Population Genetics and Hardy–Weinberg (1C) - Bio & Biochem

A large population of wildflowers is genotyped at an autosomal locus with alleles C1 and C2. Researchers calculate allele frequencies from a representative sample of seeds and then compare expected genotype frequencies ( $p^2$ , $2pq$ , $q^2$ ) to observed genotype frequencies in adult plants. Adults show a heterozygote deficit, while seed genotype frequencies are near Hardy-Weinberg. Field notes indicate that adult plants are spatially clustered by genotype due to microhabitat differences, and pollination occurs mostly among near neighbors. Hardy-Weinberg equilibrium assumes: (i) random mating, (ii) no selection, (iii) no mutation, (iv) no migration, and (v) large population size. Which factor most likely explains the deviation from expected genotype frequencies in adults?
Answer: Nonrandom mating driven by spatial structure, increasing homozygosity relative to $2pq$ This question tests the understanding of how violations of Hardy-Weinberg assumptions, particularly nonrandom mating, lead to deviations in genotype frequencies. The Hardy-Weinberg principle assumes that allele and genotype frequencies remain stable in a population with random mating, no selection, no mutation, no migration, and large size, predicting genotype frequencies as p², 2pq, and q². Here, the wildflower population shows a heterozygote deficit in adults compared to expectations from seed allele frequencies, with spatial clustering and neighbor pollination indicating nonrandom mating. Choice A is correct because this spatial structure promotes assortative mating, increasing homozygosity and reducing heterozygotes below 2pq in the observed adults. Choice C fails by incorrectly suggesting overdominant selection reduces heterozygotes, when in fact overdominance (heterozygote advantage) would increase heterozygotes above expectations, misapplying selection concepts. A transferable check for Hardy-Weinberg assumptions involves examining mating patterns for randomness, such as assessing spatial distribution or mate choice behaviors. If nonrandom mating is evident, expect deviations like heterozygote deficits due to inbreeding or population substructure.
A human genetics cohort analyzes a biallelic autosomal locus (C/c) in a metropolitan region. Genotype frequencies in newborns deviate from Hardy-Weinberg expectations, with an excess of homozygotes relative to $2pq$ . The locus is not associated with measured health outcomes in infancy, and sequencing shows no evidence of recurrent mutation at the site. Survey data indicate that individuals tend to select partners from the same linguistic community, and the region contains two large subpopulations with different allele frequencies at this locus. Assumptions for Hardy-Weinberg equilibrium: random mating; no selection; no mutation; no migration; very large population (negligible drift). Which factor most likely explains the deviation from expected genotype frequencies?
Answer: Population substructure with assortative mating, producing a Wahlund effect and homozygote excess This question tests understanding of population substructure and the Wahlund effect. The Hardy-Weinberg principle assumes random mating across the entire population, but when individuals preferentially mate within subgroups (here, linguistic communities), this creates population substructure. When subpopulations with different allele frequencies are sampled together but mate separately, the overall population shows an excess of homozygotes compared to Hardy-Weinberg expectations - this is the Wahlund effect. Choice A incorrectly claims panmixia increases homozygosity (it maintains Hardy-Weinberg proportions), while choice D wrongly suggests balancing selection for heterozygotes would create homozygote excess. To recognize population substructure effects, look for assortative mating patterns combined with different allele frequencies between subgroups, which produces homozygote excess when analyzed as one population.
A conservation team monitors a biallelic locus (D/d) in an endangered mammal restricted to a single valley. The team reports that mating appears random and that there is no detectable migration, mutation, or selection at this locus. Following a wildfire, the breeding population is reduced to a small number of survivors, and allele frequencies in the next generation differ markedly from pre-fire estimates. Which factor most likely explains the deviation from expected Hardy-Weinberg behavior across generations?
