AP Biology Quiz: Variations In Populations
20 questions · exam conditions
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Variations In PopulationsQuestion 1 of 20

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?

Allele D increased because the pesticide caused most insects to mutate from d to D during exposure.
Allele D increased because insects with allele D had higher survival and reproduction, shifting allele frequencies.
Allele D increased because insects without D left the population by choice, reducing d allele representation.
Allele D increased because all genotypes had the same fitness, so allele frequencies changed directionally.
Allele D increased because acquiring pesticide tolerance during life is inherited, raising D frequency in offspring.
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AP Biology Quiz

AP Biology Quiz: Variations In Populations

Practice Variations In Populations in AP Biology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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

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Question 1

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?

  1. Allele D increased because the pesticide caused most insects to mutate from d to D during exposure.
  2. Allele D increased because insects with allele D had higher survival and reproduction, shifting allele frequencies. (correct answer)
  3. Allele D increased because insects without D left the population by choice, reducing d allele representation.
  4. Allele D increased because all genotypes had the same fitness, so allele frequencies changed directionally.
  5. Allele D increased because acquiring pesticide tolerance during life is inherited, raising D frequency in offspring.
Explanation: 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.

Question 2

In a population of field mice, fur color is controlled by two alleles. Before a wildfire, 52% of mice are light brown (genotypes BB or Bb) and 48% are dark brown (bb). After the wildfire, the habitat is mostly blackened soil. Over the next 6 generations, surveys show dark-brown mice increase to 76% while total population size remains similar. No new alleles are detected, and mating remains random within the population. Which statement best explains the observed change in the population over generations?

  1. Dark-brown individuals acquired darker fur after the fire and passed that change to offspring.
  2. Natural selection increased the frequency of the b allele because dark fur improved survival on burned soil. (correct answer)
  3. Random mating caused the b allele to become more common because it is a recessive allele.
  4. Gene flow from a nearby population introduced many new b alleles into the mice after the fire.
  5. The population needed camouflage, so more offspring developed dark fur regardless of genotype.
Explanation: This question assesses the skill of analyzing variations in populations. The population of field mice initially had genetic variation in fur color alleles, with both light and dark phenotypes present before the wildfire. After the habitat changed to blackened soil, natural selection acted on this existing variation by favoring dark-brown mice (bb genotype), which were better camouflaged and thus had higher survival rates against predators. As a result, these individuals reproduced more successfully, increasing the frequency of the b allele over generations without new alleles or non-random mating. A tempting distractor is choice A, which reflects the misconception of Lamarckian inheritance where acquired traits are passed to offspring. To analyze evolutionary changes in populations, always evaluate how natural selection acts on pre-existing genetic variation in response to environmental pressures.

Question 3

A bacterial population contains two heritable variants: one produces an enzyme that breaks down antibiotic X, and the other does not. When antibiotic X is added to the culture, most non-enzyme bacteria die, but some enzyme-producing bacteria survive and reproduce. After several days, the culture is dominated by enzyme-producing bacteria. Which statement best explains how variation affected evolution in this population?

  1. Antibiotic X caused all bacteria to mutate into enzyme producers, so the population changed without selection.
  2. Enzyme producers survived and reproduced more, increasing the frequency of the enzyme allele in the population. (correct answer)
  3. Non-enzyme bacteria chose to stop dividing, allowing enzyme producers to become common without genetic change.
  4. The enzyme trait spread because bacteria needed it, so the population produced the enzyme in response.
  5. Because bacteria reproduce asexually, heritable variation cannot influence changes in allele frequencies over time.
Explanation: This question assesses the skill of analyzing variations in populations by examining how genetic variants in bacteria respond to selective pressures like antibiotics. The correct answer, B, describes how the preexisting enzyme-producing variant had a survival advantage in the presence of antibiotic X, allowing those bacteria to reproduce more and increase the frequency of the enzyme allele in the population over time. This demonstrates natural selection in a microbial context, where variation in a heritable trait leads to differential reproductive success without the need for new mutations during the experiment. The rapid dominance of enzyme producers underscores how strong selection can quickly alter population composition based on existing genetic diversity. A tempting distractor is A, which reflects the misconception of directed mutations, implying the antibiotic induced changes in all bacteria, but selection acts on variation already present. To approach similar questions, evaluate whether the change results from selection on heritable variation or from environmental induction of traits in individuals.

