AP Biology

## Learning Outcomes

- Explain why genetic variation is required for evolution by natural selection.
- Identify mutation and sexual reproduction as major sources of genetic variation.
- Describe how crossing over, independent assortment, and fertilization increase variation.
- Distinguish between evolution in populations and changes in individual organisms.
- Use examples such as antibiotic resistance and peppered moth coloration to explain selection on existing variation.
- Connect changes in allele frequencies to evolution over multiple generations.

## AP Biology Syllabus

In AP Biology Unit 7, natural selection is tested as a process that acts on heritable variation within populations. For this subtopic, focus on how variation arises, why it matters, and how selection changes the genetic makeup of populations over generations. Exam questions often ask you to explain a real scenario using variation, inheritance, differential reproductive success, and changes in allele frequencies.

- Genetic variation means individuals in a population differ in alleles and, often, phenotypes.
- Mutations create new alleles and are a source of new genetic variation.
- Sexual reproduction reshuffles existing alleles through meiosis and fertilization.
- Natural selection acts on phenotypes, not directly on genotypes.
- Populations evolve when allele frequencies change over time.
- Individuals do not evolve during their lifetimes in response to need or effort.
- A trait must be heritable for natural selection to change its frequency across generations.
- More genetic variation can increase the chance that some individuals survive environmental 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 can natural selection occur only when there is variation among individuals in a population?
2. How do mutations differ from crossing over and independent assortment as sources of genetic variation?
3. In an antibiotic-treated bacterial population, why is it incorrect to say that bacteria “learn” to become resistant?
4. What does it mean to say that evolution is a change in the gene pool of a population over time?
5. How could an environmental change cause one phenotype to become more common and then later become less common?

## Introduction

Natural selection cannot produce adaptation from nothing. It acts on variation that already exists in a population. If all individuals in a population had exactly the same alleles and the same phenotypes, then a change in the environment would affect them in the same way. Some might survive by chance, but there would be no particular heritable trait for selection to favor.

Genetic variation is therefore the raw material for evolution. Mutations create new alleles, and sexual reproduction rearranges alleles into new combinations. When some of these genetic differences affect phenotypes, they can influence survival or reproduction. Over generations, this can change the genetic makeup of a population.

> **Key Term: genetic variation**
> Differences in DNA sequences, alleles, or inherited traits among individuals in a population.

### Why genetic variation is necessary for natural selection

Darwin’s theory of natural selection depends on variation. Members of a population are not identical. Some individuals have phenotypes that allow them to survive and reproduce more successfully in a particular environment. Others have phenotypes that reduce their chance of survival or reproduction. Over time, the alleles associated with more successful phenotypes can become more common.

A standard AP Biology explanation of natural selection should include four connected ideas:

- Variation exists in a population.
- Some variation is heritable.
- More individuals are produced than can survive, so competition, predation, disease, or other pressures affect survival and reproduction.
- Individuals with advantageous heritable traits tend to leave more offspring, causing those traits and their alleles to become more common over generations.

> **Key Term: natural selection**
> A nonrandom process in which individuals with heritable phenotypes better suited to an environment have greater reproductive success, causing related alleles to become more common over generations.

Variation is useful only to evolution if it is heritable. For example, a plant may grow taller because it received more sunlight. That extra height, caused mainly by the environment, is not necessarily passed to offspring. By contrast, if plants differ genetically in stem growth, and taller stems help some plants access light and reproduce, then selection can change the frequency of alleles associated with taller growth.

A useful way to remember this is: natural selection acts on phenotypes, but evolution is measured in populations by changes in allele frequencies. A predator sees a moth’s wing color, not its DNA sequence. However, if wing color is inherited, predation can change the frequency of color alleles in the next generation.

> **Key Term: allele frequency**
> The proportion of a specific allele among all copies of that gene in a population.

The peppered moth example is a classic case. In England, moth populations included light and dark wing-color variants. Before heavy industrial pollution, light-colored lichens on trees helped light moths blend in, making them harder for birds to detect. Dark moths were more visible and more likely to be eaten. During the Industrial Revolution, soot darkened tree surfaces and killed much of the pale lichen. In those polluted areas, dark moths became better camouflaged, survived predation more often, and reproduced more successfully. The frequency of dark coloration increased.

When pollution controls later reduced soot and light lichens returned, the selective advantage shifted back toward lighter moths. This shows a key AP point: the environment determines which phenotypes are favorable. A trait is not automatically “better” in every setting.

> **Test Tip:**
> In free-response answers, do not simply write “the organism adapted.” State the original variation, the selection pressure, the effect on survival or reproduction, and the resulting change in allele or phenotype frequency.

