- A population of insects has variation in a detoxification enzyme due to alleles D and d. In fields treated with a new pesticide, insects with genotype DD survive at higher rates than dd insects, and D allele frequency increases from 0.30 to 0.55 over several generations. Which statement best explains how genetic variation contributed to this evolutionary response?
Answer: Allele D increased because insects with allele D had higher survival and reproduction, shifting allele frequencies.
This question examines how genetic variation enables evolutionary responses to novel selective pressures. The correct answer (B) correctly identifies that insects carrying allele D had higher survival rates when exposed to the pesticide, allowing them to reproduce more than insects with allele d who died at higher rates. This differential survival and reproduction based on genotype shifted the D allele frequency from 0.30 to 0.55 over several generations, demonstrating natural selection acting on preexisting genetic variation. Answer A incorrectly suggests the pesticide caused mutations from d to D during exposure, confusing the selective effect of pesticides (killing susceptible individuals) with mutagenic effects (which are rare and random, not directed). Remember that pesticides select for resistance alleles that already exist in the population at low frequency—they don't create the resistance alleles.
- In a bacterial population, some cells carry a plasmid with an antibiotic-resistance gene, and other cells lack the plasmid. Before antibiotic exposure, plasmid-bearing cells are 5% of the population. When the antibiotic is added, plasmid-bearing cells survive and reproduce more than plasmid-free cells. After 24 hours, plasmid-bearing cells make up 80% of the population. No evidence indicates the resistance gene arose during the 24-hour period.
Which statement best explains the population-level change?
Answer: Some cells already had resistance, and selection increased their contribution to the next generations.
This question tests understanding of how antibiotic resistance evolves through selection on existing variation in bacterial populations. The correct answer (B) recognizes that some bacterial cells already carried resistance plasmids before antibiotic exposure (5% of the population), and when the antibiotic was added, these resistant cells survived and reproduced while non-resistant cells died. This differential survival rapidly increased the frequency of plasmid-bearing cells to 80% in just 24 hours. Answer A incorrectly suggests the antibiotic caused all cells to mutate and become resistant, but the question states no new resistance arose during the experiment. The key principle is that antibiotics don't create resistance—they select for pre-existing resistant variants in the population.
- A population of bacteria contains two alleles of a gene affecting antibiotic resistance: allele X (higher resistance) and allele x (lower resistance). When an antibiotic is introduced, the frequency of X rises from 0.05 to 0.80 after many generations. Assume resistance differences are heritable and bacteria reproduce rapidly. Which statement best explains how variation influenced the evolutionary outcome?
Answer: Bacteria with allele X had higher reproductive success under antibiotic exposure, increasing X frequency in the population.
This question requires analyzing how pre-existing genetic variation enables rapid evolutionary responses to selective pressures. The bacterial population already contained both X (resistant) and x (sensitive) alleles before antibiotic exposure, though X was initially rare at 0.05 frequency. When the antibiotic was introduced, bacteria carrying allele X survived and reproduced successfully, while those with allele x died before reproducing. This extreme differential reproductive success caused the X allele to increase dramatically from 0.05 to 0.80 over many generations through natural selection. Choice A incorrectly suggests that the antibiotic causes each bacterium to acquire resistance, representing the misconception that environmental pressures directly cause beneficial mutations rather than selecting for existing variation. The strategy is to recognize that antibiotics don't create resistance—they reveal and amplify pre-existing resistant variants by eliminating sensitive individuals, allowing resistant ones to dominate the population.
- In a coastal plant population, salt tolerance is influenced by two alleles at a locus: S (higher tolerance) and s (lower tolerance). A storm permanently increases soil salinity. Over 15 generations, allele frequency changes from S = 0.40 to S = 0.71. Plants are sessile, and pollination occurs locally. Which statement best explains the role of preexisting genetic variation in the population's evolutionary response to increased salinity?
Answer: Individuals with allele S survived and reproduced more, increasing S frequency in the population over generations.
This question tests understanding of how standing genetic variation enables population-level evolutionary responses to environmental changes. The coastal plant population already contained both S (salt-tolerant) and s (salt-sensitive) alleles before the storm increased soil salinity. When salinity increased, plants with the S allele had higher survival and reproductive success in the new salty conditions, while those with the s allele struggled to survive and reproduce. Over 15 generations, this differential reproductive success caused the S allele frequency to increase from 0.40 to 0.71 through natural selection. Choice B represents the Lamarckian misconception that individuals can change their genes during their lifetime in response to environmental conditions. The key insight is that populations evolve because individuals with certain pre-existing alleles leave more offspring, gradually shifting the genetic composition of the population over generations.