Question 4

A lizard population includes individuals that differ genetically in sprint speed. During a period of increased predation, faster lizards are less likely to be captured and produce more offspring. After several generations, the mean sprint speed of hatchlings increases compared with the original population. Which statement best explains how variation contributed to evolution in this population?

  1. Predators increased sprint speed in all lizards equally, so the population changed without differences in survival.
  2. Faster lizards had higher reproductive success, increasing the frequency of alleles associated with high speed. (correct answer)
  3. Slower lizards trained to run faster during their lifetimes, and their offspring inherited the learned speed.
  4. Mean sprint speed rose because predators removed the slowest individuals, but allele frequencies stayed constant.
  5. Sprint-speed variation decreased fitness differences, preventing selection from changing the population over time.
Explanation: This question assesses the skill of analyzing variations in populations by investigating how genetic differences in sprint speed influence survival and evolutionary shifts under predation. The correct answer, B, indicates that faster lizards, due to preexisting genetic variation, had higher survival and reproductive success, leading to an increase in alleles for high speed and a higher mean sprint speed in hatchlings. This exemplifies directional selection, where the predator pressure favored one end of the speed variation spectrum, altering allele frequencies across generations. The heritable nature of sprint speed ensures the change is evolutionary, passed on to offspring. A tempting distractor is C, which embodies the misconception of use-and-disuse inheritance, implying learned behaviors are inherited, but evolution requires selection on genetic traits. A key strategy is to assess if variation is heritable and if environmental pressures create differential reproduction, enabling prediction of population changes.

Question 5

A coastal grass population contains individuals with either high or low salt tolerance due to different alleles at one gene. After a storm surge, soil salinity increases and remains high for several years. In the first year, survival is 70% for high-tolerance plants and 30% for low-tolerance plants. By year five, seedlings show a higher proportion of high-tolerance phenotypes than before the surge. Which statement best explains how preexisting variation contributed to evolutionary change in this population?

  1. High salinity increased survival of plants with high tolerance, raising their allele frequency in later generations. (correct answer)
  2. Individual plants developed higher salt tolerance during the surge and passed the acquired tolerance to offspring.
  3. The storm surge created salt-tolerance alleles in all plants at the same rate, so frequencies stayed constant.
  4. Low-tolerance plants migrated inland, so the original population evolved without any selection on tolerance.
  5. Because both phenotypes survived each year, natural selection could not change allele frequencies over time.
Explanation: This question assesses the skill of analyzing variations in populations by exploring how preexisting genetic differences affect survival and allele frequencies under environmental pressure. The correct answer, A, explains that high salinity selected for plants with preexisting high-tolerance alleles, as they survived and reproduced more, gradually increasing the frequency of those alleles in subsequent generations. This illustrates natural selection acting on heritable variation, where the storm surge created a selective pressure that favored one variant over the other without introducing new mutations. The persistence of both phenotypes initially and the shift in seedling proportions confirm that the change was evolutionary, driven by differential success based on genetic variation. A tempting distractor is B, which embodies the misconception of inheritance of acquired characteristics, implying individuals adapt during their lifetime and pass on those changes, but traits must be heritable from the start for selection to occur. For transferable strategy, when evaluating population changes, distinguish between selection on existing genetic variation and non-evolutionary mechanisms like phenotypic plasticity or migration.

Question 6

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?

  1. Fluctuations occur because selection favors different beak depths in different years, shifting trait frequencies back and forth. (correct answer)
  2. Fluctuations occur because individual birds change beak depth each season and transmit those changes genetically.
  3. Fluctuations occur because beak depth is not heritable, so allele frequencies cannot change across generations.
  4. Fluctuations occur because mutation always produces shallow-beak alleles after hard-seed years and deep-beak alleles after soft-seed years.
  5. Fluctuations occur because all beak depths have identical fitness, so the mean trait value oscillates predictably.
Explanation: 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.

Question 7

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?

  1. Antibiotics induced resistance mutations in most cells, so plasmids became common after treatment.
  2. Preexisting resistant cells survived and reproduced more, increasing the plasmid genotype frequency. (correct answer)
  3. All cells were equally resistant, so plasmid frequency increased due to uniform survival and reproduction.
  4. Nonresistant cells chose to acquire plasmids during exposure, raising resistance across the population.
  5. Antibiotics reduced reproduction in resistant cells, so plasmid frequency increased by compensatory drift.
Explanation: 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.

