AP Biology 7.1 Introduction to Natural Selection Study Notes - New Syllabus Effective 2025
AP Biology 7.1 Introduction to Natural Selection Study Notes- New syllabus
AP Biology 7.1 Introduction to Natural Selection Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
- Describe the causes of natural selection.
- Explain how natural selection affects populations.
Key Concepts:
- Natural Selection
- Artificial Selection
7.1.A Describe the Causes of Natural Selection
🎯 What Is Natural Selection?
Natural selection is a process in which organisms with traits better suited to their environment tend to survive and reproduce more successfully. Over time, these traits become more common in the population.
🌱 What Causes Natural Selection?
1️⃣ Genetic Variation
- Every population contains individuals with slight genetic differences.
- These differences arise through mutations, meiosis (crossing over & independent assortment), and sexual reproduction.
- Without variation, all individuals would respond the same to environmental pressures — no selection could occur!
Example: Some beetles are green, some brown their color makes a difference in survival depending on their surroundings.
2️⃣ Overproduction of Offspring
- Organisms often produce more offspring than can survive.
- Resources like food, water, and shelter are limited.
Example: A tree may release thousands of seeds, but only a few will grow into mature trees.
3️⃣ Competition for Resources
- Because resources are limited, organisms must compete to survive.
- This creates a struggle for existence only the best-suited individuals will survive.
Example: Predators compete for prey; plants compete for sunlight and soil nutrients.
4️⃣ Differential Survival and Reproduction (Fitness)
- Not all individuals survive or reproduce equally.
- Those with traits that give them an advantage in their environment are more likely to survive and reproduce.
- These advantageous traits get passed on to the next generation.
Example: Faster gazelles are more likely to escape predators and live to reproduce.
5️⃣ Heritability of Traits
- Traits must be heritable (passed from parents to offspring) for natural selection to act on them.
- If a trait helps an organism survive, it must be genetically encoded to affect future generations.
Note: Acquired traits (like dyed hair or muscle built from training) are not inherited and don’t affect evolution.
🔁 Key Summary
Natural selection occurs when:
- There is variation in traits,
- More offspring are produced than can survive,
- There is competition, and
- Some traits offer a survival advantage,
- And these traits are inherited by the next generation.
💡 Bottom Line:
Natural selection is driven by genetic variation, limited resources, and the differential reproductive success of individuals with favorable traits.
7.1.A.1 Natural Selection as a Major Mechanism of Evolution
Natural selection is the primary way populations evolve over time. It filters out traits that don’t help with survival or reproduction and favors traits that improve fitness in a given environment.
🔁 How It Drives Evolution: 
- Evolution means a change in allele frequencies in a population over generations.
- Natural selection leads to these changes because individuals with beneficial traits pass those traits on more often.
- Over time, advantageous alleles become more common, and populations become better adapted.
🧠 Remember:
Natural selection doesn’t create new traits — it selects from existing variation in the population.
Example:
In a bird population where some individuals have slightly longer beaks, those with longer beaks may be better at accessing food. They survive and reproduce more — leading to more birds with long beaks in the next generation.
🧬 Contrast with Other Mechanisms:
While natural selection is the main driver, other mechanisms of evolution include:
- Mutation (introduces new alleles)
- Genetic drift (random changes in allele frequencies)
- Gene flow (movement of alleles between populations)
- Sexual selection (traits favored for mating success)
🧩 Why It Matters:
Without natural selection, populations wouldn’t adapt to changing environments. It’s how species:
- Become better suited to their habitats
- Diversify over time
- Develop complex traits (e.g., camouflage, resistance to antibiotics, flight in birds)
✅ Conclusion:
Natural selection is the engine of adaptive evolution. It causes traits that increase survival and reproductive success to become more frequent — driving changes in populations over time.
7.1.A.2 Darwin’s Theory of Natural Selection: Competition & Survival
🎯 Big Idea:
Natural selection happens because resources are limited, and not all individuals survive or reproduce equally. Those with better traits (phenotypes) have a survival advantage.
🦠 Key Points:
✅ Limited Resources = Competition
- In any environment, things like food, space, mates, and shelter are limited.
- Because more offspring are produced than can survive, individuals must compete for these resources.
✅ Favorable Traits = Better Survival 
- Some individuals have variations (phenotypes) that give them an advantage (e.g., better camouflage, stronger beaks, faster speed).