Answer: Genetic drift due to a population bottleneck causing random allele frequency changes This question tests understanding of population bottlenecks and genetic drift. The Hardy-Weinberg principle requires very large population sizes to prevent random sampling effects on allele frequencies. The wildfire creates a population bottleneck, drastically reducing the breeding population size and violating this assumption. The correct answer A identifies genetic drift from the bottleneck as causing random allele frequency changes. Answer D incorrectly claims population size is irrelevant to Hardy-Weinberg equilibrium, when large population size is one of the five critical assumptions. Following catastrophic events that reduce population size, expect random changes in allele frequencies due to genetic drift, with the magnitude of change inversely related to population size.
A mosquito population is sampled before and after introduction of an insecticide. A single autosomal locus has alleles R (resistance) and r (susceptible). Prior to insecticide use, genotype frequencies were consistent with Hardy-Weinberg expectations and mating appeared random. After insecticide deployment, adult survival in treated areas is markedly higher for RR and Rr than for rr. No evidence of migration into the treated area is found. Assumptions for Hardy-Weinberg equilibrium: random mating; no selection; no mutation; no migration; very large population (negligible drift). Based on the Hardy-Weinberg principle, which prediction about allele frequencies is most consistent with the post-insecticide scenario?
Answer: The frequency of allele R will increase because differential survival violates the no-selection assumption This question tests understanding of how selection affects allele frequencies. The Hardy-Weinberg principle predicts constant allele frequencies only when there is no selection, but insecticide exposure creates strong selection pressure. Since RR and Rr mosquitoes survive better than rr in treated areas, individuals carrying the R allele have higher fitness, violating the no-selection assumption. This differential survival will increase the frequency of allele R in subsequent generations as more R-carrying individuals survive to reproduce. Choice B incorrectly claims random mating maintains constant frequencies despite selection, while choice C wrongly suggests selection favors rare alleles (selection favors beneficial alleles regardless of frequency). To predict selection outcomes, identify which genotypes have higher fitness and recognize that alleles in those genotypes will increase in frequency.
A wildlife biologist samples a large deer population at an autosomal locus with alleles H and h. The allele frequencies estimated from gamete-equivalent sampling are $p(H)=0.7$ and $q(h)=0.3$ . The observed adult genotype frequencies match the expected $p^2:2pq:q^2$ within sampling error. Hardy-Weinberg equilibrium assumes: (i) random mating, (ii) no selection, (iii) no mutation, (iv) no migration, and (v) large population size. Based on Hardy-Weinberg reasoning, which prediction is most consistent for the next generation if these assumptions continue to hold?
Answer: Allele frequencies are expected to remain approximately $p=0.7$ and $q=0.3$ This question tests predictions under Hardy-Weinberg equilibrium when assumptions hold. The Hardy-Weinberg principle states that in a large, randomly mating population with no selection, mutation, or migration, allele frequencies remain constant, and genotypes match p², 2pq, q². In this deer population, current genotypes match expectations, suggesting equilibrium. Choice A correctly predicts stable allele frequencies of p=0.7 and q=0.3 in the next generation if assumptions continue. Choice B is incorrect because heterozygotes persist via random mating, not disappear due to segregation, misconstruing meiotic effects. A transferable check for Hardy-Weinberg assumptions is to verify if observed genotypes fit expected ratios using allele frequencies. Stability over generations confirms equilibrium conditions.
In a long-term plant ecology study, a biallelic locus (C/c) is tracked in a large meadow population. The investigators claim Hardy-Weinberg assumptions are largely satisfied (random mating via wind pollination, large population size, no migration, no selection). A new point mutation is detected that converts C to c in germline cells at a measurable rate; there is no evidence of back-mutation (c to C). Over several generations, allele c frequency increases despite stable census size and no detected migration. Which factor most likely explains the deviation from Hardy-Weinberg equilibrium?
