Population Genetics
Help Questions
AP Biology › Population Genetics
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?
Nonrandom mating increasing heterozygote frequency at the neutral locus
Stabilizing selection maintaining allele $T$ near an intermediate frequency
Genetic drift producing random allele frequency changes in a small population
Mutation converting allele $t$ to allele $T$ at a variable rate
Gene flow from nearby populations repeatedly altering allele $T$ frequency
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.
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?
Increased mutation converting allele $K$ into allele $k$ each generation
Nonrandom mating increasing allele $k$ frequency without affecting fitness
Gene flow introducing allele $k$ from a neighboring population
Directional selection against allele $K$ under pesticide exposure
Genetic drift due to random sampling in a large population
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.
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?
Nonrandom mating increasing heterozygosity at the beak-shape locus
Increased mutation rate converting other alleles into allele $B$
Gene flow from the mainland population into the island population
Founder effect causing genetic drift in a small new population
Directional selection favoring allele $B$ in the island environment
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.
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?
Gene flow importing allele K from the nearby population with the same frequency
Genetic drift producing a consistent, directional increase in allele K each generation
Directional selection favoring allele K despite equal survival among genotypes
Nonrandom mating converting allele k into allele K through mate choice
Mutation introducing additional K alleles during the year of elevated mutation rate
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.
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?
Random mating increasing heterozygosity and thereby lowering R frequency
Mutation introducing new r alleles at a rate sufficient to halve R frequency
Stabilizing selection maintaining intermediate allele frequencies in the lake
Gene flow from the river population into the lake population
Genetic drift due to repeated population bottlenecks each breeding season
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.
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?
Assortative mating increasing heterozygotes and forcing allele frequency to 0.50
Stabilizing selection independently driving both lakes toward allele C frequency 0.50
Mutation pressure producing allele C in Lake 2 until it matches Lake 1
Gene flow equalizing allele frequencies through migration between lakes
Genetic drift causing both lakes to randomly reach the same allele frequency
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.
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?
Random mating increasing allele S frequency by altering genotype proportions
Mutation generating allele S repeatedly until it becomes common
Directional selection favoring allele S under pesticide exposure
Genetic drift due to random sampling effects in a very large population
Gene flow from untreated populations bringing allele S into treated areas
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.
A population of 25 rabbits on an isolated peninsula has allele P frequency 0.48. Over 15 generations, allele P reaches fixation (1.00). Throughout the period, food availability, predator presence, and reproductive output show no consistent differences among genotypes, and no migration occurs. The population remains small each generation. Which factor most directly explains fixation of allele P?
Directional selection consistently favoring allele P due to higher fitness
Gene flow adding allele P from immigrants until fixation occurs
Genetic drift leading to fixation through random sampling in a small population
Mutation pressure steadily converting the alternative allele into allele P
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.
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?
Mutation creating allele K in Population 1 and removing it in Population 2
Disruptive selection favoring opposite alleles in each population
Gene flow between populations reducing differences in allele frequencies
Nonrandom mating increasing heterozygosity and equalizing allele frequencies
Genetic drift causing both populations to move toward intermediate 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.
In a bacterial population, allele S at a gene has frequency 0.00 at time 0. After exposure to UV radiation, allele S is detected at frequency 0.001 in the next generation, while the population size remains extremely large. No migration is possible in the closed culture flask, and the environment does not differentially affect reproduction among genotypes at this gene. Which process most likely explains the appearance of allele S in the population?
Nonrandom mating increasing allele S frequency in a closed culture
Gene flow introducing allele S from another bacterial population
Genetic drift increasing allele S frequency from standing variation
Directional selection favoring allele S, causing it to originate
Mutation creating allele S from a preexisting allele in the population
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.