Variations in Populations
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AP Biology › Variations in Populations
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 without D left the population by choice, reducing d allele representation.
Allele D increased because acquiring pesticide tolerance during life is inherited, raising D frequency in offspring.
Allele D increased because insects with allele D had higher survival and reproduction, shifting allele frequencies.
Allele D increased because all genotypes had the same fitness, so allele frequencies changed directionally.
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.
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?
Gene flow from a nearby population introduced many new b alleles into the mice after the fire.
Dark-brown individuals acquired darker fur after the fire and passed that change to offspring.
Natural selection increased the frequency of the b allele because dark fur improved survival on burned soil.
The population needed camouflage, so more offspring developed dark fur regardless of genotype.
Random mating caused the b allele to become more common because it is a recessive allele.
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.
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?
Enzyme producers survived and reproduced more, increasing the frequency of the enzyme allele in the population.
Antibiotic X caused all bacteria to mutate into enzyme producers, so the population changed without selection.
Non-enzyme bacteria chose to stop dividing, allowing enzyme producers to become common without genetic change.
The enzyme trait spread because bacteria needed it, so the population produced the enzyme in response.
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.
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?
Sprint-speed variation decreased fitness differences, preventing selection from changing the population over time.
Slower lizards trained to run faster during their lifetimes, and their offspring inherited the learned speed.
Faster lizards had higher reproductive success, increasing the frequency of alleles associated with high speed.
Predators increased sprint speed in all lizards equally, so the population changed without differences in survival.
Mean sprint speed rose because predators removed the slowest individuals, but allele frequencies stayed constant.
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.
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?
Low-tolerance plants migrated inland, so the original population evolved without any selection on tolerance.
Individual plants developed higher salt tolerance during the surge and passed the acquired tolerance to offspring.
Because both phenotypes survived each year, natural selection could not change allele frequencies over time.
The storm surge created salt-tolerance alleles in all plants at the same rate, so frequencies stayed constant.
High salinity increased survival of plants with high tolerance, raising their allele frequency in later generations.
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.
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?
Fluctuations occur because selection favors different beak depths in different years, shifting trait frequencies back and forth.
Fluctuations occur because individual birds change beak depth each season and transmit those changes genetically.
Fluctuations occur because beak depth is not heritable, so allele frequencies cannot change across generations.
Fluctuations occur because mutation always produces shallow-beak alleles after hard-seed years and deep-beak alleles after soft-seed years.
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.
In a coastal snail population, shell color is controlled by two alleles. Before a storm, allele B frequency is 0.60 and allele b is 0.40. After the storm deposits dark sediment, birds more easily spot light shells; in the next generation, allele B frequency is 0.78. Which statement best explains how variation affected this change in allele frequency across generations?
The storm directly mutated most b alleles into B alleles, producing a rapid adaptive shift.
Individuals changed shell color during their lifetime, then passed the new color allele to offspring.
Random mating after the storm created new alleles for dark shells, increasing allele B frequency.
Preexisting color alleles caused differential survival and reproduction, increasing the frequency of allele B.
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).
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?
Antibiotics reduced reproduction in resistant cells, so plasmid frequency increased by compensatory drift.
Preexisting resistant cells survived and reproduced more, increasing the plasmid genotype frequency.
Antibiotics induced resistance mutations in most cells, so plasmids became common after treatment.
Nonresistant cells chose to acquire plasmids during exposure, raising resistance across the population.
All cells were equally resistant, so plasmid frequency increased due to uniform survival and reproduction.
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.
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?
The dark allele stayed common because survivors were a nonrandom sample, shifting allele frequencies by chance.
The dark allele stayed common because the population needed darker coats, so selection increased the allele.
The dark allele stayed common because equal fitness always drives allele frequencies toward 100% for one allele.
The dark allele stayed common because wildfire conditions directly converted light alleles into dark alleles.
The dark allele stayed common because mice with light coats chose to avoid mating, lowering their allele frequency.
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.
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?
Allele P rose because dominance causes alleles to increase even when they reduce fitness.
Purple allele P rose because plants with purple leaves experienced less herbivory and produced more seeds on average.
Allele P rose because plants selected mates with purple leaves, which creates new alleles in offspring.
Purple plants attracted birds, so individual green plants changed to purple and passed that trait to offspring.
Allele P rose because random drift is stronger in large populations and rapidly changes allele frequencies.
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.