Population Genetics - AP Biology

1. In a small island population of 120 lizards, allele $G$ at a color locus had frequency 0.50 in 2010. A hurricane in 2011 randomly killed most individuals, leaving 18 survivors that founded the next generations. By 2013, the allele $G$ frequency in the population was 0.78, and no consistent difference in survival or reproduction was detected among color phenotypes. Which process is most likely responsible for the observed change in allele frequency?
   Answer: Genetic drift caused by a population bottleneck after the hurricane This question tests understanding of population genetics mechanisms, specifically how random events affect allele frequencies in small populations. The hurricane drastically reduced the population from 120 to just 18 individuals, creating a severe population bottleneck where allele frequencies can change dramatically due to random sampling effects rather than selection. Since no consistent fitness differences were detected among color phenotypes, natural selection cannot explain the change from 0.50 to 0.78 frequency for allele G. Students often incorrectly choose directional selection (B) because they assume any increase in allele frequency must be due to that allele being advantageous, but the key evidence against selection is the explicit statement about no survival or reproduction differences. When population size drops severely and allele frequency changes occur without fitness differences, genetic drift through a bottleneck is the most likely explanation.
2. A large coastal plant population has allele $S$ at frequency 0.55. A fungal pathogen arrives, and over 8 generations allele $S$ declines to 0.22. Field estimates show genotypes carrying $S$ have consistently lower survival during pathogen outbreaks, and immigration is negligible. Which factor most directly explains the decrease in allele $S$ frequency?
   Answer: Directional selection acting against allele $S$ during pathogen outbreaks 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.
3. In a large moth population, allele $D$ at a wing-color locus has frequency 0.50. Over ten generations, $p(D)$ remains near 0.50, but genotype surveys show heterozygotes ($Dd$) consistently have higher survival than either homozygote. Population size is large, and there is no immigration. Which evolutionary process most likely explains the stable intermediate allele frequency?
   Answer: Heterozygote advantage maintaining both alleles via balancing selection This question illustrates balancing selection maintaining genetic variation in populations. Despite the large population size and absence of immigration, allele D remains stable at frequency 0.50 over ten generations, with heterozygotes (Dd) consistently showing higher survival than either homozygote. This heterozygote advantage is a form of balancing selection that maintains both alleles in the population because neither can go to fixation - when either allele becomes rare, it appears mostly in high-fitness heterozygotes. Students might choose disruptive selection (A), but that favors both homozygotes over heterozygotes, which is the opposite of what's described. When heterozygotes have the highest fitness, balancing selection maintains stable intermediate allele frequencies regardless of population size.
4. A rabbit population has allele $A$ at frequency 0.40 at a fur locus. Over five generations, 15% of the population each generation consists of immigrants from a nearby population where $p(A)=0.90$. No differences in survival or reproduction among genotypes are detected, and population size remains large. After five generations, $p(A)$ rises to 0.63. Which process is most likely responsible for the increase in allele $A$ frequency?
   Answer: Gene flow increasing allele $A$ frequency through immigration This question tests understanding of how gene flow affects allele frequencies in populations. The rabbit population experiences continuous immigration, with 15% of individuals each generation coming from a source population where allele A has frequency 0.90 (much higher than the initial 0.40). Over five generations, this one-way gene flow increases the frequency of allele A to 0.63, as immigrants bring their alleles into the population. The problem explicitly states there are no fitness differences among genotypes and the population remains large, ruling out selection and drift. Students might incorrectly choose balancing selection (C), but that would maintain frequencies rather than cause directional change, and there's no evidence of heterozygote advantage. When you see continuous migration from a population with different allele frequencies, gene flow is the primary driver of frequency change.
5. Two neighboring frog populations differ in allele K frequency: Population 1 has $p(K)=0.20$, and Population 2 has $p(K)=0.80$. After a wet season creates a temporary corridor, many frogs move between ponds. In the following breeding season, Population 1 has $p(K)=0.35$ and Population 2 has $p(K)=0.68$. Both populations remain large, and no genotype at this locus shows consistent differences in survival or reproduction. Which factor most directly explains the convergence in allele frequencies?
   Answer: Gene flow between populations reducing differences in allele frequencies 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.
6. In a mainland bird population, allele $B$ at a beak-shape locus had frequency 0.50. A storm then carried 12 birds to a small offshore island, founding a new population. In the first generation on the island, the frequency of allele $B$ was 0.17. No additional birds arrived afterward, and there is no evidence of different survival or reproduction among genotypes on the island. Which process is most likely responsible for the change in allele frequency in the island population?
   Answer: Founder effect causing genetic drift in a small new population 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.
