- In a small island population of birds, genetic drift causes the frequency of a neutral allele to change from 0.3 to 0.7 over several generations. Which statement best explains why this change is most likely to occur in small populations rather than large populations?
Answer: Random sampling effects have greater impact when fewer individuals contribute gametes to the next generation
When you encounter questions about genetic drift, focus on the fundamental principle that random sampling has disproportionate effects in small versus large populations. Genetic drift is the random change in allele frequencies due to sampling error when gametes form the next generation.
The correct answer is B because random sampling effects are magnified when fewer individuals reproduce. Think of it like flipping coins: if you flip 10 coins, getting 7 heads (70%) is reasonably likely by chance alone. But if you flip 1,000 coins, getting 700 heads is extremely unlikely. Similarly, when only a few birds contribute gametes to form the next generation, random chance can dramatically shift allele frequencies from 0.3 to 0.7.
Answer A incorrectly suggests small populations have higher mutation rates. Population size doesn't affect per-individual mutation rates, and mutations are typically rare events that wouldn't cause such large frequency changes quickly.
Answer C confuses natural selection with genetic drift. The question specifically states the allele is neutral, meaning natural selection doesn't act on it. Small populations don't experience stronger selection pressure simply due to their size.
Answer D mentions gene flow restriction, which can contribute to genetic drift effects, but this isn't the primary mechanism. The core issue is sampling variance, not reduced migration.
Remember this pattern: genetic drift questions often test whether you understand that random sampling error decreases as sample size increases. Small populations = large random effects; large populations = small random effects due to statistical averaging.
- A mutation introduces a new allele into a population at a rate of 1×10−5 per generation. Assuming no other evolutionary forces are acting, what factor most directly determines whether this new allele will become established in the population?
Answer: The effective population size, because genetic drift in small populations may eliminate the allele before selection can act
When you encounter questions about new mutations entering populations, think about the interplay between mutation, selection, and genetic drift. The key insight is that most new mutations start as single copies in large populations, making them extremely vulnerable to random loss.
The correct answer is C because effective population size determines the strength of genetic drift relative to other evolutionary forces. In small populations, genetic drift is powerful and can randomly eliminate even beneficial alleles before natural selection has a chance to increase their frequency. A new allele introduced at 1×10−5 per generation will likely appear as just one copy initially, making it highly susceptible to being lost purely by chance in small populations. Large effective population sizes reduce the impact of drift, giving selection more opportunity to act on the allele's fitness effects.
Choice A confuses mutation rate with establishment probability. While larger populations do experience more total mutations, this doesn't help any individual new allele become established. Choice B incorrectly assumes dominance guarantees success—many beneficial recessive alleles can spread, while harmful dominant alleles will be selected against. Choice D misses the point entirely; generation time affects how quickly evolution proceeds but doesn't influence whether a particular new allele survives its vulnerable early stages.
Remember this pattern: when evaluating the fate of new mutations, always consider genetic drift first. Small populations are "mutation graveyards" where most new alleles disappear randomly, regardless of their potential benefits. Effective population size is often the most critical factor determining evolutionary outcomes. - A biologist studying a large population finds that the effective population size (Ne) is much smaller than the census population size. Which factor most likely explains this discrepancy?
Answer: Unequal sex ratios or variation in reproductive success among individuals
When you encounter questions about effective population size (Ne) versus census population size, you're dealing with a key concept in population genetics that distinguishes between the total number of individuals counted and the number that actually contribute genes to the next generation.
Effective population size represents the number of individuals in an idealized population that would experience genetic drift at the same rate as the actual population being studied. It's almost always smaller than the census count because not all individuals reproduce equally—or at all.
Option B correctly identifies the primary cause of this discrepancy. Unequal sex ratios (like having far more males than females, or vice versa) dramatically reduce Ne because reproduction requires both sexes. Similarly, when reproductive success varies widely—perhaps only a few dominant males mate while others don't—fewer individuals actually pass on their genes despite a large total population.
Option A misunderstands the relationship entirely. High mutation rates don't affect the counting discrepancy between Ne and census size—they create genetic variation but don't change how many individuals reproduce.
