AP Biology Quiz: Population Genetics

What this quiz covers

This quiz focuses on Population Genetics, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Biology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Stabilizing selection maintaining allele $T$ near an intermediate frequency
2. Genetic drift producing random allele frequency changes in a small population (correct answer)
3. Gene flow from nearby populations repeatedly altering allele $T$ frequency
4. Mutation converting allele $t$ to allele $T$ at a variable rate
5. Nonrandom mating increasing heterozygote frequency at the neutral locus

Explanation: This question tests understanding of genetic drift in small populations. The lizard population has only about 80 adults each generation, and allele T frequency fluctuates unpredictably between 0.30 and 0.70 over 20 generations at a neutral locus. These random, directionless fluctuations in allele frequency are the hallmark of genetic drift - the random sampling of gametes from generation to generation has larger effects in small populations. The problem rules out selection (neutral locus, no fitness differences), gene flow (no immigration), and mutation (no mention of high rates). Students often expect drift only in extremely small populations, but populations of 80 individuals can experience substantial drift effects. When you see unpredictable fluctuations in allele frequencies in small populations at neutral loci, genetic drift is the driving force.

Question 2

1. Genetic drift due to random sampling in a large population
2. Directional selection against allele $K$ under pesticide exposure (correct answer)
3. Gene flow introducing allele $k$ from a neighboring population
4. Increased mutation converting allele $K$ into allele $k$ each generation
5. Nonrandom mating increasing allele $k$ frequency without affecting fitness

Explanation: This question demonstrates directional selection against a deleterious allele in response to environmental change. After pesticide introduction, genotype fitness measurements show a clear pattern: KK has lowest survival, Kk intermediate, and kk highest - indicating the K allele reduces fitness in the pesticide environment. This fitness gradient drives the decrease in K frequency from 0.55 to 0.18 over eight generations through directional selection against K. The large population size rules out drift, and the problem states no immigration occurs and mutation rates are unchanged. Students might choose gene flow (C) because the k allele increases, but the problem explicitly states no immigration occurs - the k increase comes from selection against K. When environmental changes create consistent fitness differences among genotypes, directional selection drives predictable allele frequency changes.

Question 3

1. Gene flow from the mainland population into the island population
2. Directional selection favoring allele $B$ in the island environment
3. Founder effect causing genetic drift in a small new population (correct answer)
4. Increased mutation rate converting other alleles into allele $B$
5. Nonrandom mating increasing heterozygosity at the beak-shape locus

Explanation: This question tests understanding of population genetics processes that can cause rapid allele frequency changes. The mainland population had allele B at frequency 0.50, but when only 12 birds founded the island population, allele B frequency dropped to 0.17 - a dramatic change in just one generation. This pattern is characteristic of the founder effect, where a small group establishing a new population carries only a non-representative sample of the original population's genetic variation. Since there's no evidence of differential survival or reproduction among genotypes, and no additional birds arrived, genetic drift through the founder effect is the only mechanism that explains this random sampling error. Students often incorrectly choose gene flow (A), but gene flow would require ongoing migration between populations, not a one-time founding event. When you see a small founding population with immediate frequency changes, think founder effect - a special case of genetic drift.

Question 4

1. Genetic drift producing a consistent, directional increase in allele K each generation
2. Gene flow importing allele K from the nearby population with the same frequency
3. Mutation introducing additional K alleles during the year of elevated mutation rate (correct answer)
4. Directional selection favoring allele K despite equal survival among genotypes
5. Nonrandom mating converting allele k into allele K through mate choice

Explanation: This question assesses understanding of population genetics, focusing on mechanisms that change allele frequencies. Mutation is most directly responsible because the temporary increase in mutation rate from k to K introduced additional allele K copies, causing a slight rise from 0.05 to 0.051 in one generation. At the population level, elevated mutations add new alleles, and in a large population, this input persists without rapid spread due to no fitness advantages or migration. The subsequent stability highlights the one-time mutational pulse. A tempting distractor is gene flow (B), which is wrong because it assumes importation from a population with the same frequency, misconstruing internal generation as external movement. To analyze similar problems, identify temporary rate changes to isolate mutation as a source of small, persistent shifts.

