of individuals from a larger population establish a new population. The founding individuals may colonize an island, enter an isolated lake, move into a new patch of habitat, or become separated by a barrier. Because the founders are only a small sample, their allele frequencies may differ from the source population.

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

For example, suppose a mainland insect population has two alleles for a gene: allele A and allele a. In the mainland population, allele A is common and allele a is rare. A storm carries five insects to an island. By chance, three of the five insects carry allele a. The new island population may start with a much higher frequency of allele a than the mainland population. This is not because allele a is better on the island. It is because the founders did not represent the source population accurately.

Founder effects can also reduce genetic diversity. If the founding individuals do not carry certain alleles, those alleles are absent from the new population. The population may grow rapidly, but growth copies the alleles that were present in the founders. Population size and genetic diversity are related, but they are not the same thing.

A founder effect has three main features:

- A few individuals leave or become separated from a larger population.
- Those individuals establish a new population.
- The new population’s allele frequencies are based on the founders, so they may differ from the original population by chance.

The founder effect is especially relevant to island populations. Islands are often colonized by a small number of individuals. If those individuals carry unusual allele frequencies, the island population can become genetically distinct from the mainland population. Over many generations, genetic drift, mutation, natural selection, and limited gene flow may cause the isolated population to differ even more.

Founder effects can also happen without islands. A small group of plants may colonize a disturbed field. A few animals may become isolated by a river, road, glacier, or mountain range. A human population may be established by a small group of migrants. In every case, the AP Biology focus is the same: a new population begins from a limited sample of alleles.

> **Test Tip:**
> To distinguish founder effect from bottleneck effect, ask: “Did a few individuals start a new population, or did a large population shrink sharply?” Starting a new population suggests founder effect. A sudden reduction in an existing population suggests bottleneck effect.

### Distinguishing drift from natural selection in exam questions

Bottleneck and founder effects are often tested beside natural selection because both can change allele frequencies. The key difference is the cause of the change.

Natural selection occurs when individuals with certain phenotypes survive and reproduce more successfully in a given environment. It acts on phenotypes, not directly on hidden genotypes. For example, darker moths may be less visible on dark tree bark and therefore less likely to be eaten by birds. If darker moths survive and reproduce more often, alleles associated with dark coloration may increase.

Genetic drift occurs when allele frequencies change due to chance. The individuals that survive or found the next generation may not have any adaptive advantage. For example, if a hurricane leaves only four birds on an island, the future population may reflect the alleles those four birds happened to carry. Their traits are not automatically the “best” traits.

> **Key Term: natural selection**
> A nonrandom process in which individuals with phenotypes that improve survival or reproduction in a particular environment leave more offspring.

Both drift and selection can produce evolution because both can change allele frequencies. However, selection tends to increase the frequency of alleles associated with higher reproductive success in a given environment, while drift can increase or decrease allele frequencies regardless of their effects.

AP Biology also expects you to avoid Lamarckian wording. Individuals do not evolve because they need a trait. A beetle does not develop a new allele because of a flood. A bacterium does not “learn” to resist antibiotics. Populations evolve when allele frequencies change across generations. Individual organisms may survive, reproduce, die, or fail to reproduce, but they do not evolve during their lifetime.

> **Exam Warning:**
> Avoid writing “the species adapted because it needed to” or “individuals changed their genes to survive.” Instead, write that existing variation was present, some individuals contributed more alleles to the next generation, and allele frequencies changed in the population.

A strong AP-style comparison might read:

“In natural selection, allele frequencies change because individuals with certain phenotypes have greater reproductive success. In a bottleneck effect, allele frequencies change because a random event reduces the population and the surviving individuals may not carry all alleles from the original population. Therefore, the bottleneck effect is genetic drift rather than selection.”

You may also be asked to interpret data. Suppose a mainland lizard population has allele B at a frequency of 0.60 and allele b at 0.40. A small group colonizes an island, and the island population begins with allele B at 0.20 and allele b at 0.80. If no evidence shows that b improves survival, the safest explanation is founder effect. If the question states that lizards with allele b have a phenotype that improves survival on the island and reproduce more, then natural selection may also be involved after founding.

Gene flow is another process to separate from drift. Gene flow is the transfer of alleles between populations, often through migration or pollen movement. Gene flow can add alleles to a population and may increase genetic diversity. Drift often reduces genetic diversity, especially after bottlenecks or founding events. If individuals enter a population and bring new alleles, the cause is gene flow. If a few individuals randomly determine the allele pool of a reduced or new population, the cause is drift.

A clear free-response answer should usually include three parts:

1. Identify the mechanism: bottleneck effect, founder effect, or genetic drift.
2. Explain the random sampling: survivors or founders carry only some alleles from the original population.
3. State the consequence: allele frequencies change, and genetic diversity may decrease.

For example:

“Because only a few individuals founded the island population, the alleles in the island population came from a small sample of the mainland population. Some mainland alleles were absent from the founders, while others were overrepresented. This changed allele frequencies in the new population by genetic drift and may have reduced genetic diversity.”

This wording is specific, uses correct population-level language, and avoids implying that drift produces better adaptation.

## Key Point Checklist

This article has covered the following key knowledge points:

- Population genetics studies genetic variation and changes in allele frequencies within populations.
- Evolution can be measured as a change in allele frequencies over generations.
- Genetic drift is random change in allele frequencies, not change caused by higher fitness.
- Genetic drift has stronger effects in small populations because small samples are less likely to represent the original allele pool.
- A bottleneck effect occurs when a population is greatly reduced for one or more generations.
- Bottlenecks can be caused by natural disasters or human activities such as overhunting and habitat destruction.
- A bottleneck can remove alleles and reduce genetic diversity even if population size later recovers.
- The northern elephant seal population is a common example of reduced diversity after intense hunting.
- A founder effect occurs when a few individuals from a larger population establish a new population.
- Founder populations may have allele frequencies very different from the source population by chance.
- Natural selection is nonrandom and depends on differential reproductive success linked to phenotype.
- In AP answers, write that populations evolve; individuals do not evolve.

## Key Terms and Concepts

- population genetics
- allele frequency
- genetic drift
- bottleneck effect
- genetic diversity
- founder effect
- natural selection


Population genetics - Bottleneck and founder effects