Unit 7 AP Biology Notes: How Selection Shapes Populations (Natural vs. Artificial)

Introduction to Natural Selection

What natural selection is (in plain language)

Natural selection is a mechanism of evolution in which individuals with certain heritable traits tend to survive and reproduce more successfully than others in a particular environment. Over many generations, this difference in reproductive success changes the trait (and allele) frequencies in the population.

A helpful way to think about it: the environment “filters” existing variation. If a trait helps you leave more offspring in that environment, that trait becomes more common—not because organisms choose it, but because more copies of the genes behind it get passed on.

Why it matters

Natural selection is one of the most important ideas in biology because it explains:

• How populations become adapted to their environments (for example, camouflage, drought tolerance, antibiotic resistance).
• Why organisms show both unity and diversity—they share common ancestry, but different environments favor different traits.
• How evolutionary change can happen on observable timescales (like pesticide resistance evolving over years).

In AP Biology, you’re expected to connect natural selection to genetics: selection acts on phenotypes (what you can “see” or measure), but evolution is tracked as changes in allele frequencies in populations across generations.

The core ingredients required for natural selection

Natural selection is not “just change.” It specifically requires a set of conditions. If any of these are missing, natural selection cannot cause evolution.

1. Variation: Individuals in a population differ in traits (size, speed, enzyme activity, coloration, behavior).
2. Heritability: At least some of that variation is genetic and can be passed from parents to offspring.
3. Differential survival and reproduction: Individuals with certain heritable traits leave more viable offspring than others in the current environment.
4. Time (generations): The above differences accumulate, shifting the population’s genetic makeup.

A common misconception is that the environment “creates” the needed traits. In reality, variation already exists (generated by mutation and recombination), and the environment changes which variants become more common.

Key vocabulary you must use correctly

• Population: A group of individuals of the same species living in the same area that can interbreed.
• Evolution: A change in the genetic makeup of a population over time (often described as changes in allele frequencies).
• Adaptation: A heritable trait that increases fitness in a specific environment.
• Fitness: In evolution, fitness means reproductive success—how many surviving, reproducing offspring you contribute to the next generation (not strength, speed, or “health” in the everyday sense).

How this shows up in data

In AP Biology, natural selection is often assessed through interpretation of:

• Survival curves, bar graphs of offspring number, or before-and-after trait distributions
• Experimental results (control vs. selective pressure)
• Claims supported by evidence and reasoning (CER-style)

You’ll often be asked to justify why an observed change is due to natural selection rather than chance.

Example (concept first, then application): “selection acts on individuals, but populations evolve”

Imagine a beetle population with green and brown beetles. Birds more easily spot green beetles on dark soil, so green beetles are eaten more often.

• Individual green beetles do not “turn brown” to survive.
• Instead, brown beetles leave more offspring on average.
• Over generations, the population becomes mostly brown.

The population evolved because the proportion of brown-associated alleles increased.

What goes wrong: common misconceptions

• Misconception: organisms evolve because they need to. Need does not cause heritable genetic change. Selection only changes frequencies of existing heritable variants.
• Misconception: selection gives organisms what they need. Selection has no goal; it only favors what works better now.
• Misconception: the “fittest” means the strongest. Fitness is measured by reproductive output, which can be influenced by survival, mating success, fertility, and offspring viability.

Exam Focus

• Typical question patterns:
  • Identify which of several scenarios meets the requirements for natural selection (variation, heritability, differential reproduction).
  • Explain (with evidence) how an environmental change would shift trait frequencies over generations.
  • Distinguish “adaptation” from “acclimation” or “behavioral choice.”
• Common mistakes:
  • Describing evolution as an individual changing during its lifetime instead of a population changing across generations.
  • Forgetting heritability—selection can act on a trait, but if it’s not genetic, it won’t cause evolution.
  • Using teleological language (“so that,” “in order to”) as if evolution has intent.

Natural Selection

Natural selection as a mechanism of evolution

Natural selection is the process where environmental conditions cause nonrandom differences in survival and reproduction among individuals with different heritable traits. Those differences lead to changes in allele frequencies in the next generation.

It’s important to separate two ideas:

• Mutation is random with respect to what the organism “needs.” Mutations arise without foresight.
• Selection is nonrandom: given the environment, some phenotypes consistently leave more offspring than others.

Natural selection connects ecology and evolution. Ecological interactions (predation, competition, disease, climate) create selection pressures; evolutionary responses change populations.

Step-by-step: how natural selection works

1. Genetic variation exists

   • Generated by mutation, recombination during meiosis, and sexual reproduction.
   • Variation can be subtle (enzyme efficiency) or obvious (color patterns).