### Mutation: the origin of new alleles

Mutations are the ultimate source of new genetic variation. A mutation is a change in DNA sequence. Mutations can happen because of errors during DNA replication, changes caused by mobile genetic elements such as transposons, or DNA damage from environmental factors. Many mutations have no effect on phenotype. Some are harmful. A small number can be beneficial in a particular environment.

> **Key Term: mutation**
> A change in the nucleotide sequence of DNA that can create a new allele.

Mutations are random with respect to fitness. This does not mean every mutation is equally likely or that mutations have no causes. It means mutations do not occur because an organism “needs” them. For example, bacteria exposed to an antibiotic do not decide to mutate in order to survive. Instead, a large bacterial population may already contain rare mutants with alleles that confer resistance. When antibiotic is added, susceptible bacteria die or fail to reproduce, while resistant bacteria survive and reproduce. The resistant allele becomes more common because of selection.

This distinction is heavily tested. Natural selection is not a process in which individuals change their genes in response to the environment. The environment selects among existing heritable variants.

> **Exam Warning:**
> Avoid Lamarckian wording such as “the bacteria became resistant because they needed to survive” or “the moths changed color to hide.” A stronger AP answer says that resistant bacteria or darker moths already existed due to genetic variation, then survived and reproduced at higher rates under certain conditions.

Mutations can affect protein structure, gene regulation, or chromosome organization. A mutation in a coding sequence may change an amino acid, although the redundancy of the genetic code means some DNA changes do not alter the amino acid sequence. Mutations in regulatory DNA can change when, where, or how much a gene is expressed. Mutations in gametes can be passed to offspring in sexually reproducing organisms; mutations in somatic cells usually affect only the individual and are not inherited by offspring.

In bacteria and other organisms with short generation times, mutation can generate visible evolutionary change rapidly. Antibiotic resistance is an exam-favorite example because it clearly links mutation, selection, and reproduction:

- A starting bacterial population contains variation in antibiotic resistance.
- The antibiotic is a selection pressure.
- Susceptible bacteria are killed or inhibited.
- Resistant bacteria survive and reproduce.
- The frequency of resistance alleles increases in the population.

This is evolution because the genetic composition of the population changes over generations.

### Sexual reproduction reshuffles genetic variation

Mutation creates new alleles, but sexual reproduction produces new combinations of alleles. In eukaryotes, meiosis and fertilization make offspring genetically different from their parents and siblings. This reshuffling can produce phenotypes on which natural selection can act.

> **Key Term: crossing over**
> Exchange of DNA between homologous chromosomes during prophase I of meiosis, producing chromosomes with new combinations of alleles.

During crossing over, homologous chromosomes pair and exchange corresponding DNA segments. This means alleles that were originally on one chromosome can be combined with alleles from the homologous chromosome. The result is recombinant chromosomes, which increase genetic differences among gametes.

> **Key Term: independent assortment**
> Random orientation and separation of homologous chromosome pairs during meiosis I, producing gametes with different combinations of maternal and paternal chromosomes.

Independent assortment occurs because each homologous chromosome pair lines up independently of the others during meiosis I. For a species with many chromosome pairs, this produces a very large number of possible chromosome combinations in gametes. Human gametes, for example, can receive many different mixtures of maternal and paternal chromosomes.

Fertilization adds another layer of variation because any sperm can fertilize any egg. The combination of two genetically unique gametes produces a genetically unique zygote.

These processes do not usually create brand-new alleles; mutation does that. Instead, they reorganize existing alleles into new combinations. This distinction matters on the exam. If a question asks for the origin of a new allele, answer mutation. If it asks why siblings differ or why gametes are genetically unique, discuss crossing over, independent assortment, and fertilization.

> **Test Tip:**
> When comparing mutation and meiosis, use precise verbs. Mutations “create” new alleles. Crossing over, independent assortment, and fertilization “shuffle,” “recombine,” or “rearrange” existing alleles.

Sexual reproduction can speed evolutionary response because it produces many genotype combinations. Suppose a population contains one allele that improves drought tolerance and another allele that improves resistance to an herbivore. If these alleles occur in different individuals, sexual reproduction can bring them together in some offspring. If the environment becomes dry and herbivores are common, offspring with both beneficial alleles may have higher reproductive success.

However, genetic variation does not guarantee survival. A population may still go extinct if the environment changes too quickly, if the needed variation is absent, or if population size becomes too small. Variation increases the probability that some individuals possess traits that are favorable under new conditions.

