AP Biology 7.4 Population Genetics Study Notes - New Syllabus Effective 2025
AP Biology 7.4 Population Genetics Study Notes- New syllabus
AP Biology 7.4 Population Genetics Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Explain how random occurrences affect the genetic makeup of a population.
Key Concepts:
- Population Genetics
7.4.A – How Random Occurrences Affect the Genetic Makeup of a Population
💡 Key Idea:
Evolution doesn’t always need natural selection.
Sometimes, random events alone can change a population’s genetic makeup — especially in small populations.
These random changes are part of a process called: Genetic Drift
🔄 What is Genetic Drift?
It’s when allele frequencies change by chance, not because of survival advantage.
🧠 It’s like:
- Tossing a coin 10 times (small population) vs. 1000 times (large population).
- You might get weird results just by luck in the smaller sample!
1. Bottleneck Effect
Happens when a disaster (like a flood, fire, or disease) randomly kills off a large part of a population not based on traits.
🧪 Example:
- A disease wipes out 90% of cheetahs.
- The few survivors may not carry all the genetic diversity from before.
- Result: Reduced genetic variation, maybe more health problems or inbreeding.
📉 Random who survives → random which alleles remain
2. Founder Effect
Happens when a few individuals start a new population, carrying only a small part of the original gene pool.
🧪 Example:
- 5 birds fly to a new island.
- They only bring a few of the population’s alleles.
- Future generations will only reflect that limited gene pool.
📉 Random who arrives → random gene frequencies
⚖️ Big Picture:
| Cause | Effect on Genetics |
|---|---|
| Bottleneck | Loss of alleles due to chance survival |
| Founder effect | Small group forms a new gene pool |
| Genetic drift overall | Random allele changes, esp. in small pops |
❗ Important:
- Genetic drift is most powerful in small populations (because random changes have bigger effects).
- Can reduce genetic diversity, which is risky for survival.
- Can make harmful alleles more common – purely by chance.
- It’s evolution, but not adaptive (no advantage needed).
7.4.A.1 – Random Occurrences That Drive Evolution
💡 Key Idea:
- Not all evolution happens through natural selection.
- Random processes can also shift allele frequencies especially in small populations and drive evolution in unexpected ways.
1. Mutation 🧬
Mutation = random changes in DNA (like typos in genetic code).
- Can create new alleles.
- Most are neutral or harmful, but some may be beneficial.
- It’s the ultimate source of genetic variation — gives evolution something to act on.
🧠 No mutation = no new traits = no evolution
2. Genetic Drift 🎲
- Random shifts in allele frequencies.
- Not based on fitness – just chance.
- Stronger in small populations.
- Can lead to loss of alleles or even fixation of harmful ones.
🔁 Evolution without selection
3. Bottleneck Effect 💥
- A sudden disaster (like flood, fire, hunting) kills off most of a population.
- Survivors = random sample, not necessarily the “fittest.”
- Result = loss of genetic diversity, higher risk of inbreeding.
📉 Less variation = less adaptability
4. Founder Effect 🧳
- A small group breaks off to start a new population.
- The new population’s traits reflect only the alleles of the founders.
- Can lead to rare traits becoming common just by chance.
🌱 New population = new allele pattern
5. Gene Flow (Migration) 🌍
- Movement of individuals (or gametes) into or out of a population.
- Adds or removes alleles from gene pool.
- Increases genetic diversity when alleles are added.
- Reduces differences between populations.
✈️ Migration = mixing genes = more variation
🧠 Summary Table:
| Random Process | What It Does |
|---|---|
| Mutation | Introduces new alleles |
| Genetic Drift | Random changes in allele frequency |
| Bottleneck Effect | Sharp drop in pop. size → loss of variation |
| Founder Effect | Small group colonizes → different allele mix |
| Gene Flow (Migration) | Adds/removes alleles via movement |
7.4.B – The Role of Random Processes in Evolution
🧠 Key Idea:
While natural selection is non-random, many other evolutionary forces are random — and they can dramatically affect populations, especially small ones.
🎲 What Do We Mean by “Random Processes”?
These are evolutionary changes that happen by chance, not because the trait is better or worse. They can:
- Increase or decrease certain alleles
- Make populations more or less genetically diverse
- Lead to unique evolutionary paths in small, isolated groups
🔄 Major Random Processes:
🧬 Mutations
- Random DNA changes create new alleles
- They supply raw material for evolution to act on
🎯 Genetic Drift
- Random changes in allele frequencies (especially in small populations)
- Can lead to loss of diversity or even fixation of harmful alleles
🌍 Gene Flow
- Movement of genes between populations
- Can mix or reduce genetic differences between groups
📌 Real-Life Examples:
- Island populations often evolve differently due to founder effect (a type of genetic drift)
- Natural disasters can cause bottlenecks, randomly wiping out alleles
🧬 Why It Matters:
These random forces can shift the direction of evolution even without selection. In some cases, randomness may matter more than natural selection, especially when:
- Populations are small
- The environment is changing rapidly
- Selection is weak or absent
✅ Summary:
- Evolution isn’t just about survival of the fittest sometimes it’s about survival of the luckiest.