2. The environment imposes selection pressures

   • A selection pressure is any factor that affects survival or reproduction (temperature, toxins, predators, limited food, pathogens).

3. Individuals with advantageous heritable traits leave more offspring

   • Advantage depends on environment: a trait can be beneficial in one setting and harmful in another.

4. Alleles linked to advantageous traits increase in frequency

   • Over generations, the population’s trait distribution shifts.

5. The population becomes better matched to the environment (adaptation)

   • This is why natural selection is often described as producing adaptations.

Selection acts on phenotypes, but evolution is genetic

Selection “sees” phenotypes (fur thickness, beak size, resistance to a toxin), because those affect performance. But for evolution to occur, the phenotype differences must be tied to heritable genetic differences.

This is why AP Biology often emphasizes: if a trait difference is purely environmental (for example, a plant is shorter because it grew in the shade), natural selection cannot make “shortness” more common unless there is genetic variation for height that affects reproductive success.

Types of natural selection (how trait distributions change)

Natural selection can reshape a population’s distribution of a trait in different ways.

Directional selection

Directional selection favors one extreme phenotype, shifting the population’s average.

• Common when environments change (new predator, new climate conditions, new toxin).
• Example idea: larger beaks become more common if only large, hard seeds remain after a drought.

Stabilizing selection

Stabilizing selection favors intermediate phenotypes and selects against extremes, reducing variation.

• Often occurs in stable environments.
• Example idea: very low birth weight and very high birth weight can both be associated with lower survival; intermediate birth weight has higher survival.

Disruptive selection

Disruptive selection favors both extremes over the intermediate, increasing variation and potentially contributing to divergence.

• Can happen when there are two different “niches” or food sources.
• Example idea: very small and very large beaks do well on different foods, but medium beaks do poorly.

A frequent mistake is to memorize these labels without tying them to graphs. On many AP-style questions, you’ll be shown a before-and-after distribution—your job is to describe how the curve changes and match it to the selection pattern.

Case study 1: Antibiotic resistance (natural selection you can observe)

Antibiotic resistance is a classic demonstration that natural selection acts on existing variation and changes populations.

1. Variation: In a bacterial population, random mutations can make some cells less affected by an antibiotic (for example, altering a target protein or producing an enzyme that breaks down the drug).
2. Selection pressure: The antibiotic kills susceptible bacteria.
3. Differential reproduction: Resistant bacteria survive and reproduce, passing resistance alleles to descendants.
4. Population change: After multiple generations, the population contains a much higher fraction of resistant bacteria.

What goes wrong in explanations: students often say “the bacteria mutate because they are exposed to antibiotics.” Exposure doesn’t create the specific mutation on demand; it changes which variants survive.

Case study 2: Peppered moth-style camouflage (predation as a selective pressure)

In a population with light and dark color variants:

• If tree bark is light (lichen-covered), dark moths are eaten more often.
• If pollution darkens bark, light moths become easier to spot.

In each environment, predation causes differential survival, shifting allele frequencies related to coloration. The same trait can be favored or disfavored depending on the environment, emphasizing that “advantage” is context-dependent.

Sexual selection (a special case you should recognize)

Sexual selection is selection based on differences in mating success. Traits that increase mating success (even if they reduce survival) can spread.

• Example idea: elaborate displays or coloration that attract mates.

A subtle point: sexual selection still changes allele frequencies through differential reproduction, so it’s a form of natural selection in the broad sense.

Natural selection vs. genetic drift (don’t confuse them)

AP Biology often asks you to distinguish natural selection from genetic drift.

• Natural selection: nonrandom change driven by trait effects on fitness.
• Genetic drift: random change in allele frequencies, especially strong in small populations.

If an allele becomes common because it improves survival/reproduction in that environment, that supports selection. If frequencies shift due to chance events (storm, random deaths), that supports drift.

Example: interpreting a simple dataset (worked reasoning)

Suppose two genotypes exist in a beetle population: genotype A and genotype B.

• In a dry year, 70% of A individuals survive to reproduce, but only 30% of B survive.
• If survival differences are due to a heritable trait (like cuticle thickness reducing water loss), then A has higher fitness in dry conditions.

Over generations of dry conditions, you’d predict genotype A alleles become more common. Your explanation should explicitly connect:

• Heritable trait difference → survival/reproduction difference → allele frequency change

What goes wrong: stopping at “A survives more” without stating that this produces a genetic shift in the next generation.