### From variation to evolution: gene pools and differential reproductive success

AP Biology often defines evolution as a change in the genetic makeup of a population over time. More specifically, it is a change in the gene pool across generations.

> **Key Term: gene pool**
> All alleles for all genes present in a population.

A population’s gene pool can change through several processes, including natural selection, gene flow, genetic drift, and mutation. For this subtopic, the main idea is that genetic variation provides the material on which these processes act. Without allele differences, there is little for selection, drift, or gene flow to change.

> **Key Term: differential reproductive success**
> Unequal production of surviving, fertile offspring by individuals with different heritable traits.

The phrase “survival of the fittest” can be misleading if fitness is interpreted as strength, speed, or health alone. In evolutionary biology, fitness refers to reproductive success in a particular environment. An organism with a trait that helps it produce more surviving offspring has higher fitness than an organism that produces fewer surviving offspring.

> **Key Term: fitness**
> An organism’s reproductive success relative to others in the population, usually measured by contribution to future generations.

Consider giraffe neck length. A simplified natural selection explanation is that ancestral giraffe populations varied in neck length. If longer-necked individuals could access more food during shortages, they were more likely to survive and reproduce. If neck length was heritable, alleles associated with longer necks increased in frequency over generations. The population changed; individual giraffes did not stretch their necks and pass on stretched necks.

Artificial selection follows the same genetic logic, except humans choose which individuals reproduce. In classroom artificial selection labs, fast-growing plants such as Brassica rapa may be selected for a quantifiable trait, such as number of trichomes on leaves. If plants with many trichomes are chosen as parents, and trichome number has a genetic basis, the next generation may show a higher average trichome number. This demonstrates that selection can shift trait distributions when heritable variation exists.

Selection can act in different patterns:

- Directional selection favors one extreme phenotype, shifting the population average in one direction. Peppered moth color during industrial pollution is a common example.
- Stabilizing selection favors intermediate phenotypes and selects against extremes. Bird clutch size is often used: too few eggs may leave no surviving offspring, while too many may be difficult to feed.
- major selection favors both extremes over intermediate phenotypes. For example, light and dark mice may be camouflaged in a patchy habitat, while intermediate-colored mice are more visible to predators.

All three types require variation. They differ in which phenotypes have the highest reproductive success.

Population genetics uses allele frequencies to describe these changes. In a simple two-allele system, allele frequencies can be represented as:

$$
p + q = 1
$$

where $$p$$ and $$q$$ are the frequencies of the two alleles. Under Hardy-Weinberg conditions, genotype frequencies can be predicted by:

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

Hardy-Weinberg equilibrium describes a non-evolving population under ideal conditions: no mutation, no selection, no gene flow, very large population size, and random mating. Real populations often violate these assumptions. For this subtopic, the key link is that evolution is detected when allele frequencies change from one generation to the next.

> **Exam Warning:**
> Do not say that natural selection gives organisms the traits they need. Selection can only increase or decrease the frequency of heritable variants that are already present or that arise by mutation.

In data-based questions, you may be asked to interpret graphs showing changes in phenotype frequencies. For example, if beige moths increase during drought because brown grass improves their camouflage from predators, your answer should connect the environmental change to survival and reproduction. A strong response might say: beige moths were less visible against dry brown grass, so birds ate fewer beige moths than green moths. Beige moths survived and reproduced more often, increasing the percentage of beige moths in the population.

Notice the wording: the population changes in percentage or allele frequency. The individual moths do not transform from green to beige because of drought.

## Key Point Checklist

This article has covered the following key knowledge points:

- Genetic variation is the raw material for evolution by natural selection.
- Natural selection requires heritable variation among individuals in a population.
- Mutations create new alleles by changing DNA sequences.
- Mutations are random with respect to an organism’s needs or fitness.
- Sexual reproduction increases variation by reshuffling alleles through crossing over, independent assortment, and fertilization.
- Natural selection acts on phenotypes, but evolution is measured as changes in allele frequencies.
- Differential reproductive success causes some heritable traits to become more common over generations.
- Fitness means relative reproductive success, not simply strength or survival.
- Populations evolve; individual organisms do not evolve during their lifetimes.
- Antibiotic resistance is explained by selection on preexisting resistant variants, not by bacteria learning resistance.
- Peppered moth coloration shows that environmental conditions determine which phenotype is favored.
- Artificial selection demonstrates that choosing parents with heritable traits can shift trait frequencies in offspring.

## Key Terms and Concepts

- genetic variation
- natural selection
- allele frequency
- mutation
- crossing over
- independent assortment
- gene pool
- differential reproductive success
- fitness


Variations in populations - Genetic variation as raw material