Artificial Selection
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AP Biology › Artificial Selection
A breeder selects only sheep with the finest wool fibers to reproduce. Fiber fineness is influenced by alleles W (finer) and w (coarser). At generation 0, $f(W)=0.55$. After repeated selection for fineness, $f(W)=0.90$ at generation 7. Which outcome is most likely in the population’s phenotype distribution for fiber fineness at generation 7 compared with generation 0?
The distribution shifts toward coarser fibers because selecting fine wool increases the frequency of allele w.
The distribution becomes bimodal because selection forces both alleles to equal frequencies in every generation.
The distribution shifts toward finer fibers because allele W became more common among reproducing parents.
The distribution stays unchanged because phenotype distributions cannot shift without new alleles entering by migration.
The distribution shifts toward finer fibers because each sheep produces finer wool after exposure to selective breeding.
Explanation
This question tests understanding of how changes in allele frequency affect phenotype distributions. As selection for fine wool increases allele W's frequency from 0.55 to 0.90, more sheep inherit W alleles (as WW or Ww), shifting the population's phenotype distribution toward finer fibers because genotypes producing finer wool become more common. This demonstrates the connection between allele frequency changes and observable trait distributions in populations. Option B incorrectly claims distributions cannot shift without migration; option C contradicts the selection direction; option D suggests individual phenotypic change rather than population evolution; option E misunderstands selection's effect on allele frequencies. To predict phenotypic outcomes, connect allele frequency changes to the phenotypes those alleles produce in the population.
In a herd of cattle, breeders allow only the 20% with the highest milk yield to reproduce each generation. Milk yield is influenced by a gene with alleles H (higher yield) and h (lower yield). At generation 0, allele frequencies are H = 0.40 and h = 0.60. After five generations of this breeding practice, the herd’s average milk yield increases and fewer low-yield calves are born. No new animals are introduced. Which outcome is most likely in the herd’s gene pool after five generations of selection?
Allele frequencies fluctuate randomly because breeding choices eliminate any effect of selection.
The frequency of allele H increases because individuals with more H alleles leave more offspring.
Allele h becomes more common because selection creates new h mutations in low-yield calves.
Each cow increases its own H allele proportion during life in response to frequent milking.
Both alleles remain at the same frequencies because selection changes only phenotypes, not genes.
Explanation
This question assesses the analysis of artificial selection, where humans selectively breed organisms for desired traits, leading to changes in population genetics. The correct answer is A because breeders allow only the highest milk-yield cows to reproduce, and since milk yield is influenced by allele H for higher yield, individuals carrying more H alleles contribute disproportionately to the next generation's gene pool. Over five generations, this differential reproductive success increases the frequency of H from 0.40, as low-yield calves with more h alleles are less likely to be born due to the selection pressure. This aligns with AP Biology concepts of evolution, where artificial selection mimics natural selection by favoring heritable traits that enhance reproductive output in the controlled environment. A tempting distractor is E, which is incorrect due to the misconception of Lamarckian inheritance, suggesting individuals acquire and pass on traits during their lifetime rather than through genetic selection. A transferable strategy for this question type is to identify how selective breeding amplifies alleles linked to the desired phenotype through generational reproduction, not individual adaptation.
In a captive breeding program for rabbits, only individuals with the longest ears are chosen as parents each generation. After many generations, the average ear length increases, but the range of ear lengths becomes narrower. Which outcome is most likely in the rabbit population after this selection regime?
The ear-length distribution narrows because selection cannot change allele frequencies.
Genetic variation affecting ear length decreases as alleles for shorter ears are selected against.
Ear-length alleles shift because rabbits stretch their ears more when long ears are preferred.
New ear-length alleles appear because breeders repeatedly measure ear length.
Alleles for shorter ears are maintained at the same frequency because they are recessive.
Explanation
This question assesses the skill of analyzing artificial selection by analyzing changes in trait variation and means in mammalian breeding programs. The correct answer is choice C because selecting rabbits with the longest ears as parents reduces the transmission of alleles for shorter ears, decreasing genetic variation and narrowing the ear-length range while increasing the average, per the AP Biology principle that directional selection reduces diversity by eliminating unfavored alleles. The stimulus describes a narrower range after many generations, reflecting loss of shorter-ear variants due to consistent selection pressure. Ear length as a quantitative trait shows how repeated selection compresses phenotypic variation. A tempting distractor is choice B, which is incorrect due to the misconception of structure-function confusion, suggesting physical stretching alters heredity rather than genetic selection. To approach similar questions, assess variance changes alongside means to infer selection's impact on underlying genetic diversity.
A breeder crosses only corn plants with the highest drought-survival scores to produce seed for the next generation. All plants are grown under the same drought-stress protocol each generation. After five generations, a larger fraction of plants survive the drought treatment than in generation 1. Which outcome is most likely responsible for the increased survival?
Survival increased because alleles that contribute to drought survival became more frequent.
Survival increased because the corn population evolved drought resistance within each plant’s lifetime.
