401. Population P is at Hardy-Weinberg equilibrium with \(p=0.80,\ q=0.20\). Population Q is also at equilibrium but has \(p=0.20,\ q=0.80\). Which relationship is correct?
ⓐ. P has more heterozygotes than Q because \(p\) is larger
ⓑ. Q has more heterozygotes than P because \(q\) is larger
ⓒ. Both populations have identical frequencies of both homozygous genotypes
ⓓ. same heterozygote frequency, with reversed homozygote frequencies
Correct Answer: same heterozygote frequency, with reversed homozygote frequencies
Explanation: Heterozygote frequency is \(2pq\). For both populations, the product of the allele frequencies is \(0.80\times0.20\), so each has a heterozygote frequency of \(2\times0.80\times0.20=0.32\). The homozygote distributions differ because the more common allele changes identity. In P, \(p^2=0.64\) and \(q^2=0.04\), whereas in Q those values are reversed. This relation shows that heterozygote frequency depends on the product of the two allele frequencies rather than on which named allele is common. Two populations can have the same heterozygote proportion while having opposite patterns of homozygous genotypes.
402. In a population at Hardy-Weinberg equilibrium, genotype aa initially occurs at frequency \(0.04\). Several generations later, aa occurs at frequency \(0.09\), and equilibrium proportions are again stated to hold. Which change occurred?
ⓐ. \(q\) increased from \(0.20\) to \(0.30\), while \(p\) decreased from \(0.80\) to \(0.70\)
ⓑ. \(q\) increased from \(0.04\) to \(0.09\), while \(p\) decreased from \(0.96\) to \(0.91\)
ⓒ. \(p\) increased from \(0.20\) to \(0.30\), while \(q\) decreased from \(0.80\) to \(0.70\)
ⓓ. Neither allele frequency changed because equilibrium proportions occur at both times
Correct Answer: \(q\) increased from \(0.20\) to \(0.30\), while \(p\) decreased from \(0.80\) to \(0.70\)
Explanation: At equilibrium, the recessive homozygote frequency is \(q^2\). Initially, \(q^2=0.04\), so \(q=0.20\) and \(p=0.80\). Later, \(q^2=0.09\), giving \(q=0.30\) and \(p=0.70\). The allele frequencies have changed despite both sampled populations individually fitting Hardy-Weinberg genotype proportions. Equilibrium at a particular time describes the relationship among genotypes for the allele frequencies then present; it does not guarantee that those allele frequencies have remained unchanged throughout history. The shift from \(q=0.20\) to \(q=0.30\) represents population-level evolutionary change at the locus. A Hardy-Weinberg calculation is valid only under the stated equilibrium condition; the algebra predicts genotype proportions from the current allele frequencies and does not by itself identify any evolutionary force. Counting alleles in a diploid sample requires \(2N\) allele copies. Each homozygote contributes two copies of one allele, whereas each heterozygote contributes one copy of each allele.
403. Four populations are surveyed at a two-allele locus.
| Population | Allele frequency \(p\) | Observed \(AA\) | Observed \(Aa\) | Observed \(aa\) |
|---|
| P | \(0.50\) | \(0.25\) | \(0.40\) | \(0.35\) |
| Q | \(0.70\) | \(0.49\) | \(0.39\) | \(0.12\) |
| R | \(0.80\) | \(0.64\) | \(0.32\) | \(0.04\) |
| S | \(0.60\) | \(0.30\) | \(0.54\) | \(0.16\) |
Which population exactly matches Hardy-Weinberg expectations?
ⓐ. Population P
ⓑ. Population Q
ⓒ. Population R
ⓓ. Population S
Correct Answer: Population R
Explanation: For Population R, \(p=0.80\), so \(q=0.20\). Hardy-Weinberg equilibrium predicts \(p^2=0.64\), \(2pq=0.32\) and \(q^2=0.04\), exactly matching the observed genotype frequencies. The other populations contain at least one mismatch between observed and expected values. For example, equal allele frequencies in P would predict \(0.25,\ 0.50,\ 0.25\), not the listed distribution. The task requires interpreting the full genotype pattern rather than simply checking whether frequencies sum to \(1\). Hardy-Weinberg equilibrium is a specific quantitative relation between allele frequencies and genotype frequencies, not merely a statement that all genetic categories are represented.