- Random processes like mutation, drift, and gene flow can change allele frequencies and drive evolution in ways that are completely unpredictable.
7.4.B.1 – Random Processes That Change Allele Frequencies
Random events can cause allele frequencies to shift in populations even when natural selection isn’t acting.
1. Mutations = Source of New Alleles 🔬
What they are: Random changes in DNA (errors during replication, radiation, etc.)
Why they matter:
- Introduce new genetic variations (new alleles)
- These changes are random — not always helpful or harmful
- Some mutations give rise to new phenotypes → Natural selection can act on these
Example: A mutation in a butterfly’s wing color gene might produce a new shade that helps it hide better.
2. Genetic Drift = Random Evolution 🌀
What it means: Random events change allele frequencies — not based on fitness
Common in: Small populations (like island species or endangered animals)
Two main effects:
- Founder Effect – A few individuals start a new population → gene pool may not represent original
- Bottleneck Effect – Population shrinks suddenly → rare alleles may disappear
Result: Populations can diverge genetically even if they started the same
3. Gene Flow = Mixing Populations 🌍
What it is: Movement of alleles between populations (via migration, mating, pollen transfer, etc.)
Effect:
- Increases genetic similarity between populations
- Prevents divergence and possible speciation
Example: If birds from one island migrate and mate with birds on another island, their genes mix → they stay part of the same species
✅ Summary:
- Random processes like mutation, drift, and gene flow can cause populations to evolve differently or stay genetically connected.
- These changes can happen without any selection pressure, just due to chance.
7.4.C – How a Population’s Genetic Makeup Changes Over Time
🧬 What Does “Genetic Makeup” Mean?
The genetic makeup of a population = allele frequencies (how common each version of a gene is)
Over time, these frequencies can change → this is evolution at the population level
🔁 What Causes These Changes?
Several factors can cause allele frequencies to shift:
| 🧪 Process | 🔍 What It Does |
|---|---|
| Natural Selection | Favors alleles that improve survival/reproduction → increases their frequency |
| Genetic Drift | Random chance events affect small populations → alleles may be lost/gained |
| Gene Flow | Migration between populations → mixes alleles, keeps populations similar |
| Mutation | Introduces new alleles randomly → adds variation |
| Non-random Mating | Certain traits are preferred → changes how often some alleles are passed on |
📉 Example of Change Over Time:
- In a beetle population:
- Dark-colored beetles increase over time because they hide from predators better
- Light-colored beetles become less common
- Allele for dark color becomes more frequent
🧠 Key Idea:
- Evolution = change in allele frequencies in a population across generations.
- It’s not individuals evolving – it’s the population as a whole that changes genetically.
✅ Summary:
- Evolution is the ongoing shift in genetic makeup caused by selection, mutation, drift, gene flow, and mating patterns.
- These forces interact and shape diversity over time — creating new traits, species, and adaptations.
7.4.C.1 – Changes in Allele Frequencies as Evidence of Evolution
🧬 What Are Allele Frequencies?
- Alleles = different versions of a gene (e.g., blue vs. brown eye color)
- Allele frequency = how common an allele is in a population
For example:
If 70% of alleles for eye color are for brown and 30% are for blue → Brown = 0.7, Blue = 0.3
🔍 How Do They Show Evolution?
Evolution = any change in allele frequency over generations
📌 If the frequency of an allele increases or decreases → that means evolution is happening
🧪 Real Examples:
| 🧪 Scenario | 🔄 Allele Frequency Shift |
|---|---|
| Peppered moths in polluted areas | Dark-colored allele became more common |
| Antibiotic resistance in bacteria | Resistance alleles increased in frequency over generations |
| Drought in Galápagos finches | Beak shape alleles shifted toward deeper, stronger beaks |
📊 Scientists Use This to Study Evolution
- Scientists measure allele frequencies using population samples
- By comparing across generations, they can track evolutionary trends
- Even if the change is small, it still counts as microevolution
🧠 Key Idea:
📌 Any change in allele frequency = clear evidence that evolution is occurring, even if no visible trait has changed yet.
✅ Summary:
- Allele frequencies = the heartbeat of evolution
- When frequencies shift → it means genetic change is occurring
- This is how scientists prove evolution is real and ongoing in nature