AP Biology Quiz: Natural Selection Continued

What this quiz covers

This quiz focuses on Natural Selection Continued, 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. Directional selection shifting the mean toward deeper beaks as a single extreme is favored
2. Disruptive selection increasing the frequency of both shallow and deep beaks
3. Stabilizing selection reducing extremes while maintaining a similar mean beak depth (correct answer)
4. Genetic drift increasing extremes due to random mating in a large population
5. Sexual selection causing beak depth to track mate choice rather than survival differences

Explanation: This question assesses the skill of analyzing patterns of natural selection by interpreting population-level changes in traits and allele frequencies. With consistent rainfall leading to mostly medium-hard seeds, birds with intermediate beak depths have the highest reproductive success, reducing the proportion of extreme beak depths over generations. The population mean beak depth remains stable, but variance decreases as extremes are selected against, illustrating how selection maintains an optimal intermediate trait value. This matches stabilizing selection, where variation is reduced around the mean without shifting it, as seen in the decline of shallow and deep beaks. A tempting distractor is directional selection, which is wrong because it would shift the mean toward one extreme rather than preserving it, stemming from a misconception that any fitness difference implies a directional shift. For similar problems, evaluate whether the mean trait value changes or stays constant while checking if variance increases, decreases, or becomes bimodal.

Question 2

1. Directional selection favoring taller stems, increasing the mean stem height each generation
2. Genetic drift changing stem-height allele frequencies randomly due to chance events
3. Stabilizing selection favoring intermediate stems, reducing extremes while maintaining the mean (correct answer)
4. Disruptive selection favoring short and tall stems, increasing variance and producing two peaks
5. Phenotypic plasticity alone changing stem height without any change in allele frequencies

Explanation: This question tests your ability to analyze natural selection patterns by examining fitness differences across a phenotype range. The scenario describes tall stems being eaten by grazers and short stems being outcompeted for light, with intermediate heights producing the most surviving seeds, leading to little mean change but decreased frequency of extremes. This perfectly matches stabilizing selection, where intermediate phenotypes have optimal fitness between competing selective pressures, reducing variation around the mean. Choice D incorrectly suggests disruptive selection, but that would favor both short and tall stems over intermediates, creating a bimodal distribution rather than the observed concentration around intermediate values. When you see selection favoring intermediates with reduced extreme frequencies and stable mean, recognize it as stabilizing selection.

Question 3

1. Directional selection favoring longer spines, shifting the mean upward across generations
2. Disruptive selection favoring both extremes, increasing variance and producing two peaks
3. Stabilizing selection favoring intermediate spines, reducing variance while maintaining the mean (correct answer)
4. Genetic drift changing spine length randomly because fitness differences were absent
5. Gene flow increasing intermediate spines because migrants reproduced more than residents

Explanation: This question assesses the skill of analyzing patterns of natural selection by evaluating how predation influences trait variance and mean over generations. Intermediate spine lengths had higher fitness (2.3 offspring) than short (1.0) or long (1.1), resulting in decreased variance while the mean remained stable. This pattern indicates stabilizing selection, as predators preferentially consumed extremes, favoring the intermediate optimum and narrowing the distribution. Over 25 generations, the population converged on the most adaptive spine length for evasion. A tempting distractor is choice B, disruptive selection, which is incorrect because it would increase variance by favoring extremes, but here variance fell, due to the misconception that predation on extremes always disrupts. A key strategy is to check if selection preserves the mean and reduces spread, pointing to stabilization in similar datasets.

Question 4

1. Directional selection favoring darker shells, increasing the l allele frequency over generations (correct answer)
2. Stabilizing selection maintaining intermediate shell colors and reducing phenotypic variance
3. Disruptive selection favoring both light and dark shells while decreasing intermediate phenotypes
4. Genetic drift causing random allele-frequency change unrelated to differences in reproductive success
5. Balancing selection maintaining both alleles at equal frequencies because all genotypes reproduce equally

Explanation: This question assesses the skill of analyzing patterns of natural selection by interpreting data on allele frequencies and reproductive success in response to environmental pressures. The data show that dark-shelled snails had higher reproductive success (2.0 offspring) compared to light-shelled ones (1.1), leading to an increase in the l allele frequency from 0.40 to 0.62 over generations. This shift indicates directional selection favoring darker shells, as the population's phenotype moved toward the advantageous dark trait due to predation by crabs. The trend of increasing frequency of the darker allele aligns with a consistent push in one direction, rather than maintaining or splitting the distribution. A tempting distractor is choice C, disruptive selection, which is wrong because it would favor both extremes and increase variance, but here only one extreme (dark) is favored, stemming from the misconception that any change in extremes implies disruption. To identify selection types in similar problems, examine how fitness differences correlate with shifts in mean trait values and allele frequencies over time.