404. A recipient population contains \(300\) diploid individuals with allele A at frequency \(0.70\). Another \(100\) diploid individuals migrate in from a population where A has frequency \(0.10\). If all individuals contribute equally to the mixed gene pool, what is the new frequency of A?
ⓐ. \(0.40\)
ⓑ. \(0.55\)
ⓒ. \(0.60\)
ⓓ. \(0.65\)
Correct Answer: \(0.55\)
Explanation: The recipient population contributes \(600\) allele copies, of which \(600\times0.70=420\) are A. The migrants contribute \(200\) allele copies, of which \(200\times0.10=20\) are A. After mixing, the population contains \(420+20=440\) copies of A among \(800\) total allele copies. Thus
\[
f(A)=\frac{440}{800}=0.55
\]
This calculation shows how gene flow can change allele frequency without creating a new allele. The migrants simply contribute a different proportion of alleles to the recipient gene pool. Since their A frequency is much lower, their arrival reduces the overall A frequency from \(0.70\) to \(0.55\). Gene flow can alter the frequency of an allele that already exists in both populations. A new mutation is unnecessary when the two populations begin with different frequencies of the same allele. Gene-flow calculations are weighted by genetic contribution: a larger group contributes more allele copies, so the combined frequency cannot be found by simply averaging the two starting frequencies unless group sizes are equal.
405. Populations P and Q occupy neighbouring habitats and exchange many reproducing migrants each generation. Populations R and S occupy comparable habitats but remain isolated. Local environmental conditions differ between the paired habitats. Which long-term pattern is most plausible?
ⓐ. gene flow can reduce P-Q divergence relative to isolated R-S divergence
ⓑ. Migration guarantees that P and Q become genetically identical at every locus
ⓒ. Isolation forces R and S to evolve in the same direction
ⓓ. Gene flow necessarily strengthens every local selective difference
Correct Answer: gene flow can reduce P-Q divergence relative to isolated R-S divergence
Explanation: Reproducing migrants transfer alleles between P and Q, repeatedly mixing their gene pools. Even when the two habitats impose different local selective pressures, substantial gene flow can oppose complete divergence by introducing alleles from one population into the other. R and S lack this homogenising exchange, so local selection and genetic drift can produce greater genetic differences over time. Gene flow does not guarantee complete identity as selection, mutation and drift can continue to act, but it generally provides a force that reduces differences produced in isolation. The comparison demonstrates that geographic connectivity affects how strongly populations can diverge, especially when local evolutionary forces act in opposite directions.
406. Two populations have been kept genetically similar by strong gene flow despite occupying habitats that favour different heritable phenotypes. A geographical barrier suddenly stops migration while the environmental differences remain. Which change is most likely over subsequent generations?
ⓐ. Both populations must immediately lose all genetic variation
ⓑ. Allele frequencies must remain similar because historical gene flow cannot be reversed
ⓒ. Natural selection must stop once migration ends
ⓓ. without gene flow, opposing local selection can drive stronger divergence
Correct Answer: without gene flow, opposing local selection can drive stronger divergence
Explanation: Before the barrier forms, migration continually transfers alleles between the two habitats and can reduce the genetic differences produced by local selection. Once the barrier stops gene flow, this homogenising influence is removed. If one phenotype has higher reproductive success in one habitat and another phenotype is favoured in the second, selection can now drive allele frequencies in different directions with less genetic input from the neighbouring population. Divergence need not occur instantly, and other forces may still act, but the barrier changes the balance among evolutionary processes. The scenario links geography with population genetics: interruption of gene flow can allow local selective differences to become more strongly reflected in the gene pools. Migration changes a recipient gene pool only when migrants contribute genetically to later generations; movement alone is not equivalent to gene flow in the population-genetic sense.