Question 8

A population of mice shows genetic variation at a coat-color locus. In a small isolated valley, a wildfire reduces the population to 12 survivors with a higher-than-average frequency of the dark-coat allele. Several generations later, the dark-coat allele remains common even though predation rates are similar across coat colors. Which statement best explains this change in allele frequency?

  1. The dark allele stayed common because survivors were a nonrandom sample, shifting allele frequencies by chance. (correct answer)
  2. The dark allele stayed common because mice with light coats chose to avoid mating, lowering their allele frequency.
  3. The dark allele stayed common because wildfire conditions directly converted light alleles into dark alleles.
  4. The dark allele stayed common because the population needed darker coats, so selection increased the allele.
  5. The dark allele stayed common because equal fitness always drives allele frequencies toward 100% for one allele.
Explanation: This question examines how genetic drift affects allele frequencies in small populations. The correct answer (A) correctly identifies that the 12 survivors represented a nonrandom genetic sample of the original population—by chance, they happened to have a higher frequency of the dark-coat allele than the pre-fire population. This founder effect or bottleneck changed allele frequencies through random sampling rather than selection, and the new frequency persisted because predation rates were similar across coat colors (no selection). Answer D incorrectly invokes teleological thinking by suggesting the population "needed" darker coats so selection increased the allele, when actually the change was due to random sampling. To distinguish drift from selection, check whether fitness differences exist—if not, frequency changes in small populations likely result from drift.

Question 9

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?

  1. Purple plants attracted birds, so individual green plants changed to purple and passed that trait to offspring.
  2. Purple allele P rose because plants with purple leaves experienced less herbivory and produced more seeds on average. (correct answer)
  3. Allele P rose because dominance causes alleles to increase even when they reduce fitness.
  4. Allele P rose because random drift is stronger in large populations and rapidly changes allele frequencies.
  5. Allele P rose because plants selected mates with purple leaves, which creates new alleles in offspring.
Explanation: 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.

Question 10

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?

  1. Exposure to the antibiotic caused bacteria to mutate into variant Y in order to survive.
  2. Antibiotic use selected for preexisting variant Y cells that survived and reproduced more than variant X cells. (correct answer)
  3. Variant Y increased because bacteria with variant X stopped reproducing when antibiotics were present.
  4. Variant frequencies changed because bacteria can choose which membrane protein variant to express each generation.
  5. Genetic drift increased variant Y because large populations always change rapidly in allele frequency.
Explanation: 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.

Question 11

In a population of wild rabbits, a virus spreads that causes high mortality. A gene has two alleles, R and r, associated with resistance; rabbits with genotype RR have the highest survival, Rr intermediate, rr lowest. Before the outbreak, R = 0.40. Two years later, R = 0.58, and the virus remains present. No migration is detected. Which statement best explains why allele frequencies changed in the population?

  1. The virus induced resistant mutations in infected rabbits, converting r alleles into R alleles.
  2. R increased because rabbits with more R alleles survived infection more often and contributed more offspring. (correct answer)
  3. R increased because rabbits with genotype rr learned to avoid the virus and then produced resistant offspring.
  4. R increased because allele frequencies always move toward 0.50 during disease outbreaks.
  5. R increased because the population's need for resistance caused more R alleles to appear during reproduction.
Explanation: This question assesses the skill of analyzing variations in populations. The rabbit population possessed variation in resistance alleles R and r before the virus outbreak, influencing survival rates. Rabbits with more R alleles (especially RR) survived infection better and produced more offspring, leading to an increase in R frequency. This demonstrates natural selection acting on existing genetic variation to favor resistant genotypes. A tempting distractor is choice A, which represents the misconception that pathogens directly induce beneficial mutations in hosts. To explain disease-driven evolution, focus on how selection enhances the frequency of pre-existing resistance variants in populations.

Question 12

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?

  1. Pollinators caused red alleles to form in each generation, so red flowers increased without inherited variation.
  2. Color variation led to differential pollination success, increasing the frequency of alleles for red petals over time. (correct answer)
  3. Plants changed petal color to red after being visited, and offspring inherited the changed color.
  4. Because multiple colors remained present, allele frequencies could not change across generations.
  5. Red flowers became common because individual plants selected mates with red petals, not because of pollinators.
Explanation: 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.

