AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/7/7-4)
Unit 7 · Topic 7.4
7.4 Population Genetics
Natural selection isn't the only thing that changes allele frequencies. Mutation adds new alleles, genetic drift shifts allele frequencies by chance (strongest in small populations, as in bottlenecks and founder effects), and gene flow moves alleles between populations, keeping them similar.
Key terms
- allele frequency
- genetic drift
- bottleneck effect
- founder effect
- gene flow
- mutation
Evolution as changing allele frequencies
An allele frequency is the proportion of all copies of a gene in a population that are a particular allele. If a population of 100 diploid individuals has 200 copies of a gene and 50 of them are allele b, the frequency of b is 50 ÷ 200 = 0.25. When allele frequencies change across generations, the population is evolving, so measuring allele frequencies gives direct evidence of evolution.
Natural selection changes allele frequencies in a nonrandom way: alleles that help survival and reproduction increase. The processes in this topic are random with respect to fitness.
Mutation
Mutation is the random process that creates new alleles. It's the original source of all genetic variation, and it supplies the new phenotypes that selection can later act on. In any one generation, mutation changes allele frequencies only a tiny amount, but over long times it matters a great deal.
Genetic drift
Genetic drift is a change in allele frequencies caused by chance events, not by differences in fitness. Which individuals happen to reproduce, or which alleles end up in the gametes that get fertilized, is partly luck. In a big population, the luck averages out. In a small population, it doesn't, so allele frequencies can swing a lot. An allele can be lost entirely or fixed (become the only allele) just by chance, even if it's harmful or helpful. Drift reduces genetic variation and can make a small population drift away from other populations of the same species.
Two special cases:
- Bottleneck effect: a disaster, overhunting or disease drastically shrinks a population for at least one generation. The survivors' alleles are a random sample, so some alleles are lost and diversity stays low even after the population recovers. Northern elephant seals were hunted down to possibly fewer than 100 animals by the 1890s; they've recovered to well over 100,000 but have very little genetic variation.
- Founder effect: a small group separates from a larger population and starts a new one. The new population's allele frequencies reflect only the founders' genes. Among the Old Order Amish of Lancaster County, Pennsylvania, a rare inherited form of dwarfism with extra fingers (Ellis–van Creveld syndrome) is far more common than in most populations, because the community descends from a small number of founders.
Gene flow
Gene flow is the movement of alleles into or out of a population when individuals migrate or gametes (like pollen) travel. It can add new alleles or change their frequencies. Gene flow tends to make populations more similar to each other, which works against them splitting into separate species. Cut off gene flow, and populations can diverge (7.10).
| Mechanism | Random or not? | Main effect |
|---|---|---|
| Natural selection | Not random | Increases alleles that raise fitness |
| Mutation | Random | Adds new alleles |
| Genetic drift | Random | Random changes; strongest in small populations; reduces diversity |
| Gene flow | Random with respect to fitness | Moves alleles between populations; makes them more alike |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Founder effect in numbers
In a large mainland lizard population, allele t has a frequency of 0.01. A storm carries 10 lizards to an empty island, and 3 of their 20 copies of the gene are t. What is the frequency of t in the new island population, and what process explains the difference?
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- Step 1: Ten diploid lizards carry 10 × 2 = 20 copies of the gene.
- Step 2: Frequency of t on the island = 3 ÷ 20 = 0.15.
- Step 3: That's 15 times the mainland frequency (0.15 ÷ 0.01 = 15), even though no selection happened; the founders were simply a random, unrepresentative sample.
- Step 4: This is the founder effect, a type of genetic drift.
Answer: Frequency of t = 0.15 on the island, compared with 0.01 on the mainland, because of the founder effect (genetic drift).
- Example 2
Predicting drift with a null hypothesis
Researchers set up 20 populations of 10 flies and 20 populations of 1,000 flies, all starting with allele frequency 0.5 for a gene that doesn't affect fitness. State a null hypothesis and predict what they'll see after 30 generations.
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- Step 1: Because the gene doesn't affect fitness, selection isn't acting on it; any change comes from chance.
- Step 2: Null hypothesis: population size has no effect on how much allele frequencies change.
- Step 3: Prediction based on drift: the small populations will show large, random changes. Some will lose the allele (0) or fix it (1), and they'll spread widely from each other.
- Step 4: The large populations will stay close to 0.5, because chance effects average out in big samples.
- Step 5: If that's what happens, the researchers reject the null hypothesis: smaller populations experience stronger drift.
Answer: Null hypothesis: size doesn't affect change in allele frequency. Expected: small populations drift widely (some to 0 or 1), large ones stay near 0.5, supporting stronger drift in small populations.
Common mistakes
- Saying genetic drift favors helpful alleles. Drift is random with respect to fitness; harmful alleles can increase and helpful ones can be lost.
- Mixing up bottleneck and founder effects. A bottleneck shrinks an existing population in place; a founder event starts a new population from a few individuals.
- Saying gene flow makes populations diverge. Gene flow makes them more alike; reduced gene flow lets them diverge.
- Calculating allele frequency by counting individuals instead of gene copies. Each diploid individual carries two copies.
On the exam
- Expect to state a null hypothesis or predict results for an experiment on drift or gene flow, as in the fly example.
- When explaining low genetic diversity in a population, connect it to a specific random process (bottleneck or founder effect) and its effect on allele frequencies.
Connected topics
Videos
Check yourself
4 questions on 7.4 Population Genetics. Pick an answer to see if you got it, and why.
A small group of settlers founded an isolated island community. One founder carried a rare recessive allele. Generations later, this allele is far more common on the island than in the population the settlers came from. Which process most likely explains this?
| Population size | Starting frequency of allele A | Range of final frequencies of A in 20 replicate populations after 50 generations |
|---|---|---|
| 10 | 0.50 | 0.00 to 1.00 |
| 100 | 0.50 | 0.28 to 0.71 |
| 1,000 | 0.50 | 0.44 to 0.56 |
Computer simulation data: allele A has no effect on fitness, and there is no mutation or migration.
Which conclusion is best supported by the simulation?
Two populations of the same plant species grow on opposite sides of a valley. Bees regularly carry pollen between them. Which of the following is the most likely effect of this pollen movement?
Hunting reduced a seal species to a few dozen animals in the late 1800s. After protection, the population grew to more than 100,000. Today the species has much less genetic variation than closely related seal species. Which of the following best explains this?
0 of 4 answered