Option C confuses natural selection with population counting. Strong selection might reduce population size overall, but it doesn't specifically explain why Ne would be smaller than census size among surviving individuals.
Option D incorrectly suggests that migrants inflate census counts with non-reproducing individuals, but migrants are typically not counted in local census data, and gene flow actually tends to increase effective population size.
Remember: Ne reflects breeding reality, not just body counts. Look for factors that limit actual reproduction when you see this type of question. - Two populations of fish become separated by a dam. Before separation, both populations had identical allele frequencies of 0.5 for alleles A and a. After 20 generations of separation, Population 1 has frequencies of 0.7 (A) and 0.3 (a), while Population 2 has frequencies of 0.4 (A) and 0.6 (a). Which evolutionary force most likely caused this divergence?
Answer: Genetic drift, because random sampling in finite populations caused the frequencies to change in different directions
When analyzing changes in allele frequencies between separated populations, you need to consider which evolutionary forces can operate independently and produce divergent results.
Genetic drift is the correct explanation here. When populations are finite in size, random sampling during reproduction causes allele frequencies to fluctuate unpredictably from generation to generation. The key insight is that drift acts randomly and independently in each population - one population drifted toward higher A frequency (0.7) while the other drifted toward lower A frequency (0.4). This bidirectional change from the starting point (0.5) is the hallmark of random genetic drift.
Option A incorrectly attributes the change to natural selection. While selection could cause frequency changes, the question provides no evidence of different environmental pressures or adaptive advantages. More importantly, we'd expect selection to consistently favor the same allele if environments were similar, not produce opposite trends.
Option B misunderstands gene flow. The dam prevents gene flow (movement of alleles between populations), but gene flow itself didn't cause the divergence - its absence allowed other forces to act. Gene flow typically homogenizes allele frequencies, not diversifies them.
Option D suggests mutation as the driver. However, mutation rates are typically too low to produce such substantial frequency changes in just 20 generations. Additionally, we'd need specific information about differential mutation rates to explain the divergent patterns.
Remember this pattern: when separated populations show random, bidirectional changes in allele frequencies without evidence of selection pressure, genetic drift is usually the culprit, especially in smaller populations where sampling effects are strongest.
- In a population genetics experiment, researchers establish multiple replicate populations from the same source population and allow them to evolve in isolation. After many generations, they find that different replicate populations have fixed different alleles at the same locus. This result best demonstrates which evolutionary principle?
Answer: Genetic drift can cause random changes in allele frequencies that lead to different outcomes in replicate populations
When you encounter population genetics experiments with replicate populations evolving in isolation, focus on what causes populations starting with identical genetic composition to diverge over time.
The key evidence here is that replicate populations from the same source fixed different alleles at the same locus despite starting identically. This randomness in outcomes points directly to genetic drift - the random sampling of alleles from generation to generation that occurs in all finite populations. Genetic drift causes allele frequencies to fluctuate unpredictably, and over many generations, can lead to fixation of different alleles in different populations purely by chance. Answer C correctly identifies this principle.
Answer A is wrong because natural selection in similar environments would produce similar outcomes across replicates, not the different outcomes observed. If selection were driving the changes, you'd expect the same beneficial allele to be favored in all populations.
Answer B incorrectly focuses on mutation as the primary driver. While mutation creates variation, the fixation of different existing alleles across replicates suggests random drift of alleles already present, not new mutations arising and spreading.
Answer D describes gene flow, which actually prevents divergence by homogenizing allele frequencies between populations. The experiment specifically involves isolated populations with no gene flow, making this answer contradictory to the setup.
Remember this pattern: when you see replicate populations from identical sources evolving differently in isolation, think genetic drift. The randomness and unpredictability of outcomes across replicates is the telltale signature of drift overwhelming selection in finite populations.
- A plant population experiences strong positive assortative mating, where individuals with similar flower colors preferentially mate with each other. How will this mating pattern affect the population's genetic structure compared to random mating?
Answer: The population will show increased homozygosity and decreased heterozygosity compared to Hardy-Weinberg expectations
When you encounter questions about mating patterns and population genetics, think about how different mating behaviors affect genotype frequencies while leaving allele frequencies unchanged (assuming no other evolutionary forces are acting).