Exam Focus

• Typical question patterns:
  • Analyze a graph showing trait distributions before and after selection and identify directional vs. stabilizing vs. disruptive selection.
  • Given a scenario (predator introduced, drought, antibiotic exposure), predict which phenotype increases and justify using the four conditions for selection.
  • Interpret experimental results (e.g., different survival rates) and argue whether data support natural selection versus random chance.
• Common mistakes:
  • Claiming the environment “caused” beneficial mutations to appear when needed.
  • Forgetting that selection requires differential reproductive success, not just survival (survival matters only if it leads to reproduction).
  • Using “fitness” as a subjective statement (“more fit”) without tying it to evidence like offspring number or survival to reproductive age.

Artificial Selection

What artificial selection is

Artificial selection is a process in which humans intentionally choose which individuals reproduce based on desired traits. Over generations, this changes the genetic makeup of the population.

Artificial selection uses the same core evolutionary logic as natural selection—differential reproduction of heritable traits—but the selecting agent is human preference rather than environmental pressures like predators or climate.

Why it matters

Artificial selection is important because:

• It provides a clear, understandable demonstration that selecting certain phenotypes can rapidly change populations.
• It explains the origin of many domesticated species and crop varieties.
• It has major societal impacts—food production, companion animals, biotechnology, and ethical considerations.
• It can reduce genetic diversity, which matters for disease vulnerability and future adaptability.

In AP Biology, artificial selection is often used as “evidence that evolution can occur” because it is observable and strongly linked to heredity.

How artificial selection works (step-by-step)

1. Heritable variation exists in the population.
2. Humans choose breeders based on preferred phenotypes (bigger fruit, calmer temperament, higher milk yield).
3. Those chosen individuals contribute more offspring to the next generation than unchosen individuals.
4. Over many generations, alleles for desired traits increase, shifting the population’s phenotype.

A key connection: artificial selection usually changes populations faster than natural selection because selection pressure can be extremely strong (humans may allow only a small fraction of individuals to reproduce).

Artificial selection compared to natural selection

Here’s the conceptual comparison that AP questions often target:

A common misconception is that artificial selection is “not evolution” because humans are involved. It is evolution because allele frequencies change across generations.

Examples of artificial selection (and what they teach)

Example 1: Dog breeding and diversification

Domestic dogs were produced from ancestral wolf populations through selective breeding for behavior and physical traits.

• Humans selected for tameness, trainability, size, coat type, and many other traits.
• Over time, this produced many breeds with dramatic phenotypic differences.

What this shows: large phenotypic change can arise from selecting among existing genetic variation. However, intense selection can also increase the frequency of harmful alleles in some breeds.

Example 2: Crop domestication

Humans have selected plants for traits like larger seeds, reduced bitterness, non-shattering seed heads, or synchronized ripening.

What this shows: artificial selection can increase yield and convenience, but it can also reduce genetic diversity, which can make crops more vulnerable to pests or changing climates.

Trade-offs and unintended consequences

Artificial selection often involves trade-offs—improving one trait can worsen another.

• Selecting for very rapid growth or very large body size might increase health problems.
• Selecting for uniform crops can reduce variation that might be protective against disease.

This matters because AP Biology expects you to think in systems: changing one part of a biological system (a trait) can have consequences for survival, reproduction, and resilience.

Artificial selection, genetic variation, and evolutionary potential

Artificial selection can either:

• Maintain variation (if breeders intentionally preserve diverse lines), or
• Reduce variation (if breeders repeatedly use a small number of individuals).

Reduced variation matters because natural selection can only act on existing heritable variation. If a population becomes genetically uniform, it may struggle to adapt to new diseases or environmental shifts.

Example: reasoning through a breeding scenario

Suppose a farmer consistently breeds only the top 5% of plants with the largest fruits.

• If fruit size is heritable (influenced by alleles that can be passed on), then alleles associated with larger fruits will increase in frequency.
• Over generations, the average fruit size increases.

What goes wrong: students sometimes describe the change as if all plants “respond” by making bigger fruits due to better care. That would be an environmental effect. For artificial selection to cause evolution, the key is who reproduces and whether the trait is heritable.

Exam Focus

• Typical question patterns:
  • Explain how selective breeding can change trait frequencies over generations, explicitly linking to heritability and differential reproduction.
  • Compare and contrast artificial selection with natural selection in terms of selecting agent and outcomes.
  • Evaluate potential consequences of reduced genetic diversity in domesticated populations.
• Common mistakes:
  • Treating artificial selection as “training” or “acclimation” rather than genetic change across generations.
  • Ignoring trade-offs (assuming selecting for a desired trait has no costs).
  • Forgetting that selection requires heritable variation—if the trait is not genetically influenced, breeding will not produce sustained change.