AP Biology Quiz: Variations In Populations

What this quiz covers

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.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

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

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

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

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

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

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

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 your ability to analyze how population variation leads to evolutionary change through natural selection. The correct answer (C) recognizes that preexisting genetic variation (alleles B and b) created differential survival when environmental conditions changed—dark-shelled snails with allele B were less visible to bird predators on dark sediment, so they survived and reproduced more than light-shelled snails with allele b. This differential reproduction increased the frequency of allele B from 0.60 to 0.78, demonstrating natural selection acting on existing variation. Answer A incorrectly suggests Lamarckian inheritance where individuals change traits during their lifetime and pass those acquired changes to offspring, which violates our understanding of genetics. When analyzing population changes, always check whether the explanation involves selection acting on preexisting variation (correct) versus individuals acquiring and passing on new traits (incorrect).

Question 8

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 9

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 10

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 11

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 12

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 13

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 14

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 15

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 16

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 17

1. Unbanded snails had higher fitness under predation, increasing the frequency of allele m in the population. (correct answer)
2. Banded snails removed their bands to avoid predators, which changed allele M into allele m.
3. The predator caused new unbanded mutations to arise in all banded snails, increasing m frequency immediately.
4. Allele m increased because individual snails decided to reproduce less when predators were present.
5. The change in m frequency cannot be due to selection because selection does not act on phenotypes.

Explanation: This question tests understanding of how genetic variation in defensive traits enables evolutionary responses to predation pressure. The snail population contained both M (banded) and m (unbanded) alleles before the new predator arrived, providing variation in shell appearance. When the visual predator was introduced, banded snails were more easily detected against the substrate and suffered higher predation rates than unbanded snails. This differential survival led to unbanded snails producing more offspring, causing the m allele frequency to increase over several generations through natural selection. Choice B represents the misconception that individuals can change their genes in response to threats, incorrectly suggesting that snails could convert M alleles to m alleles during their lifetime. The strategy is to recognize that predation acts as a selective force on existing variation—individuals with traits that reduce predation risk leave more offspring, shifting allele frequencies in the population over time.

Question 18

1. Plants with allele L changed their DNA to allele E after experiencing fewer pollinator visits in one season.
2. Earlier flowering individuals produced more seeds when pollinators peaked earlier, increasing allele E frequency over generations. (correct answer)
3. The population increased allele E because plants collectively adjusted flowering time to match pollinators, independent of heredity.
4. Allele E increased because pollinators preferentially visited plants at random, causing a nonselective allele frequency shift.
5. The allele frequency change occurred because every plant flowered earlier, so allele frequencies stayed constant.

Explanation: This question requires analyzing how genetic variation in timing traits enables populations to track environmental changes. The plant population already contained both E (early flowering) and L (late flowering) alleles before the climate shift, providing variation in flowering time. When pollinators peaked earlier due to climate change, plants with allele E flowered during peak pollinator abundance and received more pollinator visits, resulting in higher seed production. Plants with allele L flowered after peak pollinator activity and produced fewer seeds, creating differential reproductive success. Over 12 generations, this fitness difference caused the E allele to increase from 0.35 to 0.67 through natural selection. Choice A incorrectly suggests that plants can change their DNA in response to environmental cues, representing the misconception that organisms can alter their genes based on need. The key principle is that populations evolve when individuals with certain alleles have higher reproductive success, gradually increasing those alleles' frequencies across generations.

Question 19

1. The antibiotic caused each bacterium to acquire allele X because survival required higher resistance.
2. Bacteria with allele X had higher reproductive success under antibiotic exposure, increasing X frequency in the population. (correct answer)
3. Allele X increased because bacteria intentionally exchanged genes to raise the population's resistance level.
4. The population became resistant because individuals with allele x adapted by changing x into X during their lifetimes.
5. Allele frequencies changed mainly because antibiotic exposure prevents natural selection from acting on variation.

Explanation: 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.

Question 20

1. Crabs caused snails to grow thicker shells during attacks, and the increased thickness was inherited by offspring.
2. Variation in shell thickness produced differences in survival, increasing the frequency of alleles for thicker shells. (correct answer)
3. The predator introduction increased gene flow from other populations, which always raises the frequency of thick shells.
4. Thick shells became common because snails with thin shells stopped reproducing to conserve energy for growth.
5. Shell thickness changed because all snails responded similarly to crabs, so allele frequencies remained unchanged.

Explanation: This question assesses the skill of analyzing variations in populations by examining how genetic diversity in shell thickness evolves under predatory pressure. The correct answer, B, highlights that variation in shell thickness created survival differences, with thicker-shelled snails more resistant to crabs, resulting in natural selection that increased the frequency of thick-shell alleles over time. Breeding experiments confirming heritability underscore that the change was evolutionary, driven by differential reproduction of preexisting variants. The increased proportion of thick-shelled snails after years illustrates adaptive evolution without the need for new genetic inputs. A tempting distractor is A, which represents the misconception of induced acquired traits, implying attacks caused shell thickening that was inherited, but selection requires heritable variation from the outset. A transferable approach is to evaluate evolutionary scenarios by checking for heritable variation and selective advantages that alter population genetics.