Positive assortative mating means individuals with similar traits preferentially mate with each other. In this case, plants with similar flower colors are more likely to reproduce together. This creates a crucial effect: it increases the probability that similar alleles will combine, leading to more homozygous offspring and fewer heterozygous offspring than would occur under random mating. The Hardy-Weinberg principle assumes random mating, so any deviation toward assortative mating will increase homozygosity above Hardy-Weinberg expectations.
Looking at the wrong answers: A) is incorrect because assortative mating itself doesn't change allele frequencies - it only redistributes existing alleles into different genotype combinations. No selection pressure is inherently created. C) misunderstands the relationship between mating patterns and effective population size. While assortative mating is non-random, it doesn't reduce the number of breeding individuals, so genetic drift isn't enhanced. D) gets the effect completely backward - assortative mating decreases heterozygosity because similar individuals are mating, not promoting genetic mixing between different types.
The correct answer is B because assortative mating fundamentally shifts the population toward more homozygous genotypes.
Study tip: Remember that assortative mating affects genotype frequencies (more homozygotes) but not allele frequencies, while disassortative mating has the opposite effect (more heterozygotes). This distinction appears frequently on biology exams.
- A population of butterflies experiences a severe drought that reduces the population size from 10,000 to 100 individuals. The allele frequency of wing pattern allele W changes from 0.6 to 0.8 during this event. Which statement best describes the primary evolutionary significance of this change?
Answer: A genetic bottleneck occurred that may have reduced overall genetic variation regardless of the allele frequency change
When you encounter population genetics scenarios involving dramatic population size reductions, focus on distinguishing between different evolutionary forces and their primary effects.
This scenario describes a genetic bottleneck - a severe reduction in population size (from 10,000 to 100 individuals) that randomly eliminates most individuals regardless of their genetic makeup. The key insight is that while allele frequencies may change during a bottleneck, the most significant evolutionary consequence is the overall loss of genetic variation across the entire genome, not changes in any single allele.
Answer C correctly identifies that a genetic bottleneck occurred and emphasizes its primary significance: reduced overall genetic variation. Even though the W allele frequency increased, this change was likely due to random sampling effects (genetic drift) rather than adaptive advantage, and the population's total genetic diversity across all loci would have decreased substantially.
Answer A incorrectly assumes the frequency change resulted from natural selection favoring drought resistance. However, frequency changes during bottlenecks are typically random, and we have no evidence that W provides drought resistance.
Answer B misidentifies this as a founder effect (which occurs when a small group establishes a new population) and incorrectly claims genetic diversity increased - bottlenecks actually reduce diversity.
Answer D suggests gene flow from migration, but the scenario describes population reduction due to drought mortality, not immigration of new individuals.
Remember: In bottleneck questions, focus on the population-wide loss of genetic variation as the primary evolutionary consequence, even when specific allele frequencies change dramatically.
- In a population genetics study, researchers find that a population has significantly fewer homozygotes and more heterozygotes than predicted by Hardy-Weinberg equilibrium. Which scenario most likely explains this observation?
Answer: Population subdivision exists, with limited mating between subgroups of different allele frequencies
When you encounter population genetics problems involving deviations from Hardy-Weinberg equilibrium, focus on identifying which forces are acting on the population. Hardy-Weinberg predicts specific ratios of homozygotes to heterozygotes, so deviations tell us about evolutionary processes at work.
The key observation here is fewer homozygotes and more heterozygotes than expected. This pattern points to forces that prevent random mating or create population structure.
Population subdivision (C) perfectly explains this observation. When a population is divided into subgroups with different allele frequencies and limited mating between groups, you get what's called the Wahlund effect. Each subgroup may be in Hardy-Weinberg equilibrium internally, but when you analyze the total population, you'll find a deficiency of homozygotes and excess of heterozygotes compared to what you'd expect from the overall allele frequencies.
Choice A is backwards—inbreeding increases homozygosity, not decreases it. Choice B (assortative mating) would also increase homozygotes as similar individuals mate together. Choice D describes heterozygote advantage, but this maintains allele frequencies through natural selection rather than directly changing Hardy-Weinberg genotype ratios in the way described.
