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AP Biology 7.2 Natural Selection Study Notes

AP Biology 7.2 Natural Selection Study Notes - New Syllabus Effective 2025

AP Biology 7.2 Natural Selection Study Notes- New syllabus

AP Biology 7.2 Natural Selection Study Notes – AP Biology –  per latest AP Biology Syllabus.

LEARNING OBJECTIVE

Describe the importance of phenotypic variation in a population.

Key Concepts: 

  • Natural Selection

AP Biology-Concise Summary Notes- All Topics

7.2.A Importance of Phenotypic Variation in a Population

🔑 Concept:

Phenotypic variation = differences in the observable traits (like color, size, shape, behavior) among individuals in a population.

🌱 Without variation, natural selection has nothing to act on!

💡 Why Is Phenotypic Variation Important?

1. Provides Raw Material for Natural Selection

  • Variation allows some individuals to be better suited to the environment than others.
  • Those with favorable traits survive and reproduce → pass traits to next generation.

2. Enables Adaptation to Changing Conditions

  • Environments are constantly changing (climate, predators, diseases, etc.).
  • If all organisms are the same, one change can wipe out the whole population.
  • Variation increases the chance that some will survive.

3. Maintains Population Health

  • Reduces risk of all individuals being wiped out by a single threat.
  • Helps populations evolve new traits over time → leading to evolution and sometimes speciation.

🧠 Example: Sickle Cell Trait in Humans

  • People who carry one sickle cell allele (AS) are resistant to malaria.
  • This trait remains common in areas with malaria because it provides a survival advantage.

🔄 Types of Variation:

  • Genetic: Inherited from DNA (e.g., mutations, recombination)
  • Environmental: Caused by surroundings (e.g., sun exposure affects skin tone)

📍Three modes of natural selection acting on phenotypic variation:

1. Directional Selection → shifts traits in one direction

Example: Darker mice survive better on dark rocks

Example: Finch beaks got deeper due to more hard seeds

2. Disruptive Selection → favors both extremes, not the middle

Example: Small and large beaks are better than medium (in seed cracker finches)

3. Stabilizing Selection → favors intermediate traits, removes extremes

Example: Human babies with average birth weights survive more

7.2.A.1 Natural Selection Acts on Phenotypic Variations

🧠 What’s the Big Idea?

Natural selection doesn’t “see” genes directly – it acts on phenotypes (physical traits), which are the visible outcomes of genes.

Only traits that affect an organism’s ability to survive and reproduce will be selected for or against.

🔍 What Is Phenotypic Variation?

Phenotypic variation = Differences in observable traits among individuals in a population.

Examples:

  • Coat color in mice
  • Beak size in birds
  • Leaf shape in plants

These differences arise from:

  • Genetic variation (mutations, recombination)
  • Environmental influence (nutrition, temperature, sunlight)

⚙️ How Natural Selection Uses These Variations:

  1. Traits that improve survival (like camouflage or disease resistance) lead to higher reproductive success.
  2. These traits become more common in the population over time.
  3. Unfavorable traits may decrease in frequency or even disappear.

🧬 Example:

In a population of beetles:

  • Green beetles blend into leaves → less likely to be eaten → reproduce more.
  • Brown beetles get eaten more → fewer offspring.
  • Result: Green trait increases in frequency = evolution through natural selection.

📉 Important Note:

🔹 Natural selection only acts on existing variation.

🔹 It cannot create new traits on demand—only selects what is already there.

📝 Summary:

  • Natural selection acts on phenotypes, not directly on genotypes.
  • Visible traits that improve fitness are passed on.
  • This leads to evolutionary change in the population’s trait distribution.

7.2.A.2 Environments Change and Apply Selective Pressures to Populations

🧠 Key Concept:

The environment is not static – it keeps changing.

These changes create selective pressures that influence which traits are favorable in a population.

🔄 What Are Selective Pressures?

Selective pressures are environmental factors that influence which traits increase survival and reproduction.

They can be:

🌱 Biotic (Living):

  • Predators
  • Competition (for food, space, mates)
  • Parasites, pathogens
  • Symbiotic relationships

🌦️ Abiotic (Non-living):

  • Temperature extremes
  • Drought or flood
  • pH, salinity, soil type
  • Climate change
  • Natural disasters

🔁 How This Affects Evolution:

1. Environmental change = new pressures → Traits that were once helpful might no longer be.

2. New selective pressures = new “winners” and “losers” → Different traits are now favored.

3. Allele frequencies shift over generations → This is evolution in action!

🧬 Example:

🦎 Lizards on storm-hit islands:

  • Hurricanes wiped out vegetation.
  • Lizards with larger toe pads (better grip) survived.
  • Next generation had more lizards with that trait.

🌻 Plant root systems:

  • In dry years, deep roots = survival.
  • In wet years, shallow roots may work better.
  • Different traits are selected each year.

📊 Real-World Impact:

Environmental change doesn’t always lead to extinction – it can lead to:

  • Adaptation (new favorable traits)
  • Migration (moving to suitable habitat)
  • Speciation (new species over time)
  • Or extinction (if no traits help survival)

📝 Summary:

  • The environment shapes evolution.
  • When environments change, natural selection responds by favoring traits that best suit the new conditions.
  • These pressures cause populations to evolve over generations.