Answer: Mutation introducing new c alleles, violating the no-mutation assumption This question tests understanding of mutation as a Hardy-Weinberg violation. The Hardy-Weinberg principle assumes no mutation to maintain constant allele frequencies across generations. The scenario describes a measurable mutation rate converting C to c alleles, directly violating this assumption. The correct answer A identifies mutation as the cause of increasing c frequency despite other Hardy-Weinberg conditions being met. Answer C incorrectly suggests random mating changes allele frequencies, when random mating only affects genotype frequencies, not allele frequencies. When allele frequencies change systematically in one direction without evidence of selection, migration, or drift, consider mutation as the driving force. Unidirectional mutation (C→c without back-mutation) will steadily increase the frequency of the mutant allele.
A large bird population is sampled at an autosomal locus with alleles Q and q. Genotype frequencies match Hardy-Weinberg expectations in year 1. In year 2, a nearby population with a different $p(Q)$ begins contributing migrants each breeding season; banding data confirm consistent immigration of breeding adults. No genotype-dependent survival differences are detected. Hardy-Weinberg equilibrium assumes: (i) no migration, (ii) random mating, (iii) no selection, (iv) no mutation, and (v) large population size. Which prediction is most consistent with Hardy-Weinberg reasoning after immigration begins?
Answer: Allele frequencies in the focal population may shift toward those of the source population due to gene flow This question tests the ability to apply the Hardy-Weinberg principle to predict changes in allele frequencies when one of its assumptions is violated. The Hardy-Weinberg principle states that in a large, randomly mating population with no selection, mutation, or migration, allele and genotype frequencies remain constant across generations. In this scenario, the bird population initially meets Hardy-Weinberg expectations, but consistent immigration from a nearby population with different allele frequencies introduces gene flow, violating the no-migration assumption. The correct prediction, choice A, follows the principle because gene flow can alter allele frequencies in the focal population, shifting them toward those of the source population. A common distractor, choice D, fails by misconstruing that Hardy-Weinberg equilibrium persists despite migration, ignoring that migration prevents equilibrium by changing allele frequencies. To recognize Hardy-Weinberg assumptions in practice, check for evidence of gene flow, such as immigration data, which would invalidate predictions of constant frequencies. Additionally, confirm other assumptions like large population size and no selection are met to isolate the violating factor.
A large urban pigeon population is genotyped at an autosomal locus with alleles T and t. Researchers compute expected genotype frequencies from allele counts and find a consistent heterozygote deficit. Behavioral observations indicate that pigeons preferentially choose mates with similar plumage, and plumage is strongly associated with TT vs tt genotypes. Hardy-Weinberg equilibrium assumes: (i) random mating, (ii) no selection, (iii) no mutation, (iv) no migration, and (v) large population size. Which factor most likely explains the deviation from expected genotype frequencies?
Answer: Nonrandom mating (assortative mating) increasing homozygosity relative to $2pq$ This question tests understanding of deviations from Hardy-Weinberg equilibrium due to nonrandom mating. The Hardy-Weinberg principle states that random mating, no selection, no mutation, no migration, and large size maintain genotype expectations. In this pigeon population, preferential mating based on plumage linked to genotypes causes heterozygote deficit. Choice D correctly identifies assortative mating as the cause, violating the random mating assumption. Choice B is incorrect because drift in large populations is minimal and not consistent. A transferable check for Hardy-Weinberg assumptions is to assess mate choice; homozygote excess suggests assortative mating. Behavioral data can link mating to genotypes.
A conservation group monitors a small, fenced population of desert rodents founded by 12 individuals. The group genotypes a neutral autosomal SNP with alleles G and g each spring. No predators are present, food is supplemented, and no genotype-specific differences in survival or fecundity have been observed in captivity. The fence prevents immigration and emigration, and mating appears random. Assumptions for Hardy-Weinberg equilibrium: random mating; no selection; no mutation; no migration; very large population (negligible drift). Across 5 years, allele frequencies fluctuate irregularly (e.g., $p_G$ rises one year and falls the next) without a consistent direction. Which factor most likely explains the deviation from Hardy-Weinberg expectations in this population?
