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
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:
- Traits that improve survival (like camouflage or disease resistance) lead to higher reproductive success.
- These traits become more common in the population over time.
- 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:
Environment | Molecular Trait | Fitness Advantage |
---|---|---|
High altitude | Hemoglobin variant | Better O₂ delivery |
Cold water | Antifreeze protein | Prevents ice damage |
High-starch diet | Extra amylase | Better 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:
Trait | Variation | Benefit |
---|---|---|
Enzyme concentration | Some individuals produce more digestive enzymes (like lactase or amylase) | Better digestion in environments with certain foods |
Hemoglobin type | High-altitude populations have hemoglobin variants | More efficient oxygen transport in low-oxygen areas |
Immune system receptors | Variation in surface proteins | Better resistance to specific infections or pathogens |
Antifreeze proteins in fish | Some fish have unique proteins that prevent ice crystal formation | Survival 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