Question 5

1. Genetic drift due to repeated population bottlenecks each breeding season
2. Mutation introducing new r alleles at a rate sufficient to halve R frequency
3. Gene flow from the river population into the lake population (correct answer)
4. Stabilizing selection maintaining intermediate allele frequencies in the lake
5. Random mating increasing heterozygosity and thereby lowering R frequency

Explanation: This question assesses understanding of population genetics, specifically how gene flow can drive changes in allele frequencies between connected populations. The removal of the dam allows migration from the river population with low R frequency (0.10) into the lake, gradually diluting the lake's higher frequency from 0.55 to 0.28 over generations. At the population level, gene flow homogenizes differences by introducing alleles proportionally to the source population's frequencies, especially with regular adult movement observed. No fitness differences among genotypes support that this is migration-driven rather than selective. A tempting distractor is A, genetic drift from bottlenecks, but this misconceives the stable large population size by assuming random fluctuations without size reductions. As a strategy, look for evidence of migration between populations with differing allele frequencies when frequencies converge without selection.

Question 6

1. Stabilizing selection independently driving both lakes toward allele C frequency 0.50
2. Gene flow equalizing allele frequencies through migration between lakes (correct answer)
3. Genetic drift causing both lakes to randomly reach the same allele frequency
4. Mutation pressure producing allele C in Lake 2 until it matches Lake 1
5. Assortative mating increasing heterozygotes and forcing allele frequency to 0.50

Explanation: This question assesses understanding of population genetics, focusing on mechanisms that change allele frequencies. Gene flow is responsible because the canal allows fish migration and interbreeding, equalizing allele C frequencies from 0.80 and 0.20 toward 0.50 in both lakes. At the population level, this exchange of alleles between previously separate groups homogenizes differences without needing fitness variations, as no survival differences were observed. The convergence reflects ongoing mixing rather than independent evolution. A tempting distractor is genetic drift (C), which is wrong because it assumes random convergence to the same frequency, misconstruing systematic equalization as chance events. To analyze similar problems, look for connectivity between populations to identify gene flow as a homogenizing force.

Question 7

1. Gene flow from untreated populations bringing allele S into treated areas
2. Genetic drift due to random sampling effects in a very large population
3. Directional selection favoring allele S under pesticide exposure (correct answer)
4. Mutation generating allele S repeatedly until it becomes common
5. Random mating increasing allele S frequency by altering genotype proportions

Explanation: This question assesses understanding of population genetics, focusing on mechanisms that change allele frequencies. Directional selection is responsible because pesticide exposure favors SS and Ss genotypes with higher survival, increasing allele S frequency from 0.30 to 0.85 over seasons. At the population level, this shifts alleles toward those enhancing fitness in the altered environment, with the large population minimizing random effects. No immigration ensures the change arises from within-population selection. A tempting distractor is gene flow (A), which is wrong because it assumes importation from untreated areas, misconstruing internal fitness-driven change as external input. To analyze similar problems, examine environmental pressures and fitness data to detect selection in large populations.

Question 8

1. Mutation pressure steadily converting the alternative allele into allele P
2. Genetic drift leading to fixation through random sampling in a small population (correct answer)
3. Directional selection consistently favoring allele P due to higher fitness
4. Gene flow adding allele P from immigrants until fixation occurs
5. Disruptive selection maintaining both alleles and increasing allele P to 1.00

Explanation: This question assesses understanding of population genetics, focusing on mechanisms that change allele frequencies. Genetic drift is most directly responsible because the small population of 25 rabbits allows random sampling to drive allele P to fixation over generations without fitness differences or migration. At the population level, drift in isolated small groups can lead to loss or fixation of alleles by chance, as seen in the progression to 1.00. The consistent small size sustains these random walks to extremes. A tempting distractor is directional selection (C), which is wrong because it assumes higher fitness for P, misconstruing random fixation as adaptive favoring. To analyze similar problems, monitor long-term trends in small, isolated populations for signs of drift leading to fixation.