Answer: Genetic drift due to small effective population size, producing random allele frequency changes This question tests recognition of genetic drift in small populations. The Hardy-Weinberg principle assumes a very large population where random sampling effects are negligible, but this desert rodent population was founded by only 12 individuals and remains small. In small populations, random sampling of gametes causes allele frequencies to fluctuate unpredictably between generations, a phenomenon called genetic drift. The irregular fluctuations without consistent direction are characteristic of drift, not selection (which would show directional change) or mutation (which occurs too slowly to cause year-to-year oscillations). Choice D incorrectly suggests random mating changes allele frequencies, when it actually maintains them under Hardy-Weinberg conditions. To identify genetic drift, look for small population size combined with random, non-directional changes in allele frequencies over time.
A large butterfly population is monitored at an autosomal locus with alleles Z and z. Researchers observe that Zz individuals have higher mating success because of a display trait, but larval survival does not differ among genotypes. Over time, the adult population shows an excess of heterozygotes relative to Hardy-Weinberg expectations computed from allele frequencies in larvae. Hardy-Weinberg equilibrium assumes: (i) random mating, (ii) no selection (including sexual selection), (iii) no mutation, (iv) no migration, and (v) large population size. Which factor most likely explains the deviation from expected genotype frequencies in adults?
Answer: Sexual selection increasing reproductive contribution of Zz individuals, altering genotype frequencies among adults This question tests understanding of deviations from Hardy-Weinberg equilibrium due to sexual selection. The Hardy-Weinberg principle states that random mating, no selection (including sexual), no mutation, no migration, and large size maintain equilibria. In this butterfly population, Zz higher mating success causes adult heterozygote excess. Choice D correctly identifies sexual selection as the cause, violating assumptions. Choice B is incorrect because drift in large populations is not predictable. A transferable check for Hardy-Weinberg assumptions is to evaluate reproductive success; heterozygote excess in adults suggests sexual selection. Comparing larval and adult frequencies reveals biases.
In a coastal lizard population, a single autosomal locus (alleles A and a) was genotyped each breeding season for 5 years. The population is large and geographically continuous, and field teams report random mating based on marked-pair observations. However, a new raptor species established in year 2 and preferentially captures lizards with the aa-associated color morph. By year 4, the observed fraction of aa individuals is consistently lower than expected from $q^2$ estimated from allele counts in hatchlings. Hardy-Weinberg equilibrium assumes: (i) no selection, (ii) random mating, (iii) no mutation, (iv) no migration, and (v) very large population size. Which factor most likely explains the deviation from expected genotype frequencies in this population?
Answer: Nonrandom survival of aa individuals due to predation, altering genotype frequencies after zygote formation This question tests understanding of deviations from Hardy-Weinberg equilibrium due to natural selection. The Hardy-Weinberg principle states that in a large, randomly mating population with no selection, mutation, or migration, allele frequencies remain constant, and genotype frequencies conform to p², 2pq, and q². In this coastal lizard scenario, the introduction of a raptor that preferentially preys on aa individuals leads to lower-than-expected aa frequencies in adults compared to hatchlings. Choice C correctly identifies nonrandom survival due to predation as the factor violating the no-selection assumption, altering genotype frequencies post-zygote formation. Choice B is incorrect because random mating actually maintains heterozygosity at 2pq, not increases it above expectations, misconstruing the role of mating in equilibrium. A transferable check for Hardy-Weinberg assumptions is to calculate expected genotype frequencies from observed allele frequencies and test for significant deviations using chi-square analysis. Persistent discrepancies in specific genotypes often signal selection, while random fluctuations may indicate drift in small populations.