Question 5

1. Directional selection increasing mean mass because larger hatchlings always survive best
2. Disruptive selection widening the distribution by favoring both small and large hatchlings
3. Stabilizing selection reducing variance by favoring intermediate hatchling mass over generations (correct answer)
4. Genetic drift narrowing the distribution because survival is unrelated to mass
5. Individuals regulating mass during development so the population evolves without selection

Explanation: This question tests your ability to analyze natural selection patterns by examining changes in trait distribution characteristics. Field data show that intermediate-mass hatchlings survive to reproduce more often than very small or very large hatchlings, and after multiple generations, the mean remains similar but the distribution becomes narrower. This narrowing of the distribution while maintaining the mean is characteristic of stabilizing selection, which eliminates extreme phenotypes by favoring intermediates each generation. Students often confuse this with genetic drift (D) because both can narrow distributions, but drift acts randomly while the data shows systematic survival differences based on mass. When analyzing selection patterns, focus on both the mean and variance: if intermediates have highest fitness and variance decreases while mean stays constant, it's stabilizing selection.

Question 6

1. Stabilizing selection because intermediate song frequencies have the highest reproductive success
2. Directional selection shifting the mean toward lower song frequency across generations (correct answer)
3. Disruptive selection favoring both low and high frequencies and eliminating intermediates
4. Genetic drift because changes in song frequency are independent of mating success
5. Individuals lowering their song frequency intentionally, which changes allele frequencies directly

Explanation: This question requires analyzing natural selection patterns by examining how a behavioral trait changes over generations. In dense forest, males with lower-frequency songs achieve more matings than higher-frequency males, creating consistent fitness differences. Over seven generations, the mean song frequency decreases steadily while higher frequencies persist at low levels, demonstrating directional selection that shifts the population mean toward lower frequencies. Students might choose stabilizing selection (A) if they misinterpret the persistence of some variation, but the key indicator is the steady directional shift in the mean rather than maintenance of the original mean. To identify selection type, track the population mean: if it shifts consistently in one direction due to fitness differences, it's directional selection even if some variation remains.

Question 7

1. Directional selection shifts the population toward only the high gill-raker extreme.
2. Stabilizing selection increases intermediate phenotypes by reducing both extremes.
3. Disruptive selection favors both extremes over intermediates, increasing phenotypic bimodality. (correct answer)
4. Genetic drift causes the intermediate phenotype to decline because large populations drift faster.
5. Natural selection is absent because all phenotypes can reproduce each generation.

Explanation: This question tests your ability to identify natural selection patterns from fitness data across phenotypes. Fish with extreme gill-raker counts (very low or very high) produce 20 juveniles while intermediates produce only 7, showing both extremes have higher fitness. Over 15 generations, intermediate phenotypes decline while both extremes become more common, creating a bimodal distribution. This pattern exemplifies disruptive selection, where extremes are favored over intermediates, increasing phenotypic variance and potentially leading to evolutionary divergence. Choice D incorrectly claims genetic drift causes the pattern, but drift acts randomly and wouldn't consistently favor both extremes over intermediates across many generations. When both extreme phenotypes have higher fitness than intermediates and the population becomes more bimodal, identify this as disruptive selection.

Question 8

1. Disruptive selection increased variance by favoring both extremes over intermediate phenotypes
2. Directional selection shifted the mean toward thicker fur because thick-furred mice reproduced most
3. Stabilizing selection reduced variance by favoring intermediate fur thickness across generations (correct answer)
4. Genetic drift reduced variance because survival differences among phenotypes were equal
5. Mutation pressure increased intermediate phenotypes because new alleles arose in cold conditions

Explanation: This question assesses the skill of analyzing patterns of natural selection by evaluating changes in phenotypic variance and mean in relation to fitness data. Intermediate fur thickness conferred the highest reproductive success (1.8 offspring) compared to thin (0.6) and thick (0.7), resulting in an increased proportion of intermediate phenotypes and decreased overall variance. The mean fur thickness remained stable, which is characteristic of stabilizing selection that reduces extremes and narrows the distribution around the optimal intermediate trait. This pattern persisted over 15 cold winters, demonstrating how selection maintains the average while eliminating less fit variants. A tempting distractor is choice A, disruptive selection, which is incorrect because it would increase variance by favoring extremes, but here variance decreased, arising from the misconception that any fitness difference at extremes implies disruption. A transferable strategy is to compare pre- and post-selection trait distributions, focusing on whether the mean shifts or variance changes to distinguish selection modes.