- In a flowering plant population, petal color varies due to different alleles: red, pink, or white. A pollinator species visits red flowers more frequently than pink or white. Over multiple generations, the proportion of red-flowered plants increases, while the other colors persist at lower frequencies. No new mutations are detected in the color gene during the study. Which statement best explains how variation affected evolutionary change in this population?
Answer: Color variation led to differential pollination success, increasing the frequency of alleles for red petals over time.
This question assesses the skill of analyzing variations in populations by examining how genetic diversity in petal color affects reproductive success and allele frequencies. The correct answer, B, shows that variation in color led to differential pollination, with red flowers receiving more visits and thus higher reproductive success, increasing the frequency of red alleles over generations through sexual selection. The absence of new mutations during the study emphasizes that the change stemmed from selection on preexisting allelic variation, allowing the population to adapt without losing all diversity. The persistence of other colors at lower frequencies illustrates that selection can shift proportions while maintaining variation. A tempting distractor is C, which represents the misconception of acquired inheritance, suggesting plants change color after visitation and pass it on, but traits must be genetically determined for evolutionary change. For similar analyses, identify the selective agent and confirm it acts on heritable variation to predict shifts in population traits.
- In a bird population, beak depth shows heritable variation. During a multi-year period of drought, small soft seeds become rare and large hard seeds become common. Over 6 generations, the mean beak depth of the population increases. Which statement best explains how variation in beak depth contributed to this evolutionary pattern?
Answer: Birds with deeper beaks left more offspring when hard seeds were common, increasing alleles associated with deeper beaks.
This question tests analysis of how heritable phenotypic variation enables evolutionary change in response to environmental shifts. The bird population exhibited heritable variation in beak depth before the drought, with some individuals having deeper beaks than others due to genetic differences. During drought when only large, hard seeds were available, birds with deeper beaks could crack these seeds more efficiently, obtaining more food and energy for survival and reproduction. These deeper-beaked birds left more offspring than shallow-beaked birds, causing alleles associated with deeper beaks to increase in frequency over 6 generations. Choice B represents the misconception that phenotypic plasticity (individual changes during lifetime) equals evolution, failing to recognize that evolutionary change requires changes in allele frequencies across generations. The key principle is that natural selection acts on heritable variation—only genetic differences that affect fitness and are passed to offspring can drive evolutionary change in populations.
- In a bird population, beak depth varies and is heritable. During years with many hard seeds, birds with deeper beaks crack seeds more efficiently and produce more offspring; during years with mostly soft seeds, birds with shallow beaks produce more offspring. Across decades, the population's average beak depth fluctuates rather than steadily increasing. Which statement best explains this pattern?
Answer: Fluctuations occur because selection favors different beak depths in different years, shifting trait frequencies back and forth.
This question tests analysis of fluctuating selection maintaining variation over time. The correct answer (A) recognizes that selection alternates direction based on seed availability—deep beaks are advantageous in hard-seed years while shallow beaks are advantageous in soft-seed years, causing the population mean to shift back and forth rather than moving steadily in one direction. This temporal variation in selection pressures maintains genetic variation for beak depth because different variants are favored at different times, preventing any single optimal beak depth from fixing in the population. Answer B incorrectly suggests individual birds change beak depth seasonally and transmit these changes genetically, invoking impossible Lamarckian inheritance of acquired characteristics. When analyzing trait dynamics over time, consider whether environmental variation creates alternating selection pressures that maintain variation.
- A beetle population includes two heritable color morphs, green and brown. In a habitat of mixed vegetation, birds prey more heavily on the more visible morph in each microhabitat. Over time, both morphs persist in the overall population. Which statement best explains how variation can be maintained in this population?
Answer: Variation persists because predation favors different morphs in different microhabitats, maintaining both alleles.
This question tests understanding of how variation can be maintained in populations through balancing selection. The correct answer (A) recognizes that different color morphs have advantages in different microhabitats—green beetles are camouflaged in green vegetation while brown beetles are camouflaged in brown vegetation, so predation maintains both morphs through spatially varying selection. This frequency-dependent selection prevents either morph from being eliminated because each has higher fitness in certain conditions, maintaining genetic variation indefinitely. Answer C incorrectly suggests that selection eliminates morphs but mutations immediately restore them, misunderstanding that mutation rates are far too low to counteract strong selection. When analyzing persistent variation, look for mechanisms like balancing selection, frequency dependence, or heterozygote advantage that actively maintain multiple alleles.