Remember this pattern: when you see excess heterozygotes in population genetics, think population subdivision or migration between populations with different allele frequencies. When you see excess homozygotes, think inbreeding or population bottlenecks. This distinction will help you quickly identify the correct mechanism in Hardy-Weinberg deviation problems.
- A population of beetles has three alleles (A₁, A₂, A₃) at a locus controlling wing length. The current frequencies are: A₁ = 0.5, A₂ = 0.3, A₃ = 0.2. Climate change creates strong selection favoring longer wings (A₁ allele). If the selection coefficient against A₂ is 0.1 and against A₃ is 0.3, which statement best predicts the evolutionary outcome?
Answer: A₁ will eventually reach fixation, but A₃ will be lost more rapidly than A₂ due to stronger selection against it
When analyzing natural selection with multiple alleles, you need to consider how selection coefficients affect the rate at which alleles are eliminated from a population. Selection coefficients measure the fitness disadvantage of each genotype - higher values mean stronger selection against that allele.
In this beetle population, A₁ is favored (selection coefficient = 0), while A₂ has a selection coefficient of 0.1 and A₃ has 0.3. This means A₃ experiences three times stronger selection pressure than A₂. Under directional selection favoring A₁, both A₂ and A₃ will decline in frequency, but A₃ will decline much faster due to its higher selection coefficient. Eventually, A₁ will reach fixation (frequency = 1.0) since it has the highest fitness.
Option A is incorrect because genetic variation isn't automatically maintained - strong directional selection typically reduces variation by eliminating less fit alleles. Option C misses the crucial point that selection coefficients differ between A₂ and A₃, so they won't be lost at the same rate. Option D is wrong because this scenario describes directional selection, not balancing selection - there's no mechanism here to maintain all three alleles at equilibrium.
Option B correctly predicts that A₁ will eventually fix in the population, and that A₃ will disappear faster than A₂ due to the stronger selection against it (0.3 vs 0.1).
Study tip: Remember that selection coefficient magnitude directly correlates with elimination speed - alleles with higher selection coefficients against them disappear from populations more rapidly than those with lower coefficients.
- Two alleles (F and f) exist in a population where FF individuals have fitness 1.0, Ff individuals have fitness 1.1, and ff individuals have fitness 0.9. What type of selection is operating, and what will happen to allele frequencies over time?
Answer: Overdominance (heterozygote advantage); both alleles will be maintained in the population
When you encounter fitness values for different genotypes, you need to identify the type of natural selection by comparing the relative fitness of homozygotes versus heterozygotes.
Looking at the fitness values: FF = 1.0, Ff = 1.1, and ff = 0.9, notice that the heterozygote (Ff) has the highest fitness at 1.1. This immediately signals overdominance or heterozygote advantage. In this scenario, the heterozygote outperforms both homozygotes, creating a selective pressure that maintains both alleles in the population. Neither allele can reach fixation because eliminating either one would reduce the frequency of the superior heterozygote genotype.
Option A is wrong because directional selection favoring F would require FF individuals to have the highest fitness, but they don't (1.0 < 1.1). Option B is incorrect for the same reason regarding the f allele—ff individuals have the lowest fitness at 0.9, not the highest. Option D describes underdominance, which occurs when heterozygotes have lower fitness than both homozygotes, but here the heterozygote has the highest fitness.
The correct answer is C. With overdominance, both F and f alleles will persist in the population at stable frequencies determined by their relative fitness values. The population will reach an equilibrium where the frequency of heterozygotes is maximized.
Study tip: Remember that heterozygote advantage always maintains genetic diversity—it's evolution's way of "hedging its bets" by keeping multiple alleles available rather than allowing fixation of just one.
- A population of beetles shows the following genotype frequencies: AA = 0.25, Aa = 0.60, aa = 0.15. What can be concluded about this population's evolutionary status?
Answer: The population is evolving because there is an excess of heterozygotes compared to Hardy-Weinberg expectations
When you encounter genotype frequency problems, you need to test whether the population follows Hardy-Weinberg equilibrium by comparing observed frequencies to expected frequencies based on allele frequencies.
First, calculate the allele frequencies from the given data. Since AA = 0.25 and Aa = 0.60, the frequency of allele A is: 0.25 + (0.60/2) = 0.55. The frequency of allele a is: 0.15 + (0.60/2) = 0.45.