Question 9

1. Directional selection increases long-ear alleles because long ears always improve heat loss.
2. Disruptive selection increases variation by favoring very short and very long ears equally.
3. Stabilizing selection favors intermediate ear length, decreasing extreme phenotypes over generations. (correct answer)
4. Genetic drift explains the narrowing range because selection cannot change quantitative traits.
5. Rabbits develop intermediate ears in cold weather and pass that change to offspring.

Explanation: This question requires analyzing natural selection patterns during environmental stress. During cold winters, rabbits with intermediate ear length produce 10 offspring while those with very short or very long ears produce only 4, showing intermediates have highest fitness. Over 9 generations, the mean stays similar but the range narrows, indicating reduced variation around the optimal intermediate value. This pattern exemplifies stabilizing selection, which maintains the population mean while reducing variance by selecting against extremes. Choice A incorrectly suggests directional selection toward long ears, but the data shows intermediates have highest fitness, not long-eared individuals. When intermediate phenotypes consistently outperform extremes and population variance decreases while maintaining the mean, recognize this as stabilizing selection.

Question 10

1. Directional selection causing an increase in the dark-shell allele frequency over generations (correct answer)
2. Stabilizing selection maintaining both shell colors by favoring intermediates each generation
3. Disruptive selection increasing both shell colors because extremes have higher fitness
4. Genetic drift producing a consistent rise in $l$ because predation is nonselective
5. Individual snails choosing darker sediment and thereby changing their allele frequencies

Explanation: This question requires analyzing natural selection patterns by tracking both phenotype and allele frequency changes. In the lake with dark sediment, fish consume light snails more often, giving dark snails (with allele l) higher survival rates. Over ten generations, allele l increases dramatically from 0.40 to 0.85 and light shells become rare, demonstrating directional selection that consistently favors one phenotype and its associated allele. Students might incorrectly choose genetic drift (D) thinking the change could be random, but the consistent predation pressure on light snails creates predictable fitness differences that drive allele frequency change. To distinguish selection from drift, look for consistent environmental pressures: when one phenotype consistently has lower survival and its allele frequency decreases predictably, it's directional selection.

Question 11

1. Directional selection increasing allele $D$ because darker moths have higher reproductive success (correct answer)
2. Stabilizing selection maintaining both alleles equally by favoring heterozygotes each generation
3. Disruptive selection increasing both $D$ and $d$ by favoring extreme wing colors
4. Genetic drift increasing $D$ predictably because predators remove light moths at random
5. Moths developing darker wings after exposure and passing that acquired darkness to offspring

Explanation: This question requires analyzing natural selection patterns at the genetic level by tracking allele frequency changes. In the industrial area, birds capture more light moths than dark moths, giving darker moths (with allele D) higher survival and reproductive success. The frequency of allele D rises dramatically from 0.30 to 0.78 over eight generations, demonstrating directional selection that consistently favors one allele over another. Students often mistakenly choose genetic drift (D) because they see predation as random, but the key detail is that predators specifically capture more light moths, creating systematic fitness differences. To identify selection versus drift, look for consistent fitness differences: if one phenotype consistently survives better and allele frequencies change predictably in that direction, it's natural selection, not random drift.

Question 12

1. Directional selection increasing wax thickness because thick wax always improves survival
2. Disruptive selection increasing both thin and thick wax phenotypes relative to intermediates
3. Stabilizing selection favoring intermediate wax thickness and reducing extremes over generations (correct answer)
4. Genetic drift causing wax-thickness changes due to chance sampling of gametes each generation
5. Nonrandom mating creating more intermediates without any differences in reproductive success

Explanation: This question tests your ability to analyze natural selection patterns by examining fitness differences and distribution changes. The scenario shows that very thin wax leads to fungal infection, very thick wax reduces growth and seed production, and intermediate thickness has the highest reproductive success, with the distribution becoming more concentrated around intermediate values after twelve generations. This describes stabilizing selection, where intermediate phenotypes have optimal fitness, reducing the frequency of extremes and narrowing variation. Choice B incorrectly suggests disruptive selection, but that would increase extreme phenotypes and create a bimodal distribution, not the observed concentration around intermediates. When you see selection favoring intermediates with a narrowing distribution around the mean, identify it as stabilizing selection.

