AP Biology

## Learning Outcomes

- Define biological fitness in terms of reproductive success, not strength or survival alone.
- Explain how heritable phenotypic variation leads to differential reproductive success.
- Predict how environmental change can alter which phenotypes have the highest fitness.
- Distinguish natural selection from individual “need,” learning, or intentional adaptation.
- Apply examples such as antibiotic resistance, peppered moth coloration, and clutch size to fitness-based reasoning.
- Interpret selection scenarios as directional, stabilizing, or major selection when appropriate.
- Write AP Biology-style explanations using population-level language and evidence.

## AP Biology Syllabus

In AP Biology Unit 7, natural selection is tested as a mechanism of evolution: populations change over generations when individuals with certain heritable phenotypes leave more offspring than others. For this subtopic, the key exam skill is explaining fitness as differential reproductive success and connecting phenotype, environment, survival, reproduction, and changes in allele or trait frequencies.

- Natural selection acts on phenotypic variation within a population.
- Phenotypic variation can arise from genetic variation, including mutation and recombination.
- The environment determines which phenotypes are favorable in a particular time and place.
- Individuals with favorable phenotypes tend to survive and reproduce more successfully.
- Populations, not individuals, evolve over generations.
- Fitness is measured by reproductive success, not by size, speed, intelligence, or survival alone.
- Selection can be directional, stabilizing, or major depending on which phenotypes have the greatest reproductive success.
- Artificial and sexual selection also involve unequal reproductive success, but the selecting factor differs.

## 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. In biological terms, why can an organism that survives for a long time still have low fitness?
2. How does antibiotic use increase the frequency of antibiotic-resistant bacteria in a population?
3. Why is it inaccurate to say that individual moths “evolved darker wings” during the Industrial Revolution?
4. In a bird population, why might an intermediate clutch size have higher fitness than very small or very large clutch sizes?
5. If an environment changes, why might a phenotype that was once advantageous become disadvantageous?

## Introduction

In everyday language, “fitness” often means physical strength, health, or athletic ability. In AP Biology, fitness has a much narrower meaning: it refers to how successfully an organism passes its genes to the next generation. A small, slow, or short-lived organism can have high fitness if it produces many surviving offspring. A strong organism can have low fitness if it fails to reproduce.

Natural selection depends on differences in fitness. Individuals in a population vary, and some of that variation is heritable. When the environment favors certain phenotypes, individuals with those phenotypes are more likely to survive long enough to reproduce, attract mates, produce viable offspring, or care for offspring successfully. Over many generations, favorable traits become more common in the population.

> **Key Term: fitness**
> The reproductive success of an organism, usually measured by its ability to produce surviving, fertile offspring compared with other individuals in the population.

### Fitness means reproductive success, not survival alone

Fitness includes survival, but survival matters mainly because it can allow reproduction. On the AP Biology exam, you should avoid using “fit” as a vague synonym for “strong” or “well adapted.” A fit organism is one that contributes more genetic material to future generations.

For example, imagine two male fish in the same population. Fish A survives for six years but never mates. Fish B survives for only two years but produces 200 offspring, several of which survive and reproduce. Fish B has higher biological fitness because it leaves more descendants. Survival without reproduction does not directly increase the frequency of that individual’s alleles in future generations.

This is why AP Biology often uses the phrase differential reproductive success. Natural selection occurs when some individuals reproduce more successfully than others because of heritable traits that affect survival or reproduction in a particular environment.

> **Key Term: differential reproductive success**
> Unequal reproduction among individuals in a population, often caused by differences in heritable traits that affect survival, mating, or offspring production.

Fitness is also relative. A phenotype does not have high fitness in every possible environment. Its success depends on current conditions. Dark wing coloration may help a moth avoid predators on dark tree bark, but the same coloration may make the moth more visible on pale lichen-covered trees. Long necks may help giraffes reach food during competition for leaves, but any trait can carry costs, such as energy demands or developmental constraints.

> **Test Tip:**
> If you use the word “fitness” in a free-response answer, explain it as reproductive success. For example: “The resistant bacteria have higher fitness because they survive antibiotic exposure and reproduce, increasing the frequency of resistance alleles.”

Fitness can be affected by many stages of the life cycle:

- Ability to obtain food or avoid predators
- Resistance to disease, parasites, toxins, or environmental stress
- Ability to attract mates
- Number of offspring produced
- Survival of offspring to reproductive age
- Ability to provide parental care, where relevant
- Fertility of offspring

A trait that improves one stage but harms another may not increase overall fitness. For instance, a bright color might attract mates but also attract predators. Whether that trait spreads depends on its total effect on reproductive success.

### How natural selection changes populations

Natural selection begins with variation. Members of a population are not genetically identical, so they may show different phenotypes. Some variation is caused by mutations, independent assortment, crossing over, and random fertilization. Natural selection acts on phenotypes, but if phenotypic differences are heritable, selection can change the genetic makeup of the population over time.