Now apply Hardy-Weinberg predictions. If the population were in equilibrium, expected genotype frequencies would be: AA = (0.55)2=0.30, Aa = 2(0.55)(0.45)=0.50, and aa = (0.45)2=0.20.
Comparing observed vs. expected: AA (0.25 vs 0.30), Aa (0.60 vs 0.50), aa (0.15 vs 0.20). The population has fewer homozygotes and more heterozygotes than expected, indicating evolutionary forces are acting.
Answer A is wrong because simply summing to 1.0 is a mathematical requirement, not evidence of equilibrium. Answer C incorrectly assumes that allele frequency relationships indicate evolutionary status—they don't. Answer D misunderstands equilibrium; having both alleles present doesn't guarantee Hardy-Weinberg conditions are met.
The excess heterozygotes in answer B could result from factors like population mixing, assortative mating patterns, or recent population bottlenecks followed by recovery.
Study tip: Always calculate expected Hardy-Weinberg frequencies and compare them to observed data. Don't rely on superficial patterns—do the math to determine if evolution is occurring. - A population of 10,000 mice has an allele frequency of 0.6 for the dominant allele (A) and 0.4 for the recessive allele (a) at a coat color locus. If the population is in Hardy-Weinberg equilibrium, how many mice would be expected to have the heterozygous genotype (Aa)?
Answer: 4,800 mice
When you encounter Hardy-Weinberg equilibrium problems, you're applying a fundamental principle that predicts genotype frequencies in stable populations. The Hardy-Weinberg equation states that if p is the frequency of the dominant allele and q is the frequency of the recessive allele, then genotype frequencies are: p2 (AA), 2pq (Aa), and q2 (aa).
Given p = 0.6 for allele A and q = 0.4 for allele a, you calculate the heterozygous frequency using 2pq=2(0.6)(0.4)=0.48. In a population of 10,000 mice, this means 0.48×10,000=4,800 heterozygous mice.
Looking at the wrong answers: Choice A (2,400) represents exactly half the correct answer, suggesting a calculation error where someone forgot to multiply by 2 in the 2pq formula. Choice C (3,600) equals p2×10,000, which would be the number of homozygous dominant (AA) individuals, not heterozygotes. Choice D (6,000) equals p×10,000, confusing allele frequency with genotype frequency.
The correct answer is B: 4,800 mice.
Remember this pattern: Hardy-Weinberg problems always follow the same steps. First, identify p and q values, then apply the appropriate formula (2pq for heterozygotes), and finally multiply by population size. Watch out for the common trap of forgetting the "2" in the heterozygote formula—heterozygotes can form in two ways (A from mom, a from dad OR a from mom, A from dad). - In a population of insects, researchers observe that the frequency of the A allele is 0.4 in newborns but 0.5 in reproducing adults. The frequency of the a allele shows the opposite pattern (0.6 in newborns, 0.5 in adults). What evolutionary process best explains this observation?
Answer: Selection against the aa genotype is increasing the frequency of the A allele from birth to reproduction
When you encounter allele frequency changes within a single generation (from birth to reproductive age), you're looking at selection acting during the organism's lifetime, not between generations.
The data shows a clear pattern: the A allele frequency increases from 0.4 to 0.5, while the a allele frequency decreases from 0.6 to 0.5 as individuals mature from newborns to reproducing adults. This shift suggests that individuals with certain genotypes are less likely to survive to reproductive age.
Since both allele frequencies move toward 0.5, and we know that selection against a recessive genotype (aa) would reduce the frequency of the a allele while increasing the A allele frequency, option C correctly identifies selection against the aa genotype. Individuals with the aa genotype are dying before they can reproduce, shifting the allele frequencies in the surviving adult population.
Option A is incorrect because genetic drift causes random changes between generations, not the systematic, directional change within a generation that we observe here. Option B is wrong because heterozygote advantage would maintain stable frequencies across generations, not cause this within-generation shift. Additionally, both alleles converging on 0.5 isn't evidence of heterozygote advantage unless we see this pattern maintained over multiple generations. Option D is impossible because mutations don't occur at rates high enough to cause such dramatic frequency changes during an individual's lifetime.