A mosquito population is monitored at a single insecticide-resistance locus with alleles R (resistant) and r (susceptible). Investigators report that before insecticide deployment, the population met Hardy-Weinberg assumptions (random mating, large population, no migration, no mutation, no selection). After insecticide use begins, adult survival surveys show that rr individuals are disproportionately absent among survivors, while RR and Rr are overrepresented. Based on Hardy-Weinberg reasoning, which prediction about allele frequencies in subsequent generations is most consistent with these observations?
Answer: Allele R frequency will increase because rr individuals have reduced survival (directional selection against r) This question tests understanding of natural selection's effect on allele frequencies under Hardy-Weinberg conditions. The Hardy-Weinberg principle assumes no selection, but this scenario introduces differential survival where rr individuals die from insecticide exposure. When selection acts against a recessive homozygote (rr), the frequency of the recessive allele (r) decreases while the dominant allele (R) increases, making answer B correct. Answer C incorrectly claims random mating alone maintains constant allele frequencies, ignoring that selection violates Hardy-Weinberg assumptions. Answer A reverses the direction of selection's effect. To predict selection outcomes, identify which genotypes have reduced fitness and remember that selection against a genotype reduces the frequency of its constituent alleles.
A large population of rabbits is genotyped at an autosomal locus with alleles Y and y. Allele frequencies are $p(Y)=0.6$ and $q(y)=0.4$ in zygotes. Researchers later sample only adult survivors after a harsh winter and observe fewer yy individuals than expected from $q^2$ based on zygote allele frequencies. Hardy-Weinberg equilibrium assumes: (i) no selection, (ii) random mating, (iii) no mutation, (iv) no migration, and (v) large population size. Which factor most likely explains the deviation from expected genotype frequencies in the adult sample?
Answer: Viability selection during winter disproportionately reducing survival of yy individuals This question tests understanding of deviations from Hardy-Weinberg equilibrium due to viability selection. The Hardy-Weinberg principle states that no selection, random mating, no mutation, no migration, and large size maintain genotype expectations. In this rabbit population, fewer yy adults after winter suggest survival differences. Choice C correctly identifies viability selection against yy as the cause, violating no-selection. Choice B is incorrect because random mating does not increase homozygotes. A transferable check for Hardy-Weinberg assumptions is to sample across seasons; post-selection deficits indicate viability effects. Survival data links genotypes to selection.
A captive breeding program maintains a large population of wolves. At an autosomal locus with alleles V and v, managers intentionally pair close relatives to preserve a rare pedigree line. After several generations, genotyping shows an increase in homozygotes relative to Hardy-Weinberg expectations computed from allele frequencies, while allele frequencies themselves have changed little. Hardy-Weinberg equilibrium assumes: (i) random mating, (ii) no selection, (iii) no mutation, (iv) no migration, and (v) large population size. Which factor most likely explains the deviation from expected genotype frequencies?
Answer: Inbreeding (nonrandom mating) increasing homozygosity without necessarily changing allele frequencies immediately This question tests understanding of deviations from Hardy-Weinberg equilibrium due to inbreeding. The Hardy-Weinberg principle states that random mating, no selection, no mutation, no migration, and large size preserve genotype ratios. In this wolf program, pairing relatives increases homozygotes without major allele changes. Choice D correctly identifies inbreeding as the cause, violating random mating. Choice B is incorrect because favoring heterozygotes would decrease, not increase, homozygosity. A transferable check for Hardy-Weinberg assumptions is to calculate inbreeding coefficients; homozygote excess suggests nonrandom mating. Pedigree analysis can quantify inbreeding levels.
A clinical genetics group screens a large, geographically isolated human community for a recessive metabolic disorder caused by genotype aa at a biallelic locus (A/a). The investigators state that Hardy-Weinberg assumptions are approximately met (random mating, large population, no migration, no mutation, no selection). Unexpectedly, the observed frequency of affected individuals (aa) is higher than predicted from the measured allele frequency of a. Pedigree review identifies a high rate of marriages among second cousins within the community. Which factor most likely explains the deviation from expected genotype frequencies?