- In a fish population, a gene has alleles A and a that influence tolerance to low oxygen. When a lake becomes eutrophic, low-oxygen events become frequent. Fish with genotype aa have the highest survival and produce more offspring than AA fish. After multiple generations, allele a frequency increases. Which statement best explains the role of variation in this change?
Answer: Allele a increased because differential survival and reproduction among genotypes shifted allele frequencies.
This question examines how genetic variation at a single locus enables evolutionary responses to environmental stress. The correct answer (B) identifies that fish with different genotypes (AA, Aa, aa) had different survival rates during low-oxygen events, with aa fish having the highest survival and reproductive success. This genotype-based differential survival and reproduction shifted allele frequencies in the population, increasing the frequency of allele a over multiple generations. Answer A incorrectly suggests that low oxygen caused new mutations to allele a at high rates, confusing the action of selection (which changes frequencies of existing alleles) with mutation (which creates new alleles rarely). To analyze allele frequency changes correctly, focus on how environmental conditions create fitness differences among existing genotypes rather than inducing specific mutations.
- A lizard population includes heritable variation in leg length. A new invasive predator is introduced, and lizards that can sprint faster are more likely to survive and reproduce. After several generations, the frequency of alleles associated with longer legs increases. Which statement best explains the connection between variation and evolution in this scenario?
Answer: Preexisting variation allowed some individuals to leave more offspring, increasing the frequency of long-leg alleles over generations.
This question tests understanding of how standing genetic variation in a trait enables evolutionary responses to new selective pressures. The lizard population already exhibited heritable variation in leg length before the predator arrived, with some individuals having longer legs than others due to genetic differences. When the invasive predator was introduced, lizards with longer legs could sprint faster and escape predation more successfully, leading to higher survival and reproduction rates. Over several generations, this differential fitness caused alleles associated with longer legs to increase in frequency through natural selection. Choice E represents the misconception that individuals evolve during their lifetimes and pass acquired traits to offspring, confusing individual development with population-level evolutionary change. The key insight is that populations evolve because individuals with certain pre-existing genetic variants have higher fitness and contribute more alleles to future generations, gradually shifting the population's genetic composition.
- In a bird population, beak depth varies and is heritable. During a drought, only large, hard seeds remain abundant. Birds with deeper beaks crack these seeds more efficiently and leave more surviving offspring. After four breeding seasons, the distribution of beak depth shifts toward deeper beaks. Which statement best explains how variation affected evolution in this population?
Answer: Heritable variation in beak depth led to differential reproductive success, changing allele frequencies across generations.
This question assesses the skill of analyzing variations in populations by investigating how heritable beak depth influences adaptation during environmental stress. The correct answer, C, describes how preexisting heritable variation in beak depth led to differential reproductive success, with deeper-beaked birds better at cracking hard seeds during drought, shifting allele frequencies and the trait distribution toward deeper beaks. This directional selection acts on the continuous variation, favoring extremes and resulting in evolutionary change across generations. The shift in beak depth distribution after breeding seasons confirms the role of genetic diversity in adaptation. A tempting distractor is A, which illustrates the misconception of Lamarckian inheritance through use and disuse, but natural selection operates on innate genetic variation. For similar questions, identify heritable variation and the selective mechanism to explain how populations evolve in response to challenges.
- A bacterial population contains two genotypes: one carries an antibiotic-resistance plasmid, and one does not. Before antibiotic exposure, resistant cells are 2% of the population. After treatment, the population rebounds and 85% of cells carry the plasmid. Which statement best explains how variation influenced the population's evolution?
Answer: Preexisting resistant cells survived and reproduced more, increasing the plasmid genotype frequency.
This question examines how genetic variation enables rapid evolutionary responses in bacterial populations. The correct answer (B) correctly identifies that the 2% of cells carrying resistance plasmids before treatment had a massive survival advantage during antibiotic exposure, allowing them to reproduce while non-resistant cells died. This differential survival and reproduction dramatically shifted the population composition from 2% to 85% resistant cells, demonstrating strong selection on preexisting variation. Answer A incorrectly suggests that antibiotics induced new resistance mutations in most cells, confusing the selective process (which acts on existing variation) with mutagenesis (which creates new variation rarely and randomly). Remember that antibiotics select for resistance but don't cause resistance mutations—the variation must exist before selection can act on it.