> **Key Term: natural selection**
> A mechanism of evolution in which individuals with favorable heritable phenotypes survive and reproduce more successfully than individuals with less favorable phenotypes.

A standard natural selection explanation has four parts:

1. Individuals in a population vary in phenotype.
2. Some phenotypic differences are heritable.
3. The environment favors some phenotypes over others.
4. Individuals with favored phenotypes have higher reproductive success, so the traits and associated alleles become more common over generations.

Consider antibiotic-resistant bacteria. In a bacterial population, some cells may already carry a mutation or gene that gives resistance to an antibiotic. When the antibiotic is applied, susceptible bacteria are more likely to die. Resistant bacteria are more likely to survive and reproduce. As reproduction continues in the presence of the antibiotic, the proportion of resistant bacteria increases.

The antibiotic does not cause individual bacteria to “try” to become resistant. It acts as a selective pressure. Resistance was already present or arose by mutation before selection increased its frequency.

> **Key Term: selective pressure**
> An environmental factor that affects which phenotypes have higher or lower reproductive success.

The peppered moth is another classic example. In England before heavy industrial pollution, many trees were covered with light-colored lichens. Light moths were harder for birds to see, so they were less likely to be eaten and more likely to reproduce. During the Industrial Revolution, pollution darkened tree bark and killed much of the pale lichen. Dark moths then had better camouflage, suffered less predation, and reproduced more successfully. Later, pollution controls allowed lichens to return, and light moths again gained an advantage in many areas.

The key AP Biology reasoning is not that moths changed color because they needed camouflage. The population already contained variation in wing color. The environment changed which wing color had higher fitness.

> **Exam Warning:**
> Do not write Lamarckian explanations such as “the moths evolved darker wings because the trees became darker” or “bacteria learned to resist antibiotics.” Individuals do not evolve in response to need. Populations evolve when heritable traits become more or less common over generations.

Natural selection can increase adaptation, but it does not create perfect organisms. Selection can act only on existing variation, and traits may have trade-offs. Environmental conditions can also change, so a trait that increases fitness at one time may decrease fitness later.

> **Key Term: adaptation**
> A heritable trait that increases fitness in a particular environment.

### Directional, stabilizing, and major patterns of fitness

Fitness differences across a range of phenotypes can produce different patterns of selection. AP Biology questions may show graphs of trait distributions before and after selection, or they may describe which individuals reproduce most successfully.

> **Key Term: directional selection**
> Selection in which one extreme phenotype has the highest fitness, causing the population’s trait distribution to shift in one direction.

Directional selection occurs when one end of a phenotypic range is favored. The peppered moth during industrial pollution is a strong example: darker coloration became more common because dark moths were better camouflaged on dark bark. Antibiotic resistance is also often directional: in the presence of an antibiotic, resistant bacteria have higher fitness than susceptible bacteria.

Directional selection does not always mean “bigger is better” or “more is better.” It means one side of the trait range is favored under the current conditions. If the environment changes, the direction of selection can reverse.

> **Key Term: stabilizing selection**
> Selection in which intermediate phenotypes have the highest fitness and extreme phenotypes are selected against.

Stabilizing selection reduces variation around an optimum. A common example is clutch size in birds, meaning the number of eggs produced in a reproductive cycle. If a bird lays too few eggs, there is a higher chance that none of its offspring survive. If it lays too many eggs, the parents may be unable to feed and care for all of them, reducing offspring survival. In robins, an intermediate clutch size of about five to six eggs is often favored. The intermediate phenotype has higher reproductive success than either extreme.

This example shows why fitness is not always maximized by producing the largest possible number of offspring. Reproductive success depends on surviving offspring, not just gametes, eggs, or births.

> **Key Term: major selection**
> Selection in which both extreme phenotypes have higher fitness than intermediate phenotypes.

major selection favors both extremes of a phenotypic range. Imagine mice living in a habitat with light sandy soil and dark rocky patches. Light fur may camouflage mice on sand, and dark fur may camouflage mice on rocks. Intermediate fur may match neither background well, making those mice more visible to predators. If both light and dark mice survive and reproduce more successfully than intermediate mice, the population may become more divided in phenotype.

Selection patterns are based on reproductive success. To identify the pattern, ask: which phenotype leaves the most surviving offspring?

> **Test Tip:**
> When interpreting a selection graph, focus on the change in phenotype frequencies across generations. A shift toward one extreme suggests directional selection; narrowing around the middle suggests stabilizing selection; increased frequencies at both extremes suggest major selection.

### Reproductive success in artificial and sexual selection

Natural selection is not the only process involving differential reproductive success. Artificial selection and sexual selection are also tested in AP Biology because they show how unequal reproduction can change populations.

> **Key Term: artificial selection**
> Selection in which humans choose which organisms reproduce based on desired traits.