7. Two fish populations live in separate lakes. In Lake 1, allele C frequency is 0.80; in Lake 2, allele C frequency is 0.20. A canal is opened, allowing fish to move freely between lakes and breed. After several generations, both lakes show allele C frequency near 0.50, with no consistent differences in survival among genotypes within either lake. Which process is most likely responsible for the convergence in allele frequencies?
   Answer: Gene flow equalizing allele frequencies through migration between lakes 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.
8. In a population of 500 prairie grasses, allele H has frequency 0.50. A prolonged drought reduces the population to 40 plants that produce most of the seeds for recolonization. In the next generation of 500 plants, allele H frequency is 0.57. Measurements show no consistent differences in seed production among genotypes at this locus during the drought, and no seeds arrive from outside populations. Which process is most likely responsible for the allele-frequency shift?
   Answer: Genetic drift following a bottleneck that altered allele frequencies by chance This question assesses understanding of population genetics, specifically how genetic drift via bottlenecks alters allele frequencies randomly. The drought reduces the grass population to 40 plants, creating a bottleneck where allele H frequency shifts from 0.50 to 0.57 by chance in the recolonizing generation. Population-level logic highlights that in small survivor groups, random sampling leads to non-adaptive changes, especially without genotype differences in seed production. Isolation from outside seeds rules out gene flow. A tempting distractor is A, directional selection for H in drought, but this misconceives the lack of fitness variances by assuming adaptive survival when none is evidenced. As a strategy, examine drastic population reductions without selective evidence as cues for bottleneck-induced drift in frequency shifts.
9. In a large snail population, allele K has frequency 0.05. A nearby population with allele K frequency 0.05 sends no migrants, and survival does not differ among genotypes. However, a chemical mutagen increases the mutation rate from allele k to K for one year. In the following generation, allele K rises slightly to 0.051, then remains near that value afterward. Which factor most directly explains the small increase in allele K frequency?
   Answer: Mutation introducing additional K alleles during the year of elevated mutation rate 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.
10. A small, isolated lizard population on a desert outcrop has allele $T$ at frequency 0.52 at a neutral locus. Over 20 generations, with no immigration and no detected genotype differences in survival or reproduction, $p(T)$ fluctuates unpredictably between 0.30 and 0.70. The census population size remains around 80 adults each generation. Which factor most directly explains these allele frequency fluctuations?
    Answer: Genetic drift producing random allele frequency changes in a small population 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.
11. In a large insect population, allele S starts at frequency 0.30. A pesticide is applied each season. After five seasons, allele S increases to 0.85. Field data show individuals with genotype SS and Ss are more likely to survive to reproduction than ss individuals in treated areas. No new individuals enter the population during the study. Which process is most likely responsible for the increase in allele S frequency?
    Answer: Directional selection favoring allele S under pesticide exposure 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.
12. In a population of 1,200 coastal mice, fur color is controlled by alleles B and b. Before a storm, the b allele frequency was 0.40. A hurricane randomly killed many individuals, leaving 60 survivors that founded the next generation. In the offspring produced by these survivors, the b allele frequency is 0.62. No evidence indicates differential survival by fur color, and no new mice immigrated during recovery. Which process is most likely responsible for the change in allele frequency between generations?
    Answer: Genetic drift caused by a population bottleneck after the hurricane This question assesses understanding of population genetics, specifically how evolutionary forces like genetic drift can alter allele frequencies in populations. The drastic reduction from 1,200 to 60 mice represents a population bottleneck, where the small number of survivors leads to random sampling of alleles, causing the b frequency to shift from 0.40 to 0.62 by chance alone. Genetic drift is particularly strong in small populations, and since no differential survival by fur color was observed, the change is not due to selection but random loss of individuals. The population-level logic here is that bottlenecks reduce genetic diversity and cause unpredictable allele frequency changes without any adaptive basis. A tempting distractor is C, natural selection favoring b, but this misconceives the scenario by assuming differential survival when evidence shows none. To approach similar problems, always check for sudden population size reductions without selective pressures as indicators of genetic drift.
13. A population of freshwater snails includes alleles $A$ and $a$ at a shell-pattern locus. Over 25 generations, allele frequencies remain near $A=0.50$ and $a=0.50$. Field data show that predators more often consume snails with the common shell pattern each year, while the rarer pattern has higher survival until it becomes common. Population size is large and migration is minimal. Which factor best explains the long-term maintenance of both alleles?
    Answer: Frequency-dependent selection maintaining both alleles in the population 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.
14. A large insect population has allele $K$ at frequency 0.55. After a pesticide is introduced, genotype fitness estimates show $KK$ has the lowest survival, $Kk$ is intermediate, and $kk$ has the highest survival. Over eight generations, $p(K)$ decreases to 0.18. No immigration occurs, and the mutation rate is unchanged. Which factor most directly explains the decline in allele $K$ frequency?