407. Researchers establish several replicate populations under two conditions. In Treatment P, carriers of allele R consistently produce more offspring, and R increases in nearly every replicate. In Treatment Q, R is neutral and its frequency rises in some replicates but falls in others. Which interpretation best fits the experiment?
ⓐ. Treatment P demonstrates drift, while Treatment Q demonstrates directional selection
ⓑ. Both treatments demonstrate only mutation
ⓒ. P shows repeatable selection, whereas Q's variable trajectories fit drift
ⓓ. Q proves that neutral alleles cannot change frequency
Correct Answer: P shows repeatable selection, whereas Q's variable trajectories fit drift
Explanation: Treatment P supplies both a fitness difference and a repeatable population response. Allele R is associated with greater reproductive success and increases consistently across replicate populations, which is the expected signature of natural selection. In Q, R has no fitness advantage, yet its frequency changes in different directions among independent populations. That stochastic variation is consistent with genetic drift. Replication is especially useful given that it helps distinguish repeatable directional effects from chance sampling. A neutral allele can certainly change frequency, so the Q result does not imply genetic stability. The experiment illustrates how selection and drift can generate superficially similar frequency changes in a single population but produce different patterns when replicated. Drift is strongest as a proportional sampling effect in small populations; a large frequency shift does not by itself prove that the affected allele is adaptive.
408. A new allele X first appears in Population P through mutation. Several generations later, X is detected for the first time in Population Q immediately after migrants from P reproduce there. Which relation is most appropriate?
ⓐ. mutation explains X in P; gene flow explains its later arrival in Q
ⓑ. Gene flow created X in P and mutation transferred it to Q
ⓒ. Recombination must independently create X in every population where it appears
ⓓ. Natural selection is required before an allele can move between populations
Correct Answer: mutation explains X in P; gene flow explains its later arrival in Q
Explanation: A genuinely new allelic state must originate through mutation or an equivalent source of new genetic sequence. In the scenario, X first appears in P, making mutation the relevant origin process. Once X exists, it need not mutate independently in every other population. Migrants carrying X can enter Q, reproduce and place the allele into the recipient gene pool. That later event is gene flow. The scenario separates the first origin of an allele from its geographical spread. Selection could later increase or decrease X depending on its fitness effects, but selection is not required simply for the allele to move. Evolutionary histories often involve several processes operating sequentially on the same variant.
409. Consider the following statements.
I. Founder effect is a form of genetic drift.
II. A neutral allele can become unusually common in founders by chance.
III. Founder effect requires the overrepresented allele to improve reproductive success.
IV. Genetic drift can also occur in small populations that were not newly founded.
ⓐ. I and III only
ⓑ. II and III only
ⓒ. I, II and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Founder effect occurs when a small group establishes a new population and, by chance, carries allele frequencies different from those of the source population. It is a special form of genetic drift and does not require the affected allele to be advantageous. Even a neutral allele can become common if it happens to be strongly represented among the founders. Genetic drift is broader than founder effect, however, because random allele-frequency changes can occur in any sufficiently small population through chance sampling. Statement III incorrectly turns a random sampling process into adaptive selection. The remaining statements correctly distinguish the specific founding situation from the more general phenomenon of genetic drift. The founder effect is a sampling form of drift: a small founding group can carry allele frequencies unlike those of the source population before local selection has had any opportunity to act.
410. Which record below is incorrectly matched with its most direct evolutionary process?
| Record | Observation | Assigned process |
|---|
| P | A new allele appears that was absent from the ancestral gene pool | Mutation |
| Q | Reproducing migrants introduce an existing allele into another population | Gene flow |
| R | A neutral allele fluctuates unpredictably in a small isolated population | Natural selection |
| S | Existing alleles occur in new combinations among gametes | Recombination |
ⓐ. Record P
ⓑ. Record R
ⓒ. Record Q
ⓓ. Record S
Correct Answer: Record R
Explanation: Records P, Q and S are correctly assigned. Mutation can generate a new allelic state, gene flow transfers alleles between populations through reproducing migrants, and recombination produces new combinations of existing alleles. Record R is different. An unpredictable change in a neutral allele within a small isolated population is characteristic of genetic drift, not natural selection. Selection requires a systematic difference in reproductive success associated with heritable variation. Since the allele is specified as neutral and the frequency trajectory is stochastic, no such fitness-based mechanism is supplied. The incorrect match is Record R. The table tests whether evolutionary processes are distinguished by their mechanisms rather than merely by the fact that all can influence genetic variation or population composition.