- A population of bacteria contains two variants of a membrane protein: variant X and variant Y. Before antibiotic use, 10% of cells carry variant Y. A hospital begins using an antibiotic that targets the membrane protein; after 3 months, 65% of sampled bacteria carry variant Y. The total number of bacteria sampled each month is similar, and sequencing shows no new variants appeared during the period. Which statement best explains the change in variant frequencies in the population?
Answer: Antibiotic use selected for preexisting variant Y cells that survived and reproduced more than variant X cells.
This question assesses the skill of analyzing variations in populations. The bacterial population had pre-existing variation with 10% carrying variant Y before antibiotic exposure. When antibiotics targeted the membrane protein, cells with variant Y were resistant and survived better, allowing them to reproduce and increase in frequency over time. This selection on existing variation explains the rise to 65% without new variants emerging. A tempting distractor is choice A, which reflects the misconception of directed mutation where the environment induces beneficial changes in response to need. For analyzing rapid population changes, consider how selection filters pre-existing variants rather than assuming mutations arise on demand.
- In a population of mice, coat thickness is influenced by multiple genes and shows continuous heritable variation. A region experiences colder winters for several decades. Over time, the distribution of coat thickness shifts so that thicker coats become more common in the population. Which statement best explains how variation contributed to this population-level evolutionary change?
Answer: Mice with thicker coats had higher survival and reproductive success, increasing alleles associated with thicker coats.
This question examines how continuous heritable variation enables populations to respond to gradual environmental changes. The mouse population showed heritable variation in coat thickness controlled by multiple genes, creating a range of phenotypes from thin to thick coats. When winters became colder over several decades, mice with thicker coats had better insulation, leading to higher survival rates through winter and greater reproductive success. This differential fitness caused alleles associated with thicker coats to increase in frequency over time, shifting the population distribution toward thicker coats. Choice B incorrectly suggests that acquired characteristics (growing thicker coats in response to cold) can change genes, representing the Lamarckian misconception that environmental responses alter heredity. The strategy is to recognize that for polygenic traits, natural selection gradually shifts the frequency of many alleles that contribute to advantageous phenotypes, causing the population mean to evolve over generations.
- A fish population in a river has two alleles at a locus affecting tolerance to low oxygen: H and h. In upstream pools with frequent low-oxygen events, H = 0.62; in downstream fast-flowing water, H = 0.29. Over time, a dam reduces flow downstream, and low-oxygen events become common there. Five years later, downstream H increases to 0.51. Which statement best explains the role of variation in the downstream population's change?
Answer: Fish increased H frequency because individuals with allele H survived low oxygen more often and produced more offspring.
This question assesses the skill of analyzing variations in populations. The fish population had variation in alleles H and h, affecting oxygen tolerance, with different frequencies upstream and downstream initially. After the dam reduced flow and caused low-oxygen events downstream, fish with the H allele had higher survival and reproductive rates in that habitat. Natural selection thus increased H frequency downstream by amplifying this pre-existing advantageous variant. A tempting distractor is choice A, which illustrates the misconception of environmentally induced allele switching rather than selection. When tracking allele changes across habitats, examine how selection differentially promotes existing variants that match local conditions.
- A plant population contains two pigment alleles affecting leaf color: G (green) and P (purple). In a shaded forest understory, birds eat more insects on green leaves than on purple leaves, reducing herbivory on purple plants. Over 12 generations, P increases from 0.22 to 0.47. No new alleles are detected and population size stays large. Which statement best explains how variation in leaf color affected evolution in this population?
Answer: Purple allele P rose because plants with purple leaves experienced less herbivory and produced more seeds on average.
This question assesses the skill of analyzing variations in populations. The plant population had variation in leaf color alleles G and P, resulting in green or purple phenotypes that affected herbivory rates. Purple-leaved plants (with P allele) experienced less insect damage due to birds eating more insects on green leaves, leading to higher seed production. Natural selection therefore increased P frequency over generations by favoring this pre-existing trait. A tempting distractor is choice A, which reflects the misconception of individual phenotypic change and inheritance of acquired traits. For herbivory-related evolution, analyze how selection on genetic variation reduces predation pressure and boosts fitness.