Question 9

1. Genetic drift causing both populations to move toward intermediate allele frequencies
2. Gene flow between populations reducing differences in allele frequencies (correct answer)
3. Mutation creating allele K in Population 1 and removing it in Population 2
4. Disruptive selection favoring opposite alleles in each population
5. Nonrandom mating increasing heterozygosity and equalizing allele frequencies

Explanation: This question assesses understanding of population genetics, specifically how gene flow reduces genetic differences between populations. The wet season corridor enables frog migration, causing allele K frequencies to converge from 0.20 and 0.80 toward intermediates (0.35 and 0.68) through allele exchange. At the population level, gene flow mixes gene pools, equalizing frequencies proportionally to migration rates in large populations without selection. No genotype fitness differences confirm this non-adaptive homogenization. A tempting distractor is D, disruptive selection favoring opposites, but this misconceives convergence by assuming divergence despite observed mixing. For similar scenarios, prioritize evidence of migration between differing populations when frequencies move toward each other without fitness effects.

Question 10

1. Gene flow introducing allele S from another bacterial population
2. Genetic drift increasing allele S frequency from standing variation
3. Mutation creating allele S from a preexisting allele in the population (correct answer)
4. Directional selection favoring allele S, causing it to originate
5. Nonrandom mating increasing allele S frequency in a closed culture

Explanation: This question assesses understanding of population genetics, specifically how mutation introduces new alleles into populations. The UV radiation exposure likely induces mutations, creating the S allele from preexisting variants, appearing at 0.001 frequency in a large bacterial population. At the population level, mutations are the ultimate source of new genetic variation, especially in closed systems where migration is impossible and no differential reproduction occurs. The sudden appearance post-UV without environmental favoritism points to mutation over other forces. A tempting distractor is D, directional selection favoring S, but this misconceives the lack of reproductive differences by assuming selection creates alleles rather than acts on them. For transferable strategy, consider mutation as the explanation for novel alleles in isolated, large populations without fitness variances.

Question 11

1. Strong gene flow that exactly balances selection each generation
2. Hardy-Weinberg equilibrium with no major evolutionary forces acting (correct answer)
3. High mutation rates maintaining constant allele frequencies despite drift
4. Founder effect producing stable allele frequencies after colonization
5. Disruptive selection increasing homozygotes while keeping allele T constant

Explanation: This question assesses understanding of population genetics, specifically the conditions for Hardy-Weinberg equilibrium where allele frequencies remain stable. With a large population regrowing quickly after the wildfire and no observed immigration, selection, or other forces, the allele T frequency stays near 0.60, indicating equilibrium. At the population level, random mating and equal fitness among genotypes prevent changes, maintaining frequencies across generations. The lack of consistent survival or reproductive differences supports that no major evolutionary mechanisms are at play. A tempting distractor is A, strong gene flow balancing selection, but this misconceives the absence of immigrants and fitness variances by assuming opposing forces. A useful strategy is to confirm the absence of all four evolutionary forces—mutation, selection, drift, and gene flow—when allele frequencies show minimal change in large populations.

Question 12

1. Gene flow between subpopulations after the road increased allele differences
2. Stabilizing selection maintaining allele $B$ at 0.40 in both groups
3. Independent genetic drift in isolated subpopulations after fragmentation (correct answer)
4. Mutation converting allele $b$ to $B$ at different rates in each group
5. Nonrandom mating causing allele $B$ to increase in both subpopulations

Explanation: This question examines population genetics following habitat fragmentation, specifically how genetic drift operates in isolated populations. The road splits one large population into two isolated subpopulations of 1,000 rabbits each, and after 30 generations they show very different allele frequencies (B=0.18 vs B=0.63) despite starting from the same source and showing no fitness differences. The lack of migration prevents gene flow from homogenizing frequencies, and the absence of fitness differences rules out selection. Students might think 1,000 individuals is too large for drift, but genetic drift affects all finite populations, and its effects accumulate over many generations. When populations become isolated and diverge in allele frequencies without selection or gene flow, independent genetic drift in each subpopulation is the driving mechanism.