Question 13

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?

  1. The habitat change caused light-coated mice to become dark during development, increasing dark allele frequency.
  2. Existing coat-color variation led to differential survival, changing allele frequencies through natural selection. (correct answer)
  3. Owls preferentially removed light mice, which increased dark allele frequency but did not involve heritable traits.
  4. The dark allele increased because it was needed in the new habitat, so it became more common in each litter.
  5. Because both alleles were present initially, the population could not experience any evolutionary change in color.
Explanation: 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.

Question 14

A rabbit population shows heritable variation in fur thickness: thin, medium, and thick. A series of unusually cold winters occurs. Field data show that rabbits with thick fur have higher overwinter survival and contribute more offspring to the next spring's population. After six winters, the average fur thickness in newborn rabbits increases. Which statement best explains how variation influenced evolution in this population?

  1. Cold winters increased survival of thicker-furred rabbits, shifting allele frequencies toward thicker fur in offspring. (correct answer)
  2. Individual rabbits grew thicker fur during winter and passed that acquired thickness directly to their offspring.
  3. The winters caused random changes in fur alleles that were unrelated to survival differences among rabbits.
  4. Rabbits with thin fur moved to warmer areas, so the remaining rabbits became thicker without selection.
  5. Because fur thickness is a phenotype, it cannot reflect underlying genetic variation that selection can act on.
Explanation: This question assesses the skill of analyzing variations in populations by exploring how heritable traits like fur thickness evolve under climatic pressures. The correct answer, A, explains that cold winters favored rabbits with thicker fur due to higher survival and reproduction, leading to a shift in allele frequencies toward thicker fur in offspring over generations. This process relies on preexisting genetic variation in fur thickness, allowing natural selection to increase the average phenotype without individuals acquiring thicker fur during their lifetimes. The increase in newborn fur thickness confirms the evolutionary change was driven by differential success of heritable variants. A tempting distractor is B, which illustrates the misconception of Lamarckian evolution, where acquired traits are inherited, but selection operates on genetic variation present at birth. A transferable strategy is to trace evolutionary changes back to heritable variation and selection, distinguishing them from non-genetic adaptations or random events.

Question 15

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?

  1. Mice with thicker coats had higher survival and reproductive success, increasing alleles associated with thicker coats. (correct answer)
  2. Colder winters caused each mouse to grow a thicker coat, and this acquired trait changed the population's genes.
  3. The shift occurred because the environment directly increased mutation rates only in genes for coat thickness.
  4. The population changed because mice with thin coats chose warmer shelters, preventing selection on coat thickness.
  5. The change shows that coat thickness variation is irrelevant because all individuals experience the same winter temperatures.
Explanation: 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.

Question 16

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?

  1. Exposure to toxin caused r alleles to mutate into R in most exposed fish, increasing R frequency.
  2. Fish developed resistance as a needed response, and this response was inherited as allele R.
  3. Individuals carrying allele R produced more surviving offspring, so the R allele increased in frequency over generations. (correct answer)
  4. Genetic drift is the best explanation because selection cannot change allele frequencies in fewer than 10 generations.
  5. The increase in R frequency occurred because all fish had equal fitness, keeping allele frequencies balanced.
Explanation: 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.

Question 17

A population of insects includes two heritable color morphs controlled by alleles G (green) and T (tan). In a drought year, grasses dry and become tan. After the drought, the frequency of T rises from 0.30 to 0.62 over several generations, while the population size remains large. Field data show tan individuals leave more offspring than green individuals during drought conditions. Which statement best explains how variation affected evolution in this population?

  1. The environment caused green alleles to convert into tan alleles within individual insects.
  2. Tan insects learned to avoid predators, and this learned behavior increased the T allele frequency.
  3. Natural selection increased the T allele frequency because tan individuals had higher reproductive success during drought. (correct answer)
  4. The T allele increased because all insects needed tan coloration, so the population produced more T alleles.
  5. Genetic drift is the most likely cause because large populations change allele frequencies rapidly by chance.
Explanation: This question requires analyzing how pre-existing genetic variation allows populations to respond to environmental changes through natural selection. The insect population already possessed both G (green) and T (tan) alleles before the drought, providing variation for selection to act upon. During drought conditions when grasses turned tan, tan insects had better camouflage from predators, resulting in higher survival and reproductive success compared to green insects. This differential fitness caused the T allele frequency to increase dramatically from 0.30 to 0.62 over several generations. Choice D incorrectly suggests that organisms can produce alleles based on need, representing the common misconception that evolution is goal-directed or that organisms can will genetic changes. The strategy here is to identify that natural selection changes allele frequencies by favoring individuals with advantageous traits who leave more offspring, not by creating new variations or through individual adaptation.