Answer: Nonrandom mating (inbreeding) increasing homozygosity without requiring a change in allele frequencies This question tests recognition of nonrandom mating as a Hardy-Weinberg violation. The Hardy-Weinberg principle requires random mating, large population size, no migration, no mutation, and no selection to maintain equilibrium genotype frequencies. The scenario describes a population with high rates of cousin marriages, which represents inbreeding - a form of nonrandom mating. The correct answer D identifies that inbreeding increases homozygosity (both AA and aa) while decreasing heterozygosity (Aa) without changing allele frequencies. Answer B incorrectly suggests balancing selection would increase aa frequency, when balancing selection actually maintains heterozygotes. When observed homozygote frequencies exceed Hardy-Weinberg predictions while allele frequencies remain stable, suspect inbreeding or other forms of nonrandom mating.
A field team genotyped a biallelic locus (alleles A and a) in an isolated island bird population over two breeding seasons. During the interval, the population remained large (estimated >10,000 breeders), no immigration was detected by banding, and mating observations were consistent with random pairing. The locus is autosomal and not linked to known fitness traits. However, a viral outbreak occurred between seasons and disproportionately reduced survival of juveniles from nests located near freshwater. Investigators noted that nest location is correlated with a maternally inherited habitat preference affecting where eggs are laid, and that preference is associated with allele A at the locus due to linkage disequilibrium with a nearby marker. Assumptions for Hardy-Weinberg equilibrium: random mating; no selection; no mutation; no migration; very large population (negligible drift). Which factor most likely explains a deviation from Hardy-Weinberg genotype frequencies at this locus in season 2?
Answer: Nonrandom survival associated with habitat-linked genotypes, consistent with selection acting on correlated traits This question tests understanding of how selection violates Hardy-Weinberg assumptions. The Hardy-Weinberg principle states that allele and genotype frequencies remain constant in a population when five assumptions are met: random mating, no selection, no mutation, no migration, and very large population size. In this scenario, a viral outbreak disproportionately killed juveniles from freshwater nests, and these nests are associated with allele A through linkage disequilibrium with a habitat preference marker. This creates differential survival based on genotype-linked traits, violating the no-selection assumption. Choice B incorrectly claims random mating increases homozygosity (it actually maintains Hardy-Weinberg proportions), while choice C misunderstands genetic drift, which is negligible in very large populations. To identify selection as a Hardy-Weinberg violation, look for differential survival, reproduction, or fitness associated with specific genotypes or linked traits.
A coastal fish species occupies two adjacent bays connected by a narrow channel. A biallelic autosomal locus (T/t) is genotyped. Bay 1 has high T frequency, Bay 2 has low T frequency. Over several spawning seasons, researchers observe that juvenile fish increasingly mix between bays due to dredging that widened the channel. No evidence suggests genotype-dependent survival or fecundity at this locus, and mating within each bay is approximately random. Assumptions for Hardy-Weinberg equilibrium: random mating; no selection; no mutation; no migration; very large population (negligible drift). Which prediction about allele frequencies is most consistent with the Hardy-Weinberg framework given the dredging?
Answer: Allele frequencies in each bay will tend to converge because migration violates the no-migration assumption This question tests understanding of migration's effect on allele frequencies. The Hardy-Weinberg principle assumes no migration between populations, but the widened channel allows juvenile fish to move between bays with different initial allele frequencies. Migration causes gene flow, which tends to homogenize allele frequencies between connected populations - Bay 1's high T frequency and Bay 2's low T frequency will converge toward intermediate values. Choice B incorrectly claims gene flow increases divergence (it reduces it), while choice D wrongly suggests migration amplifies drift rather than causing predictable convergence. To predict migration effects, recognize that gene flow between populations with different allele frequencies will cause those frequencies to become more similar over time, violating the no-migration assumption.