Survival increased because drought conditions caused all plants to develop the same protective allele.
Survival increased because plants that died still passed their alleles to the next generation.
Survival increased because selection always increases genetic diversity at survival loci.
Explanation
This question assesses the skill of analyzing artificial selection by determining how breeding for survival traits enhances population resilience in crops. The correct answer is choice B because crossing only high drought-survival corn plants increases the frequency of alleles conferring drought resistance, leading to higher survival rates after five generations, according to the AP Biology concept that selection enriches populations for beneficial alleles under consistent conditions. The stimulus specifies identical drought-stress protocols each generation, ensuring the observed increase in survival stems from genetic improvements. As a heritable trait, drought survival responds to selection by shifting the population toward more resistant genotypes. A tempting distractor is choice E, which is incorrect due to a level-of-organization error, confusing within-lifetime phenotypic plasticity with heritable evolutionary changes across generations. To approach similar questions, isolate genetic effects by noting controlled environments and track how selection favors alleles improving fitness-related traits.
A breeder maintains two lines of mice from the same starting population. In Line 1, only the fastest runners (top 10%) are used as parents each generation. In Line 2, breeders choose parents randomly with respect to running speed. Both lines are housed identically. After 15 generations, Line 1 has a higher mean running speed than Line 2. Which statement best accounts for the difference?
Line 2 had more mutations because random breeding increases mutation rates at running genes.
Line 2 mice ran slower because random mating removes alleles for fast running from populations.
Line 1 mice ran faster because they trained harder after being selected as parents.
Line 1 had increased frequency of alleles contributing to fast running due to nonrandom breeding.
The difference occurred because populations change traits to match breeder preferences over time.
Explanation
This question assesses the skill of analyzing artificial selection by comparing outcomes in selected versus control lines to isolate selection effects. The correct answer is choice C because Line 1's nonrandom breeding from the fastest 10% increases alleles for fast running, raising mean speed over 15 generations, consistent with the AP Biology concept that artificial selection drives evolutionary change by favoring specific genotypes. The stimulus contrasts Line 1 with randomly bred Line 2 under identical housing, attributing the speed difference to selection pressure. Running speed as a heritable trait demonstrates directional selection's impact. A tempting distractor is choice A, which is incorrect due to the misconception of teleology, suggesting mice train to meet selection criteria rather than inheriting advantageous alleles. To approach similar questions, use control groups to confirm selection's role and predict trait divergence based on breeding methods.
In a herd of 200 cattle, ranchers allow only the 20 bulls with the greatest muscle mass to sire calves each year for 10 years; other bulls are excluded from breeding. Calves are raised similarly regardless of parent. Muscle mass varies continuously, and the trait is heritable. After 10 years, the average muscle mass in the calf population is higher than at the start. Which outcome is most likely in the population’s gene pool after this artificial selection?
Alleles associated with greater muscle mass increase in frequency across generations of calves.
The cattle population becomes genetically identical because selection removes all variation.
Allele frequencies stay constant because only the environment determines muscle mass.
New alleles for greater muscle mass arise because selection creates beneficial mutations.
Each calf develops greater muscle mass because ranchers chose muscular bulls as parents.
Explanation
This question assesses the skill of analyzing artificial selection by examining how human-directed breeding changes trait distributions in populations over generations. The correct answer is choice A because ranchers selectively breed only the most muscular bulls, leading to calves inheriting alleles that promote greater muscle mass, and over 10 years, these alleles increase in frequency as less muscular individuals are excluded from reproduction, aligning with the AP Biology concept that artificial selection alters allele frequencies by favoring heritable traits. The stimulus specifies that muscle mass is heritable and varies continuously, indicating polygenic inheritance, where nonrandom mating increases the prevalence of favorable allele combinations in the gene pool. Since calves are raised similarly, environmental factors are controlled, confirming that the shift in average muscle mass results from genetic changes due to selection pressure. A tempting distractor is choice C, which is incorrect due to the misconception of teleology, suggesting selection creates mutations to meet needs rather than acting on existing variation. To approach similar questions, identify how selection acts on pre-existing genetic variation to shift allele frequencies without introducing new mutations or individual adaptations.
In a flock of chickens, a breeder allows only the 20% with the highest egg production to reproduce each generation. Egg production is influenced by a gene with alleles H (higher production) and h (lower production). After six generations, the proportion of chicks showing high egg production increases from 35% to 70% in the flock. Which outcome is most likely in the flock’s gene pool after these generations of artificial selection?
New H alleles arise during breeding because selection causes mutations that improve egg production.
Egg production rises because the population needs more eggs, causing H to appear in most individuals.
Allele H increases in frequency because individuals with allele H contributed more offspring to the next generations.
Allele h increases in frequency because selection favors maintaining both alleles at equal frequencies over time.
Each chicken develops allele H during its lifetime in response to higher egg-laying conditions.