In artificial selection, humans act as the selecting force rather than the natural environment. Farmers and breeders choose individuals with preferred phenotypes and allow them to reproduce. Over generations, those traits become more common.

A common plant example involves wild cabbage, Brassica oleracea. Human selection for different traits produced familiar crops: larger leaves gave rise to kale, enlarged flower clusters contributed to cauliflower, lateral buds produced Brussels sprouts, and selection for stems and flowers contributed to broccoli. These changes occurred because humans repeatedly gave some plants higher reproductive success than others.

AP Biology lab questions may describe artificial selection using Fast Plants, Brassica rapa. Students might select plants with many trichomes, which are small hairlike structures on leaves. If plants with more trichomes are chosen as parents, the next generation may show a higher average number of trichomes, provided the trait is heritable. In such questions, be ready to identify the independent variable, dependent variable, control group, and evidence that selection changed the trait distribution.

Sexual selection is another special case.

> **Key Term: sexual selection**
> Selection in which individuals with traits that improve mating success have higher reproductive success.

Sexual selection occurs when certain traits increase the chance of attracting mates or winning competition for mates. Examples include bright coloration, songs, courtship behaviors, antlers, or displays. These traits may not always improve survival. In fact, they may make an organism more visible to predators or require extra energy. However, if they greatly increase mating success, they can still increase fitness.

This point is often tested because it separates survival from reproduction. A trait can reduce survival slightly but still spread if it increases reproductive success enough. Conversely, a trait that improves survival but prevents mating may not increase fitness.

> **Exam Warning:**
> Do not assume that the most common trait is always the “best” trait in an absolute sense. A trait’s fitness depends on the environment, mating system, predators, resources, and other conditions at that time.

When answering free-response questions, connect sexual or artificial selection to differential reproduction. For example: “Males with longer tail feathers attracted more mates, so they produced more offspring. If tail length is heritable, alleles associated with longer tails would increase in frequency over generations.”

### Writing exam answers about fitness

AP Biology questions often ask you to explain, justify, predict, or analyze data. A strong answer about fitness should link cause and effect in a clear chain:

Phenotypic variation → environmental or mating advantage → greater survival or mating success → more surviving offspring → increased frequency of trait or allele in the population.

For example:

> **Question:**
> During a drought, grass in a moth habitat turns brown. Beige moths become more common, while green moths decrease. Explain how natural selection caused this change.

A high-scoring response would state that the moth population contained variation in color. During the drought, beige moths were better camouflaged against brown grass, so birds were less likely to eat them. Beige moths therefore survived and reproduced more successfully than green moths. Because coloration is heritable, the frequency of beige moths increased in the population over generations.

Notice the answer includes variation, selection by the environment, reproductive success, heredity, and population change. It does not say that green moths changed color or that beige moths appeared because they were needed.

Quantitative questions may ask you to compare percentages, rates, or graph trends. If a graph shows two moth colors as percentages of the population, the two percentages may add to 100% if those are the only two color categories. If beige moths rise from 20% to 80%, describe the increase using the data, then explain why that increase suggests higher reproductive success under the stated conditions.

For experimental design, use fitness-related dependent variables that can actually be measured. Examples include number of offspring, number of surviving offspring, proportion surviving to reproductive age, mating success, or frequency of a trait in the next generation. Avoid vague dependent variables such as “adaptation” unless you define how it is measured.

A strong prediction also includes a justification. For example: “If the antibiotic is removed, the frequency of resistance may decrease if resistance carries an energy cost, because nonresistant bacteria may reproduce faster in the antibiotic-free environment.” This answer recognizes that fitness depends on conditions and that traits can involve trade-offs.

## Key Point Checklist

This article has covered the following key knowledge points:

- Fitness means reproductive success, especially production of surviving, fertile offspring.
- Survival contributes to fitness only if it increases the chance of reproduction.
- Natural selection requires heritable variation within a population.
- The environment determines which phenotypes have higher fitness in a given setting.
- Individuals with favorable phenotypes reproduce more successfully, causing traits and alleles to become more common over generations.
- Populations evolve; individual organisms do not evolve during their lifetimes.
- Antibiotics select for resistant bacteria that already possess resistance traits or genes.
- Peppered moth coloration shows how environmental change can alter which phenotype has higher fitness.
- Stabilizing selection favors intermediate phenotypes, such as moderate clutch size in some birds.
- Directional selection favors one extreme phenotype, while major selection favors both extremes.
- Artificial selection occurs when humans choose which organisms reproduce.
- Sexual selection occurs when traits increase mating success, even if they do not always improve survival.

## Key Terms and Concepts

- fitness
- differential reproductive success
- natural selection
- selective pressure
- adaptation
- directional selection
- stabilizing selection
- major selection
- artificial selection
- sexual selection


Natural selection - Fitness and reproductive success