Question 13

1. Directional selection favored faster speeds, shifting the mean speed higher each generation
2. Stabilizing selection favored intermediate speeds, reducing variance and producing a single peak
3. Genetic drift caused bimodality because all speed classes had similar reproductive success
4. Disruptive selection favored both slow and fast speeds, reducing intermediate phenotypes over time (correct answer)
5. Gene flow increased extremes because migrants with intermediate speeds reproduced least in all habitats

Explanation: This question assesses the skill of analyzing patterns of natural selection by examining fitness and resulting distribution changes on a new substrate. Both slow and fast running speeds had higher offspring (2.0 and 1.8) than intermediates (0.7), leading to a bimodal distribution and sharp decline in intermediates after 11 generations. This reflects disruptive selection, as the volcanic terrain likely created niches where extremes were advantageous, increasing variance and splitting the trait. The pattern reduced the middle of the spectrum without a net shift in mean speed. A tempting distractor is choice B, stabilizing selection, which is incorrect because it would create a single peak by reducing variance, but here bimodality appeared, stemming from the misconception that low intermediate fitness stabilizes. A practical strategy is to identify disruption when fitness favors extremes, leading to multimodality in trait distributions.

Question 14

1. Directional selection increasing mean body mass because larger mice consistently leave more offspring
2. Disruptive selection increasing both extremes because small and large mice avoid competition
3. Stabilizing selection reducing extremes and decreasing variance around an intermediate mass (correct answer)
4. Genetic drift changing body mass distribution due to random allele sampling in a huge population
5. Nonrandom mating alone narrowing the distribution without any differences in survival or fecundity

Explanation: This question assesses the skill of analyzing patterns of natural selection by interpreting population-level changes in traits and allele frequencies. In a stable environment, intermediate body mass mice have higher survival and reproduction, leading to a narrowing distribution over generations. The mean body mass stays similar, but extremes decline, with associated alleles decreasing in frequency, reducing overall variation. This demonstrates stabilizing selection, which preserves an optimal intermediate by selecting against deviations. A tempting distractor is disruptive selection, incorrect because it would increase extremes rather than decrease them, arising from a misconception that any environmental pressure favors divergence. For future analyses, assess if selection reduces variance around a stable mean to identify stabilization across traits.

Question 15

1. Directional selection favoring darker fur, shifting allele frequencies toward one extreme (correct answer)
2. Stabilizing selection favoring intermediate fur, reducing variation around the mean
3. Disruptive selection favoring both extremes, increasing the frequency of light and dark alleles
4. Genetic drift from random sampling, changing allele frequencies without fitness differences
5. Balancing selection maintaining all alleles at similar frequencies through heterozygote advantage

Explanation: This question assesses the skill of analyzing patterns of natural selection by interpreting population-level changes in traits and allele frequencies. The data show that dark-fur rabbits have the highest reproductive success, followed by medium, then light, indicating a fitness gradient favoring darker fur in the post-wildfire dark environment. Over generations, the dark-fur allele frequency increases significantly while the light-fur allele decreases, demonstrating a shift toward one extreme without evidence of migration or random changes. This pattern aligns with directional selection, where selection pressures push the population mean toward darker fur by favoring alleles associated with that trait. A tempting distractor is disruptive selection, which is incorrect because it would increase both light and dark extremes rather than systematically favoring only darker fur, reflecting a misconception about when extremes are bimodally favored. To distinguish selection patterns in future questions, always compare trends in fitness across the trait range and track how allele frequencies and phenotypic distributions shift over time.

Question 16

1. Directional selection favoring darker fur, shifting allele frequencies toward one extreme (correct answer)
2. Stabilizing selection favoring intermediate fur, reducing variance around the mean
3. Disruptive selection favoring both extremes, increasing frequency of light and dark alleles
4. Genetic drift causing random allele changes unrelated to predation differences
5. Individuals darken their fur during life and pass that acquired trait to offspring

Explanation: This question tests your ability to analyze natural selection patterns by examining changes in allele frequencies and phenotype distributions. The scenario describes darker mice surviving better after a wildfire blackens the soil, with the dark-fur allele frequency increasing from 0.35 to 0.72 and average fur darkness increasing—both indicators that the population is shifting toward one extreme (darker fur). This matches directional selection, where one extreme phenotype has higher fitness, causing the population mean to shift in that direction over generations. Choice E incorrectly suggests Lamarckian inheritance where acquired traits are passed on, but natural selection acts on existing genetic variation, not traits developed during an individual's lifetime. When you see consistent shifts in both allele frequency and population mean toward one extreme, identify it as directional selection.