Question 13

1. Founder effect causing persistent sampling error in allele frequencies
2. Gene flow repeatedly reintroducing alleles from neighboring lakes
3. Frequency-dependent selection maintaining both alleles in the population (correct answer)
4. High mutation rate continually producing both alleles at equal rates
5. Genetic drift in a small population keeping allele frequencies constant

Explanation: This question examines population genetics mechanisms that maintain genetic variation, specifically frequency-dependent selection. The key evidence is that predators preferentially consume snails with the common shell pattern, giving a survival advantage to whichever pattern is rare at any given time - when A becomes common, a becomes advantageous, and vice versa. This negative frequency-dependent selection maintains both alleles at intermediate frequencies (around 0.50 each) over 25 generations. Students might incorrectly choose founder effect (A) thinking it explains persistent patterns, but founder effects cause random deviations from source populations, not stable maintenance of specific frequencies. The strategy for identifying frequency-dependent selection is looking for fitness that changes based on an allele's frequency in the population, creating a balanced polymorphism.

Question 14

1. Gene flow from offshore populations replacing allele $m$ rapidly
2. Mutation pressure reducing allele $m$ frequency within a few generations
3. Genetic drift associated with a population bottleneck event (correct answer)
4. Directional selection against allele $m$ due to the toxin
5. Nonrandom mating increasing homozygosity for alleles other than $m$

Explanation: This question illustrates how population genetics processes can cause dramatic allele frequency shifts during demographic events. The fish population crashed from 10,000 to just 120 individuals - a severe population bottleneck - then rebounded to 9,500. During this bottleneck, allele m frequency dropped from 0.48 to 0.11, despite being at a neutral locus with no fitness differences among genotypes. This random change in allele frequency during a population size reduction is classic genetic drift, specifically a bottleneck effect where random sampling of survivors drastically alters genetic composition. Students often incorrectly choose directional selection (D) because of the toxic spill, but the problem explicitly states this is a neutral locus with no genotype fitness differences. When populations experience severe size reductions, genetic drift through bottlenecks can cause large, random changes in allele frequencies regardless of the cause of population decline.

Question 15

1. Genetic drift increasing allele $R$ frequency by random sampling
2. Gene flow introducing allele $R$ repeatedly from other populations
3. Directional selection favoring allele $R$ under antibiotic exposure (correct answer)
4. High mutation rates creating many new $R$ alleles each generation
5. Assortative mating increasing the frequency of allele $R$ in bacteria

Explanation: This question examines population genetics in the context of antibiotic resistance evolution. A rare resistance allele R increases from frequency 0.001 to 0.35 in just 30 generations after antibiotic treatment begins - an extremely rapid change. The problem states the population remains very large (ruling out drift) and mutation rates are low and unchanged (ruling out mutation pressure). This leaves directional selection as the only mechanism that can drive such rapid frequency increase: bacteria carrying allele R survive antibiotic exposure better than those without it, giving them a massive fitness advantage. Students might choose genetic drift (A), but drift cannot cause such consistent directional change in large populations, especially not this rapidly. When you see rapid allele frequency increases coinciding with environmental changes that affect survival, think directional selection.

Question 16

1. Genetic drift causing random allele-frequency changes in a small population (correct answer)
2. Directional selection alternately favoring allele G and allele g each generation
3. Gene flow repeatedly adding and removing allele G from nearby populations
4. Mutation converting allele g to G and then G to g at high rates
5. Nonrandom mating changing allele frequencies while keeping genotype fitness equal

Explanation: This question assesses understanding of population genetics, focusing on mechanisms that change allele frequencies. Genetic drift is most directly responsible because the small population size of 60 plants allows random sampling errors to cause unpredictable fluctuations in allele G frequency over generations. At the population level, drift leads to non-directional changes without fitness differences or immigration, as seen in the erratic pattern from 0.55 to 0.47. The stable but small size amplifies these random effects each generation. A tempting distractor is directional selection (B), which is wrong because it assumes alternating favoring of alleles, misconstruing random fluctuations as adaptive shifts. To analyze similar problems, assess patterns of change for randomness versus directionality to pinpoint drift in small populations.