- These favorable phenotypes help them survive longer and reproduce more.
✅ Differential Reproduction = Evolution
- Those with favorable traits pass on their genes to more offspring.
- Over generations, the frequency of favorable traits increases in the population.
- This leads to evolution by natural selection.
🧪 Example: Galapagos Finches
- Drought → fewer small seeds available.
- Only birds with stronger, deeper beaks could crack large seeds and survived.
- Over time, average beak size increased in the population.
🔁 Summary of Natural Selection (Darwin’s Logic)
- Variation exists in populations.
- More offspring are produced than can survive.
- Competition for limited resources occurs.
- Individuals with favorable traits survive and reproduce more.
- Those traits become more common over generations.
7.1.B How Natural Selection Affects Populations
🧠 Key Idea:
Natural selection doesn’t act on individuals directly it acts on their traits (phenotypes), and this influences the entire population over time.
🔄 How It Works:
1. Individuals don’t evolve populations do.
- A single organism can’t change its genes or adapt during its life.
- But if individuals with beneficial traits survive and reproduce, their traits spread through the gene pool.
2. Favorable traits become more common over generations.
- If a trait helps survival/reproduction, it’s passed on more often.
- This causes a shift in allele frequency (evolution at the population level).
3. Unfavorable traits decrease.
- Traits that reduce fitness (like being too slow, too visible, or poorly adapted) are less likely to be passed on.
- These alleles may disappear over time.
🧬 Example: Peppered Moths
- Before the Industrial Revolution, light-colored moths were more common (they blended in).
- After pollution darkened trees, dark-colored moths survived better.
- Over time, the population shifted toward darker coloration.
📈 Result of Natural Selection:
- Populations become better adapted to their current environments.
- Genetic makeup (allele frequencies) changes over time.
- This is one mechanism of evolution.
❗Important:
Natural selection depends on:
- Variation in traits
- Heritability of those traits
- Select pressure from the environment
7.1.B.1 Evolutionary Fitness
🌱 What is Evolutionary Fitness?
🧠 Definition:
Evolutionary fitness refers to an organism’s ability to survive, reproduce, and pass on its genes to the next generation.
👉 Key Point:
It’s not about being the fastest, strongest, or smartest — it’s about having more viable offspring that can also reproduce.
📊 How is Fitness Measured?
✅ More Offspring = Higher Fitness
- An individual who produces more surviving, fertile offspring has higher evolutionary fitness than one who produces fewer.
- Even if a trait helps you survive, if you don’t reproduce, it doesn’t affect fitness.
Example: Bird Beak Size 
- In a drought year, birds with medium-sized beaks can eat the most available seeds.
- They survive and reproduce more.
- These birds have higher fitness, and more birds in the next generation have medium beaks.
🧬 Natural Selection & Fitness
Natural selection favors individuals with higher fitness.
These individuals:
- Are better adapted to their environment
- Contribute more genes to the gene pool
- Cause the population to evolve over time
🧠 Summary:
Fitness in evolution = reproductive success, not physical strength.
7.1.B.2 How the Environment Affects Evolution
🔁 Environment Is Always Changing
🧠 Key Idea:
Both biotic (living) and abiotic (non-living) environmental factors can fluctuate — and this changes how evolution works in each generation.
🔬 Biotic Factors (Living Things)
These are interactions with other organisms:
- Predators
- Prey
- Competitors
- Parasites
- Mates
Example: If a new predator appears, individuals with better camouflage may now have higher fitness.
🧊 Abiotic Factors (Non-living things)
These are physical or chemical aspects of the environment:
- Temperature
- Rainfall
- Natural disasters
- Soil pH or salinity
- Climate change
Example: A cold winter might select for thicker fur in animals.
🔄 Changing Conditions = Shifting Selection
- The traits that are “favorable” can change with the environment.
- This leads to different genetic variations being selected in each generation.
🧬 Example:
- In one year, drought favors deep roots.
- In a wet year, shallow-rooted plants may thrive.
📈 Result: Evolution Is Not Fixed
- Natural selection doesn’t always favor the same traits.
- Direction of evolution may shift as the environment changes.
- Over time, this can lead to new adaptations or even new species.
💡 Summary:
Natural selection is not static it responds to the changing biotic and abiotic environment, affecting which traits are passed on in each generation.