Question 17

1. Directional selection favoring longer beaks, shifting the mean upward each generation
2. Disruptive selection favoring extreme beaks, increasing variation and producing two peaks
3. Stabilizing selection favoring intermediate beaks, reducing extremes while maintaining the mean (correct answer)
4. Gene flow from another population introducing new extreme beak alleles each generation
5. Individuals adjust beak size to match prey and transmit those changes to offspring

Explanation: This question tests your ability to analyze natural selection patterns by examining changes in phenotype distribution and variance. The scenario shows that birds with intermediate beaks have higher reproductive success than those with extreme beaks, and after eight generations, the mean remains similar but variation narrows—classic signs of stabilizing selection. Stabilizing selection favors intermediate phenotypes and reduces the frequency of extremes, concentrating the population around an optimal value without shifting the mean. Choice A incorrectly suggests directional selection, but the mean isn't shifting upward; instead, the distribution is becoming narrower around the existing mean. When you see reduced variation with a stable mean and selection against extremes, recognize it as stabilizing selection.

Question 18

1. Stabilizing selection favoring intermediates, narrowing variation around a single peak
2. Directional selection favoring darker fish, shifting the entire distribution toward one extreme
3. Genetic drift producing random changes in coloration unrelated to predator detection
4. Disruptive selection favoring both extremes, reducing intermediates and increasing bimodality (correct answer)
5. Fish choose habitats that change their color during life, altering allele frequencies directly

Explanation: This question tests your ability to analyze natural selection patterns by examining changes in phenotype frequency distributions. The scenario describes intermediate-colored fish being more easily detected by predators, while extreme phenotypes (pale and dark) have camouflage advantages, leading to decreased intermediate frequencies and increased extreme frequencies with a bimodal distribution. This perfectly describes disruptive selection, where extreme phenotypes have higher fitness than intermediates, creating two peaks in the distribution. Choice A incorrectly suggests stabilizing selection, but that would favor intermediates and create a single narrow peak, not the observed bimodal pattern. When you see selection against intermediates producing a bimodal distribution with increased extreme frequencies, identify it as disruptive selection.

Question 19

1. Disruptive selection that increases the frequency of both extreme seed sizes
2. Directional selection that shifts the mean seed size toward larger seeds
3. Stabilizing selection that reduces extremes and maintains an intermediate mean (correct answer)
4. Genetic drift that changes seed-size allele frequencies independent of survival differences
5. Mutation pressure that consistently converts intermediate-seed alleles into extreme-seed alleles

Explanation: This question tests your ability to analyze natural selection patterns by examining fitness differences across phenotypes. The scenario shows that very small seeds produce seedlings with high mortality, very large seeds are eaten by rodents, and intermediate seeds have the highest reproductive success, with the mean changing little but extremes becoming rarer. This describes stabilizing selection, where intermediate phenotypes have the highest fitness, reducing variation around an optimal value. Choice B incorrectly suggests directional selection toward larger seeds, but the data shows large seeds have lower fitness due to rodent predation, and the mean isn't shifting. When you see selection favoring intermediates with reduced frequency of extremes and a stable mean, recognize it as stabilizing selection.

Question 20

1. Directional selection favoring later emergence, shifting the mean later each generation
2. Stabilizing selection favoring intermediate emergence, decreasing variance around the mean
3. Disruptive selection favoring early and late emergence, reducing intermediate phenotypes (correct answer)
4. Gene flow homogenizing emergence timing by introducing intermediate phenotypes from neighbors
5. Insects changing emergence timing in response to wasps and transmitting the change genetically

Explanation: This question tests your ability to analyze natural selection patterns by examining changes in phenotype frequency distributions. The scenario shows that insects emerging mid-season have reduced reproductive success due to parasitoid wasps, while early and late emergers increase in frequency and intermediate phenotypes decline—creating a bimodal distribution. This perfectly describes disruptive selection, where extreme phenotypes (early and late emergence) have higher fitness than intermediates, splitting the population into two peaks. Choice B incorrectly suggests stabilizing selection, but that would favor intermediates and create a single peak, opposite to the observed pattern of declining intermediate frequencies. When you see selection against intermediates with increasing extreme frequencies, recognize it as disruptive selection.