411. Assertion: A population that matches Hardy-Weinberg genotype proportions at one sampling time must never have experienced evolutionary change in the past.
Reason: Hardy-Weinberg proportions at one time describe the relationship between current allele frequencies and current genotype frequencies, not the entire prior history of those allele frequencies.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Assertion is false, but Reason is true
Explanation: The Assertion is false as a population can undergo an allele-frequency change and later display genotype proportions consistent with its new frequencies. Hardy-Weinberg equilibrium at a particular sampling point does not erase earlier evolutionary events. The Reason is true and explains the limitation of a single-time observation. The equations \(p^2\), \(2pq\) and \(q^2\) describe the expected genotype distribution for the allele frequencies currently present. To determine whether allele frequencies changed historically, samples from different times or other evidence are required. A later equilibrium state can follow migration, selection, drift or another event that previously altered the gene pool. When an observed recessive homozygote frequency is supplied, the square root gives the corresponding recessive allele frequency before \(p+q=1\) and the heterozygote relation can be used.
412. A graph has phenotypic value on the x-axis and frequency on the y-axis. Before selection, the population has one broad central peak. After selection, the central region is greatly reduced and two peaks appear near opposite extremes, while neither extreme is consistently favoured over the other. The pattern is:
ⓐ. stabilising selection
ⓑ. directional selection
ⓒ. disruptive selection
ⓓ. genetic drift
Correct Answer: disruptive selection
Explanation: The decisive graphical feature is the decline of the intermediate phenotype accompanied by increased representation at both ends of the distribution. When both extremes have greater reproductive success than intermediate individuals, selection produces a two-peaked or bimodal pattern. That is disruptive selection. Stabilising selection would do the opposite by concentrating the distribution around the centre, whereas directional selection would favour only one side and shift the population peak toward that extreme. The description also supplies a systematic phenotype-fitness relationship, making genetic drift an inappropriate interpretation. Drift can change frequencies randomly, but it does not predict a consistent two-extreme fitness pattern. The post-selection curve reflects disruption of the original central distribution. The founder effect is a sampling form of drift: a small founding group can carry allele frequencies unlike those of the source population before local selection has had any opportunity to act.
413. Three populations show the following changes in a heritable quantitative phenotype.
| Population | Mean after selection | Distributional change |
|---|
| P | Approximately unchanged | Narrower around the centre |
| Q | Shifted toward higher values | Peak displaced toward one end |
| R | Central class reduced | Two prominent extreme peaks |
Which mapping is correct?
ⓐ. P-stabilising, Q-directional, R-disruptive
ⓑ. P-directional, Q-disruptive, R-stabilising
ⓒ. P-disruptive, Q-stabilising, R-directional
ⓓ. P-stabilising, Q-disruptive, R-directional
Correct Answer: P-stabilising, Q-directional, R-disruptive
Explanation: Population P retains approximately the same central value but becomes more concentrated around it, the characteristic outcome of stabilising selection. Population Q moves toward one side of the phenotype range, which identifies directional selection. Population R loses representation in the centre while both extremes become prominent, matching disruptive selection. The three patterns are best distinguished by how reproductive success is distributed across the phenotype range and how that alters the frequency curve. Stabilising selection reduces extremes, directional selection favours one extreme, and disruptive selection favours both extremes relative to intermediates. The table integrates changes in mean, spread and curve shape rather than requiring recognition of a single isolated graphical feature.