- A lizard population includes individuals with short or long hind limbs due to genetic variation. After an invasive plant increases branch density, long-limbed lizards more often fall while foraging, and short-limbed lizards leave more offspring. Over generations, the short-limb phenotype becomes more common. Which statement best explains how variation affected this outcome?
Answer: Short limbs became common because individuals with short limbs had higher reproductive success in the new habitat.
This question tests analysis of how morphological variation affects fitness in changed environments. The correct answer (B) recognizes that short-limbed lizards had higher reproductive success in the new habitat because they fell less often while foraging in dense branches, allowing them to survive better and produce more offspring than long-limbed lizards. This differential reproductive success based on limb-length variation caused the short-limb phenotype to increase in frequency over generations through natural selection. Answer E incorrectly invokes Lamarckian inheritance by suggesting limb length is acquired during life and transmitted to offspring, which violates genetic principles—phenotypic traits must be heritable through genes, not acquired characteristics. When analyzing trait frequency changes, verify that the explanation involves heritable variation and differential reproduction, not acquired characteristics.
- A coastal snail population includes shells with either thick or thin walls. A gene with two alleles influences shell thickness; heterozygotes show intermediate thickness. Before an increase in crab predators, allele frequencies are T = 0.55 and t = 0.45. After 10 years, T = 0.72 and t = 0.28, and crab abundance is high each year. No evidence of migration or mutation is found during the period. Which statement best explains how variation contributed to the change in allele frequencies?
Answer: Selection favored thicker shells, so snails with more T alleles left more offspring, raising T frequency.
This question assesses the skill of analyzing variations in populations. The snail population had pre-existing genetic variation in shell thickness alleles T and t, leading to phenotypic differences in shell wall thickness. With the increase in crab predators, natural selection favored individuals with thicker shells (higher T allele frequency), as they were less likely to be crushed and thus survived to reproduce more offspring. This differential reproductive success shifted the allele frequencies toward higher T over time, without migration or mutation altering the gene pool. A tempting distractor is choice A, which embodies the misconception that environmental pressures cause directed mutations or acquired changes passed to offspring. When examining allele frequency shifts, focus on how selection amplifies advantageous pre-existing variants rather than assuming induced adaptations.
- A fish population has genetic variation at a locus affecting toxin resistance: allele R (resistant) and r (nonresistant). A factory begins releasing a toxin into the river, and within 8 generations the frequency of R increases from 0.10 to 0.46. Researchers find no evidence of increased mutation rate at this locus. Which statement best explains how genetic variation enabled the population-level change?
Answer: Individuals carrying allele R produced more surviving offspring, so the R allele increased in frequency over generations.
This question examines how genetic variation present in a population before an environmental change enables evolutionary adaptation. The fish population already contained both R (resistant) and r (non-resistant) alleles at low frequency before the factory began releasing toxins. When toxins entered the river, fish carrying the R allele survived and reproduced at much higher rates than those with the r allele, leading to differential reproductive success. This natural selection process caused the R allele frequency to increase from 0.10 to 0.46 in just 8 generations, demonstrating rapid evolution. Choice A incorrectly suggests that environmental exposure can cause directed mutations, a common misconception that confuses the random nature of mutation with the non-random process of selection. The strategy is to recognize that selection acts on existing variation—the R allele was already present at low frequency and increased because R-carriers had higher fitness, not because new mutations arose in response to the toxin.
- A population of mice has two alleles for coat color, and both alleles are present at the start of a long-term study. When the habitat shifts from light sand to dark volcanic rock, owls capture light-coated mice more often than dark-coated mice. Over ten generations, the frequency of the dark-coat allele increases. Which statement best explains how variation affected evolutionary processes in this population?
Answer: Existing coat-color variation led to differential survival, changing allele frequencies through natural selection.
This question assesses the skill of analyzing variations in populations by exploring how coat-color alleles respond to habitat changes and predation. The correct answer, B, explains that preexisting variation in coat color resulted in differential survival, with dark-coated mice better camouflaged on volcanic rock, leading to natural selection that increased the dark allele's frequency over generations. This adaptive shift demonstrates how environmental changes can act on genetic diversity to drive evolution without new mutations. The presence of both alleles initially and the gradual frequency change confirm the role of heritable variation in the process. A tempting distractor is A, which reflects the misconception of environmentally induced development, suggesting habitat caused color changes within lifetimes, but selection acts on genotypes. To solve comparable problems, determine if the trait is heritable and how selection pressures on variation lead to allele frequency changes.