    Answer: Directional selection against allele $K$ under pesticide exposure 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.
15. In a large laboratory population of bacteria, allele M is absent at generation 0. A single DNA replication error creates allele M in one cell. No migration occurs, and allele M does not change growth rate relative to the ancestral allele. After many generations, allele M is present at low frequency (about 0.002) rather than remaining at zero. Which factor most directly explains the appearance of allele M in the population?
    Answer: Mutation introducing a new allele into the population's gene pool 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.
16. In a lake fish population, allele R has frequency 0.55. A dam is removed, connecting the lake to a river population where allele R frequency is 0.10. Over five breeding seasons, the lake population's allele R frequency declines steadily to 0.28 while population size remains large and stable. Field surveys detect regular movement of adult fish from the river into the lake each year. No consistent differences in survival or fecundity among genotypes are observed. Which process is most likely responsible for the allele-frequency change in the lake?
    Answer: Gene flow from the river population into the lake population 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.
17. In a large population of insects, allele $R$ confers pesticide resistance. Before pesticide use, $R$ frequency was 0.02. After a pesticide is applied each season for 6 years, allele $R$ frequency increases to 0.46. Laboratory assays show that individuals with $R$ have higher survival when exposed to the pesticide, and migration into the population is negligible. Which process is most likely responsible for the allele-frequency change?
    Answer: Directional natural selection favoring allele $R$ under pesticide exposure This question tests understanding of population genetics, specifically how natural selection changes allele frequencies when there are fitness differences. The pesticide creates a selective environment where individuals carrying the resistance allele R have higher survival, causing R to increase from 0.02 to 0.46 over six years of pesticide application. The large population size rules out genetic drift, negligible migration eliminates gene flow, and the dramatic increase is too rapid for mutation alone. Students often confuse any environmental change with drift (A), but drift is random and doesn't consistently favor specific alleles based on their effects. When an environmental factor (pesticide) creates consistent survival differences among genotypes, and the advantageous allele increases in frequency, directional selection is the driving force.
18. Two separate populations of the same fish species live in similar lakes with no migration between them. Both populations are large. At a neutral microsatellite locus, allele $M$ starts at frequency 0.50 in each lake. After 40 generations, lake 1 has $M=0.12$ and lake 2 has $M=0.87$, with no consistent fitness differences among genotypes detected. Which factor most likely explains the divergence in allele frequency between lakes?
    Answer: Independent genetic drift causing random allele-frequency changes in each lake This question examines population genetics in isolated populations, specifically how genetic drift causes divergence. Despite starting with identical allele frequencies (M=0.50) and living in similar environments with no fitness differences among genotypes, the two lake populations diverged dramatically (lake 1: M=0.12, lake 2: M=0.87) over 40 generations. The absence of migration prevents gene flow, and the lack of fitness differences rules out selection. Students might incorrectly assume large populations are immune to drift, but even large populations experience some random sampling effects, and over many generations these can accumulate to produce substantial divergence. The key insight is that isolated populations will diverge through independent genetic drift even when they start identically and experience similar selective pressures.
19. In a large plant population, allele T has frequency 0.60. After a wildfire, the population quickly regrows to a similar size from surviving seeds in the soil. Genetic sampling shows allele T frequency is 0.59 in the first postfire generation and 0.60 in the second postfire generation. No immigrants are observed, and no consistent differences in survival or reproduction among genotypes are detected. Which evolutionary mechanism best explains the minimal change in allele frequencies across generations?
    Answer: Hardy-Weinberg equilibrium with no major evolutionary forces acting 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.
20. In a large population of wildflowers, genotype fitnesses at a locus are $W_{AA}=0.70$, $W_{Aa}=1.00$, and $W_{aa}=0.65$ across multiple years. Starting with allele $A$ frequency 0.60, the population shifts toward an intermediate allele frequency and then remains stable. Migration is negligible and population size is large. Which evolutionary mechanism most directly explains the stable intermediate allele frequency?
    Answer: Heterozygote advantage (overdominance) maintaining a balanced polymorphism This question tests understanding of population genetics, specifically how heterozygote advantage maintains genetic variation. The fitness values show that heterozygotes (Aa) have the highest fitness (1.00) compared to both homozygotes (AA=0.70, aa=0.65), creating a balanced polymorphism where both alleles are maintained at intermediate frequencies. Starting from A frequency 0.60, the population evolves toward a stable equilibrium where neither allele can be eliminated because heterozygotes carrying both alleles have the highest fitness. Students often incorrectly choose directional selection (B) when they see fitness differences, but directional selection requires one allele to consistently increase fitness, not the heterozygous combination. When heterozygotes have higher fitness than both homozygotes, natural selection maintains both alleles through overdominance or heterozygote advantage.

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