Question 17

1. Mutation introducing a new allele into the population's gene pool (correct answer)
2. Gene flow importing allele M from another bacterial population
3. Directional selection favoring allele M because it increases bacterial fitness
4. Genetic drift creating allele M by random sampling of existing alleles
5. Nonrandom mating increasing allele M frequency despite no new allele origin

Explanation: This question assesses understanding of population genetics, focusing on mechanisms that change allele frequencies. Mutation is most directly responsible because a single DNA replication error introduced allele M into the gene pool where it was previously absent, allowing it to appear at low frequency after generations. At the population level, mutations create novel genetic variation, and even without fitness advantages or migration, the new allele persists at low levels due to its origin. The large population size suggests the spread is not driven by random sampling alone. A tempting distractor is genetic drift (D), which is wrong because it assumes random sampling created the allele, misconstruing drift as a source of new alleles rather than a shuffler of existing ones. To analyze similar problems, trace allele origins to distinguish introduction mechanisms like mutation from redistribution processes.

Question 18

1. Directional selection increasing allele T because T genotypes leave more offspring (correct answer)
2. Genetic drift causing random increases in allele T in a large population
3. Gene flow importing allele T from an unsampled neighboring population
4. Mutation converting allele t to T at a rate sufficient to reach 0.60 quickly
5. Nonrandom mating alone shifting allele frequencies without fitness differences

Explanation: This question assesses understanding of population genetics, focusing on mechanisms that change allele frequencies. Directional selection is responsible because the fungal pathogen creates an environment where TT and Tt genotypes have higher fitness, increasing allele T frequency from 0.10 to 0.60 through greater seed production. At the population level, this adaptive process shifts alleles toward those conferring survival advantages in the new conditions, with the large population size minimizing random effects. No migration ensures the change stems from within-population dynamics rather than external input. A tempting distractor is gene flow (C), which is wrong because it assumes importation from unsampled populations, misconstruing internal fitness differences as external allele movement. To analyze similar problems, evaluate fitness data across genotypes to identify selection as the driver of directional changes.

Question 19

1. Mutation converting C alleles to c alleles, lowering c frequency overall
2. Genetic drift due to a severe bottleneck reducing population size each year
3. Gene flow bringing in many C alleles from a nearby population
4. Natural selection acting against genotype cc and reducing allele c frequency (correct answer)
5. Random mating increasing heterozygotes and thereby decreasing allele c frequency

Explanation: This question assesses understanding of population genetics, specifically how natural selection can reduce frequencies of disadvantageous alleles. The new predator imposes selection against the cc genotype, which contributes fewer offspring, leading to a decrease in p(c) from 0.30 to 0.12 over years. Population-level logic shows that when homozygous recessive individuals have lower fitness, the recessive allele's frequency declines as those genotypes are underrepresented in subsequent generations. The large population size and no immigration minimize drift or gene flow effects. A tempting distractor is B, genetic drift from bottlenecks, but this misconceives the stable large population and directional change by assuming random rather than fitness-based shifts. A key strategy is to identify consistent fitness disadvantages for specific genotypes as hallmarks of selection in allele frequency declines.

Question 20

1. Genetic drift caused by random sampling in a small population each year
2. Directional selection acting against allele $S$ during pathogen outbreaks (correct answer)
3. Gene flow bringing in more non-$S$ alleles from neighboring populations
4. Mutation converting allele $S$ into other alleles at unusually high rates
5. Assortative mating reducing heterozygosity and thereby lowering allele $S$

Explanation: This question tests understanding of population genetics, specifically how natural selection reduces the frequency of deleterious alleles. The arrival of a fungal pathogen creates a selective environment where genotypes carrying allele S have consistently lower survival during outbreaks, causing S to decline from 0.55 to 0.22 over 8 generations. The large population size rules out genetic drift as a major factor, and negligible immigration eliminates gene flow as an explanation. Students might incorrectly choose genetic drift (A) because they see any frequency decrease as random, but the key evidence is the consistent fitness disadvantage of S-carrying genotypes during pathogen outbreaks. When an environmental factor consistently reduces the survival or reproduction of individuals carrying a specific allele, directional selection against that allele drives the frequency change.