Remember: when you see allele frequencies changing within a single generation, think selection pressure acting on survival or reproduction, not drift or mutation.
- The frequency of a recessive lethal allele in a population is 0.01. Assuming Hardy-Weinberg equilibrium and that homozygous recessive individuals die before reproducing, what will be the frequency of this allele in the next generation?
Answer: 0.0099
When you encounter Hardy-Weinberg problems involving lethal alleles, you need to account for how selection against lethal genotypes changes allele frequencies between generations.
Let's work through this step-by-step. Initially, the recessive lethal allele has frequency q=0.01, so the dominant allele has frequency p=0.99. In the current generation, genotype frequencies are: AA=p2=0.9801, Aa=2pq=0.0198, and aa=q2=0.0001.
Since homozygous recessive individuals (aa) die before reproducing, only AA and Aa individuals contribute to the next generation's gene pool. The surviving population consists of 0.9801 AA individuals and 0.0198 Aa individuals, totaling 0.9999 survivors.
Among survivors, the frequency of the recessive allele comes only from heterozygotes (since AA individuals carry no recessive alleles). The Aa individuals contribute 0.0198×0.5=0.0099 recessive alleles per total survivor. Therefore, q′=0.99990.0099≈0.0099.
Answer A (0.005) incorrectly halves the original frequency. Answer C (0.01) assumes no change, ignoring the lethal effect. Answer D (0.0198) mistakenly uses the heterozygote frequency rather than calculating the actual allele frequency among survivors.
The correct answer is B (0.0099).
Study tip: For recessive lethal alleles, use the formula q′=1+qq where q is the current allele frequency. This saves time on calculations and helps you recognize the pattern that lethal alleles always decrease in frequency. - In a large randomly mating population, the frequency of a dominant allele (D) is 0.7 and the frequency of the recessive allele (d) is 0.3. If individuals with the dd genotype have a 20% reduction in fitness compared to DD and Dd individuals, what will be the approximate frequency of the d allele after one generation of selection?
Answer: 0.27
This question tests your understanding of Hardy-Weinberg equilibrium and natural selection. When you see allele frequencies and fitness differences, you need to calculate how selection pressure changes allele frequencies over time.
Start with the initial conditions: D frequency = 0.7, d frequency = 0.3. The dd genotype has 20% reduced fitness, meaning its relative fitness is 0.8 compared to 1.0 for DD and Dd genotypes.
First, calculate initial genotype frequencies using Hardy-Weinberg: DD = (0.7)2=0.49, Dd = 2(0.7)(0.3)=0.42, dd = (0.3)2=0.09.
Next, apply selection by multiplying each genotype frequency by its fitness: DD contributes 0.49×1.0=0.49, Dd contributes 0.42×1.0=0.42, and dd contributes 0.09×0.8=0.072 to the next generation.
The mean population fitness is 0.49+0.42+0.072=0.982. After selection, the new genotype frequencies are: DD = 0.49/0.982=0.499, Dd = 0.42/0.982=0.428, dd = 0.072/0.982=0.073.
The new d allele frequency equals the dd frequency plus half the Dd frequency: 0.073+(0.428/2)=0.287≈0.27.
Answer B (0.27) is correct. Answer A (0.24) underestimates the remaining d alleles, C (0.30) incorrectly assumes no selection occurred, and D (0.33) paradoxically increases the selected-against allele.
Remember: selection against recessive alleles is relatively slow because most recessive alleles hide in heterozygotes where they're not selected against. - A marine biologist compares two populations of fish: Population X (effective size = 50) and Population Y (effective size = 5,000). Both start with the same allele frequencies. After 100 generations with no selection, migration, or mutation, which prediction about genetic diversity is most accurate?
Answer: Population Y will have much higher genetic diversity because genetic drift effects are minimal in large populations
When you encounter questions about population genetics over time, focus on how population size affects genetic drift—the random changes in allele frequencies that occur in all populations.
Genetic drift has an inverse relationship with effective population size. In small populations, random sampling effects are magnified because each individual represents a larger fraction of the gene pool. When only 50 individuals reproduce (Population X), chance events can dramatically shift allele frequencies between generations. In contrast, when 5,000 individuals reproduce (Population Y), random fluctuations average out, and allele frequencies remain much more stable.