Explanation
This question tests your ability to analyze artificial selection and its effects on allele frequencies in populations. In this chicken breeding scenario, the breeder allows only the highest-producing 20% to reproduce, which means individuals with allele H (higher production) are overrepresented among parents compared to their frequency in the general population. This differential reproduction causes allele H to increase in frequency over generations because H-carrying individuals contribute more gametes to the gene pool, shifting the population from 35% to 70% high producers. Option B incorrectly suggests balancing selection, which would maintain both alleles rather than increase one; option C commits the misconception that selection causes beneficial mutations rather than acting on existing variation; options D and E represent Lamarckian errors where individuals acquire or develop traits during their lifetime. When analyzing artificial selection problems, identify which individuals reproduce more and trace how this differential reproduction changes allele frequencies across generations.
In a herd of cattle, a breeder uses artificial insemination from only the top 5% of bulls for muscle mass each year. Muscle mass is influenced by alleles M (higher mass) and m (lower mass) at a locus. Over 10 years, the frequency of allele M rises from 0.30 to 0.75 in calves. Which statement best describes why allele M increased in frequency?
Allele M increased because using a few bulls increases mutation rates that generate more M alleles.
Allele M increased because allele frequencies automatically move toward 0.75 in any large population.
Allele M increased because selection directly changed m alleles into M alleles in fertilized eggs.
Allele M increased because calves needed more muscle mass, so more M alleles formed during growth.
Allele M increased because bulls with M were overrepresented among parents, shifting allele frequencies in offspring.
Explanation
This question examines artificial selection through selective breeding in cattle using artificial insemination. When only the top 5% of bulls for muscle mass serve as fathers, bulls carrying allele M are dramatically overrepresented among parents, causing their alleles to dominate the gene pool and increasing M's frequency from 0.30 to 0.75 in just 10 years. This exemplifies how artificial selection works: individuals with desired traits contribute disproportionately more alleles to future generations through differential reproduction. Option B incorrectly suggests alleles form in response to need (Lamarckian error); option C wrongly claims selection directly converts alleles; option D misunderstands allele frequency dynamics; option E incorrectly links breeding practices to mutation rates. To analyze artificial selection scenarios, identify which individuals reproduce and calculate how this skews allele transmission to offspring.
In a population of corn, kernels can be red or yellow. A farmer saves seeds only from plants with the reddest kernels to plant the next year. Kernel color is influenced by alleles R (red) and r (yellow). Over four planting cycles, the fraction of red-kernel plants rises from 40% to 85%. Which change in the population is most likely responsible for this shift?
The frequency of allele R increased because saving seeds causes new R mutations to occur more often.
The frequency of allele R increased because plants sensed selection and converted r alleles into R alleles.
The frequency of allele R increased because allele frequencies change only in response to environmental stress.
The frequency of allele R increased because the population required red kernels, so most offspring became red.
The frequency of allele R increased because plants producing red kernels contributed more seeds to the next generation.
Explanation
This question examines artificial selection in crop breeding through seed saving practices. When the farmer saves seeds only from plants with the reddest kernels, plants carrying allele R contribute disproportionately more seeds (and thus alleles) to the next generation, causing R's frequency to increase and shifting the population from 40% to 85% red-kernel plants over four cycles. This demonstrates how human selection of parents based on phenotype drives evolutionary change in agricultural populations. Option B incorrectly suggests plants can sense and respond by converting alleles (Lamarckian misconception); option C wrongly links seed saving to mutation rates; option D misunderstands selection triggers; option E commits a teleological error about population requirements driving inheritance. When analyzing agricultural selection, trace how choosing certain plants as seed sources changes allele frequencies in subsequent plantings.
A breeder maintains two lines of rabbits: Line 1 is produced by mating only rabbits with very long ears, while Line 2 is produced by mating rabbits at random regardless of ear length. Ear length is influenced by alleles L (longer) and l (shorter). After 12 generations, Line 1 shows a much narrower range of ear lengths than Line 2. Which explanation best accounts for the difference between the lines?
Line 1 narrowed variation because rabbits evolved longer ears within their lifetimes before reproducing.
Random mating in Line 2 caused individuals to change from allele l to L when exposed to longer-eared parents.
Line 2 lost variation because random mating prevents alleles from being passed to offspring consistently.
Line 1 gained variation because selection increases recombination rates that create new ear-length alleles.
Directional selection in Line 1 reduced allelic diversity at ear-length loci compared with the randomly bred Line 2.
Explanation
This question analyzes how artificial selection affects genetic variation compared to random mating. Line 1 experiences directional selection for long ears, which increases the frequency of allele L while decreasing l, reducing allelic diversity and narrowing the phenotypic range as the population becomes more genetically uniform at ear-length loci. In contrast, Line 2 maintains both alleles through random mating, preserving genetic variation and a wider range of ear lengths. Option B incorrectly suggests individuals change alleles (Lamarckian error); option C wrongly claims selection increases variation through recombination; option D misunderstands random mating's effect on allele transmission; option E represents within-lifetime change rather than population evolution. To compare selection regimes, recognize that directional selection reduces variation while random mating maintains it.