7.2 Natural Selection

Natural Selection AP Biology Summary

Natural selection acts on phenotypic variation in a population, favoring traits that help organisms survive and reproduce in a given environment. Because environments change and apply different selective pressures, the same trait can raise fitness in one setting and lower it in another, and even variation in molecules inside cells can shape an organism's reproductive success.

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Why This Matters for the AP Biology Exam

Natural selection runs through the entire AP Biology course, so getting the logic right here pays off everywhere. You need to connect three ideas: variation must already exist, the environment applies selective pressure, and heritable traits that boost reproductive success become more common over time.

On the exam, you may be asked to describe why phenotypic variation matters, explain how a selective pressure changes a population, or use data to support a claim about fitness. Precise language counts. Avoid statements that sound Lamarckian (organisms do not develop traits because they need them), and do not just toss out the word "fitness" without explaining that it means reproductive success. Remember that natural selection acts on individuals, but populations are what evolve.

Key Takeaways

• Natural selection acts on phenotypic variation that already exists in a population; it cannot create new traits on demand.
• Environments change and apply selective pressures from both living (biotic) and nonliving (abiotic) factors.
• A phenotypic variation can increase fitness in one environment and decrease it in another.
• Fitness means reproductive success, not just strength or survival.
• Variation in the number and types of molecules within cells (like enzymes and proteins) can help a population survive and reproduce in different environments.
• For selection to drive change, the favored traits must be heritable so they pass to offspring.

Phenotypic Variation Is the Raw Material

A phenotype is any observable trait, such as color, shape, size, or a behavior. Within a population, individuals show a range of these traits. Those differences may look minor, but they decide who survives and reproduces when the environment poses a challenge.

Without phenotypic variation, natural selection has nothing to act on. If every individual were identical, they would all face an environmental change the same way. Natural selection works by favoring existing variations that give an advantage in a particular environment.

For selection to change a population over time, three things must be true:

• Phenotypic variation exists in the population.
• Some of that variation affects survival and reproduction.
• The variation is heritable, so it can be passed to offspring.

Over generations, traits tied to higher reproductive success become more common, while traits tied to lower reproductive success become rarer. The environment acts as the selector, not a conscious force.

Environmental Change and Selective Pressure

A selective pressure is any environmental factor that makes organisms with certain traits more likely to survive and reproduce. These pressures come from biotic factors (predators, competitors, pathogens) and abiotic factors (temperature, rainfall, pollution).

Because environments fluctuate, selective pressures shift too. A trait that helped one generation can hurt the next. This is why evolution is ongoing: populations keep adapting as conditions change. Examples of pressures include climate shifts, new predators or competitors, habitat changes, pollution, and disease.

Molecular Variation and Fitness

Variation is not just about traits you can see from the outside. Differences in the number and types of molecules within cells, such as proteins and enzymes, can give a population a greater ability to survive and reproduce across different environments.

For example, different enzyme versions, made from variations in genes, can change how efficiently a cell carries out a reaction. A more effective enzyme variant might let an organism process a nutrient or handle a stress better, which connects directly to its reproductive success. So molecular-level variation feeds the same selection process that acts on whole-organism traits.

When the Same Trait Helps or Hurts

The effect of a variation depends on the environment. A trait that raises fitness in one setting can lower it in another.

A variation that increases fitness might improve camouflage, help an organism find or compete for food, boost disease resistance, increase mating success, or improve tolerance of conditions like heat or drought. A variation that decreases fitness might do the opposite, such as making an organism easier for predators to spot or more vulnerable to disease.

Example: Sickle Cell and Malaria Resistance

This is an AP example you should be able to reason through, not a separate rule to memorize. Sickle cell anemia comes from a mutation in the gene for hemoglobin.

• People with two copies of the mutation often have sickle cell anemia, which lowers fitness.
• People with one copy usually avoid severe symptoms and also have increased resistance to malaria.
• In regions where malaria is common, carriers can have higher fitness than people with no copies (vulnerable to malaria) or two copies (severe disease).

The result is that the allele stays at higher frequency in malaria-prone regions and stays rare where malaria is absent. This shows clearly how environment decides whether a variation helps or hurts.

Example: DDT Resistance in Insects

Insecticide resistance shows natural selection on a short timescale.

1. Before DDT use, insect populations already had genetic variation, including rare individuals with resistance.
2. When DDT was applied, most insects died, but resistant ones survived.
3. Survivors reproduced and passed resistance to offspring.
4. Each generation had a higher proportion of resistant individuals.
5. Over time, resistance spread through populations.

Notice that DDT did not create resistance. It selected for variation that was already present, which is the key point graders look for.

Example: Flowering Time and Climate Change

Flowering time is an AP example of selection driven by a changing environment. As temperatures rise, plants that flower at a time matching pollinator activity and favorable weather tend to have higher reproductive success. Variation in flowering time gives a population the raw material to shift earlier or later as conditions change. Populations with more variation in this trait have a better chance of tracking the new conditions.

How to Use This on the AP Biology Exam

Free Response

When you explain a case of natural selection, build your answer around the logic chain:

1. State that heritable phenotypic variation already exists in the population.
2. Name the selective pressure and the environment.
3. Explain that individuals with the favorable phenotype have higher reproductive success.
4. Conclude that the favorable trait increases in frequency over generations, so the population evolves.

Define fitness as reproductive success when you use the word. Make clear that the trait was present before the pressure, not created by it.

Data Analysis

You may get a graph or table showing trait frequencies, survival rates, or allele frequencies over time. Use the data to support a claim: identify which phenotype is favored, connect it to the selective pressure, and predict how the population changes next. Tie your reasoning back to reproductive success, not just survival in one moment.

Common Trap

Avoid Lamarckian wording. Do not write that organisms "develop" or "gain" a trait because they need it, or that they "try" to adapt. Selection acts on variation that is already there.

Common Misconceptions

• "Organisms adapt during their lifetime because they need to." Individuals do not evolve. Natural selection changes trait frequencies across a population over generations, acting on variation that already exists.
• "Fitness means being the strongest or living the longest." Fitness means reproductive success. An organism that survives but never reproduces has low fitness.
• "DDT or the environment caused the resistance mutation." Selective pressures do not create the favorable variation. They favor variation that was already present.
• "A beneficial trait is always beneficial." The same variation can raise fitness in one environment and lower it in another, like the sickle cell allele in malaria versus non-malaria regions.
• "Only visible traits matter for selection." Variation in molecules inside cells, such as enzymes and proteins, can also affect survival and reproduction.
• "Natural selection has a goal or plans ahead." It is not a conscious force; it is just the outcome of differences in reproductive success.

Related AP Biology Guides

• 7.1 Introduction to Natural Selection
• Unit 7 Overview: Natural Selection
• 7.4 Population Genetics
• 7.3 Artificial Selection
• 7.6 Evidence of Evolution
• 7.5 Hardy-Weinberg Equilibrium