7.2.A.3 Some Phenotypic Variations Can Increase or Decrease Fitness

🧠 What Is “Fitness”?

Fitness means how well an organism can survive and reproduce in its environment.

➡️ So, phenotypic variations (differences in physical traits) affect fitness based on how useful they are in a specific environment.

Phenotypic Variation That Increases Fitness:

These traits give an advantage—individuals with them survive better and produce more offspring.

📌 Examples:

  • Camouflage in arctic hares → helps hide from predators in snow
  • Deeper beaks in finches → crack tougher seeds during drought
  • Antibiotic resistance in bacteria → survive drug treatment and reproduce

 

Phenotypic Variation That Decreases Fitness:

These traits make survival or reproduction harder.

📌 Examples:

  • Bright-colored insects in predator-heavy areas → easier targets
  • Genetic diseases like sickle cell anemia in non-malarial zones
  • Fish with overly large fins in fast-moving water → harder to swim

⚖️ It’s All About Context

🌀 A trait that helps in one environment may hurt in another.

📍 Example:

  • Sickle cell trait:
    • In Africa (malaria zones): Helpful — gives resistance to malaria
    • Outside malaria zones: Harmful — causes health problems

📝 Summary:

  • Not all variations are equal
  • Some phenotypes make organisms more likely to survive and reproduce
  • Others may reduce survival chances
  • Natural selection favors phenotypes that improve fitness, but fitness is always environment-dependent!

7.2.B How Molecular Variation Affects Organism Fitness

🧠 Big Idea:

Variation isn’t just seen in physical traits like color or size – it also occurs at the molecular level, like in proteins, enzymes, and DNA sequences. These differences can affect how well an organism functions, survives, and reproduces.

🧬 1. Molecules Matter for Survival

Cells rely on molecules like:

  • Enzymes
  • Hormones
  • Structural proteins
  • Receptors
  • DNA/RNA sequences

Small changes in these molecules (due to mutations or inherited variation) can affect how efficiently they work — this directly impacts fitness.

⚙️ 2. Examples of Molecular Variation & Fitness

✅ Helpful Variations:

  • Hemoglobin mutation in high-altitude populations ➤ More efficient oxygen transport → better survival in thin air
  • Amylase gene copy number variation in humans ➤ More enzyme copies → better digestion of starch-rich diets
  • Antifreeze proteins in Arctic fish ➤ Prevents freezing → survival in icy water

❌ Harmful Variations:

  • Faulty enzymes in genetic disorders (like Tay-Sachs or PKU) ➤ Cells can’t break down molecules properly → leads to disease
  • Receptor mutations that affect immunity ➤ Less effective defense → increased infection risk

🔄 3. Why This Matters for Natural Selection:

  • Natural selection favors organisms with molecular traits that improve performance in a given environment.
  • Over generations, these helpful molecular variations spread in the population.

📌 If a molecule works better, the organism tends to:

  • Live longer
  • Reproduce more
  • Pass on that molecular trait

🔎 Real-World Examples:

EnvironmentMolecular TraitFitness Advantage
High altitudeHemoglobin variantBetter O₂ delivery
Cold waterAntifreeze proteinPrevents ice damage
High-starch dietExtra amylaseBetter digestion

✅ Summary:

  • Molecular variation = differences in important molecules
  • These differences affect how well an organism survives & reproduces
  • Natural selection acts on these invisible traits too, not just physical ones!

7.2.B.1 Molecular Variation Helps Populations Adapt

🌟 Key Idea:

The types and amounts of molecules (like proteins, enzymes, and genetic material) inside cells can vary between individuals in a population. This molecular variation can help some individuals survive and reproduce better in certain environments.

🧠 Why It Matters:

  • When environments change (like climate, food sources, or disease), not all organisms are equally prepared.
  • Some individuals may have molecular traits that help them function better in those new conditions.
  • These individuals are more likely to survive, reproduce, and pass on those helpful traits.

🧪 Examples of Molecular Variation:

TraitVariationBenefit
Enzyme concentrationSome individuals produce more digestive enzymes (like lactase or amylase)Better digestion in environments with certain foods
Hemoglobin typeHigh-altitude populations have hemoglobin variantsMore efficient oxygen transport in low-oxygen areas
Immune system receptorsVariation in surface proteinsBetter resistance to specific infections or pathogens
Antifreeze proteins in fishSome fish have unique proteins that prevent ice crystal formationSurvival in freezing waters

🌍 In Different Environments:

Different environments = different selective pressures → So different molecular traits are beneficial.

Cold = antifreeze proteins
High altitude = better oxygen-binding hemoglobin
High-starch diet = more amylase enzyme
Disease-prone = better immune receptors

🔄 Evolutionary Impact:

Over time, these helpful molecular traits become more common in the population due to natural selection.

  • This increases the population’s fitness (survival + reproduction)
  • Populations become better adapted to their environment

📝 Summary:

  • Variation in molecules (like enzymes, receptors, or proteins)
  • Leads to better performance in different conditions
  • Increases survival and reproductive success
  • Helps the whole population adapt over time
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