Question 18

A coastal plant population includes individuals with either high leaf salt-gland density or low salt-gland density. A storm surge increases soil salinity for several growing seasons. Plants with high gland density have higher survival and set more seeds than plants with low gland density under these conditions. In the next generation, the proportion of seedlings with high gland density increases. Genetic analyses show both phenotypes were present before the storm surge.

Which statement best explains the role of variation in this evolutionary change?

  1. The salt surge induced high gland density in all plants, and the new phenotype was inherited.
  2. Existing variation in gland density led to differential reproductive success, changing trait frequencies in the population. (correct answer)
  3. Low gland density plants increased gland density by using more salt, so their offspring had higher density.
  4. The population increased gland density because individuals needed to remove salt to survive.
  5. Random mating alone increased high gland density without any differences in survival or seed set.
Explanation: This question requires analyzing how existing variation in populations enables evolutionary responses to environmental change. The correct answer (B) recognizes that genetic variation in salt-gland density was already present before the storm surge, and this variation led to differential reproductive success when salinity increased—plants with high gland density survived better and produced more seeds. This differential reproduction changed the trait frequencies in the next generation, with more seedlings having high gland density. Answer A incorrectly suggests the salt induced the trait in all plants (acquired characteristic), but the question explicitly states both phenotypes existed before the storm. To solve variation problems, always check whether the variation was preexisting and heritable, then trace how differential survival or reproduction changes population frequencies.

Question 19

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?

  1. Antibiotic exposure caused all cells to mutate into resistant cells, increasing resistance frequency.
  2. Some cells already had resistance, and selection increased their contribution to the next generations. (correct answer)
  3. Plasmid-free cells chose to acquire plasmids, so resistance increased through individual behavior.
  4. Resistance became common because the population needed it, regardless of initial variation.
  5. Genetic drift alone explains the rapid increase, because survival was equal with and without antibiotic.
Explanation: 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.

Question 20

In a large coastal snail population, shell color is controlled by a single gene with two alleles: snails with genotype BBBB or BbBb have dark shells, and snails with genotype bbbb have light shells. Before a storm, allele BB frequency is 0.60 and allele bb is 0.40. The storm deposits dark sediment, after which birds more easily spot and eat light-shelled snails, so dark-shelled snails are more likely to survive and reproduce. No snails migrate into or out of the population. Five generations after the storm, allele BB frequency is 0.78. Which statement best explains how variation affected this change in allele frequency across generations?

  1. Individuals changed shell color during their lifetime, then passed the new color allele to offspring.
  2. Random mating after the storm created new alleles for dark shells, increasing allele B frequency.
  3. Preexisting color alleles caused differential survival and reproduction, increasing the frequency of allele B. (correct answer)
  4. The storm directly mutated most b alleles into B alleles, producing a rapid adaptive shift.
  5. All snails benefited equally from dark sediment, so allele frequencies changed without selection.
Explanation: This question tests how natural selection acting on preexisting heritable variation changes allele frequencies. Before the storm, the population already contained both alleles. Because BBBB and BbBb snails have dark shells, they are camouflaged on dark sediment, while light-shelled bbbb snails are more easily found by birds. Dark-shelled snails therefore survive and reproduce at higher rates, and they pass BB alleles to their offspring. Over several generations, each round of differential survival and reproduction raises the frequency of BB, from 0.60 to 0.78. Because the population is large and closed to migration, drift and gene flow are unlikely explanations, so choice C is correct. Choice A describes inheritance of acquired characteristics; shell color is determined by inherited alleles, not changed by individuals during their lifetimes. Choice B is incorrect because mating recombines existing alleles but does not create new ones. Choice D is incorrect because the storm did not cause mutations; selection acted on alleles that were already present. Choice E is incorrect because light-shelled snails were eaten more often, which is selection. When analyzing population change, check whether the explanation involves selection acting on preexisting heritable variation rather than individuals acquiring new traits.