Over 100 generations without selection, migration, or mutation, Population Y will retain much higher genetic diversity because genetic drift effects are minimal in large populations, making answer C correct.
Answer A incorrectly assumes starting conditions determine final outcomes—but genetic drift operates continuously, regardless of initial allele frequencies. Answer B contains a fundamental misconception: while small populations may show faster changes in allele frequencies, this "evolution" represents random loss of genetic variation, not adaptive improvement or increased diversity. Answer D provides specific percentages that seem precise but aren't supported by the general relationship described—the exact values would require complex calculations involving initial allele numbers and specific drift formulas.
Study tip: Remember that genetic drift always reduces genetic diversity over time, but small populations lose diversity much faster than large ones. When you see effective population size comparisons, immediately think about drift intensity—smaller populations experience stronger drift and greater genetic diversity loss.
- In a large population of flowering plants, two alleles (R and r) control flower color. The fitness values are: RR = 1.0, Rr = 0.95, rr = 0.8. If the initial frequency of the r allele is 0.4, what will happen to this allele frequency over time?
Answer: It will decrease because the r allele reduces fitness in both heterozygous and homozygous conditions
When you encounter fitness values in population genetics, you're dealing with natural selection acting on different genotypes. Fitness measures reproductive success, where 1.0 represents maximum fitness and lower values indicate reduced survival or reproduction.
To determine how allele frequency changes, examine how selection affects each genotype. Here, RR individuals have the highest fitness (1.0), while Rr individuals have intermediate fitness (0.95), and rr individuals have the lowest fitness (0.8). Since the r allele contributes to reduced fitness in both heterozygous (Rr) and homozygous (rr) conditions, natural selection will consistently favor the R allele over time.
With an initial r allele frequency of 0.4, individuals carrying r alleles will produce fewer offspring on average than those with R alleles. This means the proportion of R alleles will increase in subsequent generations while r allele frequency decreases.
Option A is incorrect because intermediate heterozygote fitness doesn't cause allele frequency increases—what matters is the overall selective disadvantage of r. Option B misunderstands natural selection; allele presence doesn't guarantee constant frequencies when fitness differences exist. Option D describes balancing selection, which would require heterozygote advantage (fitness > 1.0) or other special conditions not present here.
Option C correctly identifies that the r allele reduces fitness in both Rr and rr genotypes, making it selectively disadvantageous.
Study tip: When analyzing selection problems, always compare each genotype's fitness to determine which alleles are favored. Declining fitness associated with an allele predicts its frequency will decrease over time.
- Gene flow occurs between two populations of the same species. Population A has an allele frequency of 0.8 for allele X, while Population B has an allele frequency of 0.2 for allele X. If 10% of Population B consists of migrants from Population A each generation, what will be the new allele frequency for X in Population B after one generation of migration?
Answer: 0.26
When you encounter gene flow problems, you're dealing with how migration changes allele frequencies in populations. The key is understanding that migrants bring their own allele frequencies, which then mix with the recipient population's frequencies.
To solve this, use the gene flow equation: p′=p(1−m)+pm⋅m, where p′ is the new frequency, p is the original frequency in the recipient population, m is the migration rate, and pm is the migrant frequency.
Here, Population B (recipient) has frequency 0.2, Population A (source) has frequency 0.8, and migration rate is 0.1 (10%). Plugging in: p′=0.2(1−0.1)+0.8(0.1)=0.2(0.9)+0.08=0.18+0.08=0.26
Choice A (0.26) is correct - this represents the weighted average of the two populations' frequencies based on their proportional contributions.
Choice B (0.32) might result from incorrectly adding the migration effect twice or miscalculating the weights. Choice C (0.50) represents the simple average of both frequencies, ignoring that only 10% are migrants. Choice D (0.68) appears to reverse the populations or incorrectly weight the calculation heavily toward the migrant frequency.
Remember this formula pattern: the new frequency equals the recipient population's contribution plus the migrant population's contribution, each weighted by their respective proportions. Gene flow problems always involve this weighted averaging concept.