A field study examines a biallelic locus (G/g) in a bird species across two adjacent habitats separated by a narrow ridge. In Habitat 1, genotype frequencies match Hardy-Weinberg expectations based on local allele frequencies. In Habitat 2, the observed heterozygote frequency (Gg) is substantially lower than expected, while both homozygotes are more common; allele frequencies are similar in both habitats. Researchers note that Habitat 2 has strong site fidelity and frequent mating among close relatives. Which factor most likely explains the deviation from expected genotype frequencies in Habitat 2?
Answer: Nonrandom mating (inbreeding) reducing heterozygosity without requiring allele frequency differences This question tests recognition of inbreeding as nonrandom mating. The Hardy-Weinberg principle requires random mating, but Habitat 2 shows frequent mating among close relatives (inbreeding). Inbreeding increases homozygosity at the expense of heterozygosity without changing allele frequencies, explaining why Habitat 2 has fewer heterozygotes than expected. The correct answer C identifies inbreeding as the cause, noting it reduces heterozygosity without requiring allele frequency differences between habitats. Answer B incorrectly suggests selection for heterozygotes would increase Gg frequency, opposite to the observed pattern. When heterozygote deficiency occurs with excess homozygotes but similar allele frequencies, inbreeding is the most likely explanation.
A pathogen surveillance lab tracks an autosomal locus in a fungal population with alleles V and v. In year 1, genotype frequencies match Hardy-Weinberg expectations. In year 2, sequencing reveals that a nearby gene conferring drug tolerance has undergone a selective sweep after widespread antifungal use, and the marker locus is physically close enough to show strong linkage to the tolerance allele. No migration is detected, and the fungal population size remains large. Assumptions for Hardy-Weinberg equilibrium: random mating; no selection; no mutation; no migration; very large population (negligible drift). Which factor most likely explains the deviation from Hardy-Weinberg expectations at the marker locus in year 2?
Answer: Selection at a linked locus (genetic hitchhiking), violating the no-selection assumption for the marker This question tests understanding of genetic hitchhiking and linked selection. The Hardy-Weinberg principle assumes no selection at the locus being studied, but selection can act indirectly through physical linkage to selected genes. When the nearby drug tolerance gene underwent a selective sweep due to antifungal use, the physically linked marker locus was carried along (hitchhiked) despite being selectively neutral itself. This violates the no-selection assumption for the marker locus and changes its allele frequencies. Choice C incorrectly claims random mating changes allele frequencies through recombination (it maintains them), while choice A misunderstands drift in large populations. To recognize hitchhiking effects, look for selection on nearby linked loci that can drag neutral markers along during selective sweeps.
A large population of insects is sampled at an autosomal locus with alleles W and w. A pesticide is introduced that kills insects regardless of genotype at this locus; survival assays confirm no genotype-dependent differences. After the pesticide introduction, allele frequencies at W/w remain stable and genotype frequencies continue to match Hardy-Weinberg expectations. Hardy-Weinberg equilibrium assumes: (i) no selection at the locus, (ii) random mating, (iii) no mutation, (iv) no migration, and (v) large population size. Based on Hardy-Weinberg reasoning, which statement best accounts for the continued match to expected genotype frequencies?
Answer: Selection is effectively absent at the W/w locus, so genotype frequencies can remain at $p^2:2pq:q^2$ under random mating This question tests why equilibrium can persist despite external mortality using Hardy-Weinberg reasoning. The Hardy-Weinberg principle states that no selection at the locus, random mating, no mutation, no migration, and large size maintain frequencies. In this insect scenario, genotype-independent pesticide allows continued equilibrium. Choice A correctly explains absent locus-specific selection permits matching expectations. Choice B is incorrect because non-locus-specific mortality does not violate assumptions. A transferable check for Hardy-Weinberg assumptions is to test locus-specific fitness; neutral loci remain in equilibrium. Post-event genotyping confirms stability.

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