414. Match each reproductive pattern with its expected selection outcome. A Column II entry is used once.
| Column I | Column II |
|---|
| P. Intermediate phenotypes reproduce most successfully | 1. Peak shifts toward one end |
| Q. One extreme has the greatest reproductive success | 2. No consistent phenotype-based selective shift |
| R. Both extremes outperform intermediates | 3. Distribution narrows around the centre |
| S. All phenotype classes have equal reproductive success | 4. Two extreme peaks increase |
ⓐ. P-1, Q-3, R-2, S-4
ⓑ. P-4, Q-2, R-3, S-1
ⓒ. P-2, Q-4, R-1, S-3
ⓓ. P-3, Q-1, R-4, S-2
Correct Answer: P-3, Q-1, R-4, S-2
Explanation: When intermediate phenotypes reproduce most successfully, both extremes are selected against and the distribution becomes narrower around the centre, giving P-3. If one extreme has the greatest reproductive success, the population shifts toward that end, so Q-1. When both extremes outperform intermediates, their representation increases on both sides and the distribution can become two-peaked, giving R-4. If all phenotype classes have equal reproductive success with respect to the trait, there is no consistent selection favouring one region of the distribution, so S-2. The mapping links the fitness pattern to the expected population-level curve rather than relying only on memorised names of selection types. Fitness is context-dependent reproductive success. A genotype favoured in one environment can lose its advantage after conditions change, so the direction of selection can also change. Population evolution is detected across generations. An individual can survive or change physiologically without that change becoming an inherited shift in population composition.
415. In several replicate populations, genotype H is favoured in Environment P and increases in frequency. Researchers then transfer the populations to Environment Q, where genotype L consistently leaves more offspring, and the allele-frequency trend reverses in nearly every replicate. What does this result most strongly support?
ⓐ. Random drift acting in the same direction in both environments
ⓑ. environment-dependent selection can reverse frequency-change direction
ⓒ. Mutation rates determining which genotype is reproductively successful
ⓓ. Permanent superiority of genotype H
Correct Answer: environment-dependent selection can reverse frequency-change direction
Explanation: The experiment changes the environment and observes a corresponding reversal in both reproductive success and allele-frequency trajectory. In Environment P, H has the fitness advantage and rises. In Environment Q, L becomes reproductively superior and the population trend reverses. Replication strengthens the interpretation given that the same environment-dependent pattern recurs across independent populations. Random drift would not be expected to produce such a consistent reversal aligned with the measured fitness difference. The experiment demonstrates that fitness is conditional rather than permanently attached to one genotype. Natural selection sorts heritable variants according to their consequences under current conditions, so changing the environment can change both the favoured genotype and the direction of evolution.
416. A small island population begins with allele frequencies strongly different from the mainland because of founder effect. Several generations later, a permanent land connection forms and large numbers of mainland individuals migrate to the island and reproduce every generation. What is the most plausible long-term effect?
ⓐ. The founder effect becomes stronger because migration increases isolation
ⓑ. The island must retain its original founder frequencies permanently
ⓒ. Migration creates new alleles but cannot change frequencies of existing ones
ⓓ. sustained gene flow can reduce the founder-related island-mainland difference
Correct Answer: sustained gene flow can reduce the founder-related island-mainland difference
Explanation: Founder effect establishes an unusual starting gene pool through chance sampling by a small colonising group. That historical signature can persist when the population remains isolated. Once a permanent connection permits large-scale repeated migration, however, mainland alleles continually enter the island population through reproducing immigrants. This gene flow can shift island allele frequencies toward those of the mainland and progressively weaken the original founder-associated difference. The founder event still occurred historically, but later evolutionary processes can modify its genetic consequences. Population history is dynamic: an early drift event can be followed by extensive gene flow that partly erases the initial divergence. Isolation and connectivity have opposite effects on the persistence of founder signatures.
417. Two populations begin with the same allele frequencies. Population P remains small and isolated, while Population Q receives many migrants every generation from a large neighbouring population with similar allele frequencies. Which contrast is most appropriate?
ⓐ. Both conditions necessarily increase divergence from neighbouring populations
ⓑ. drift can diverge P, while repeated gene flow limits divergence in Q
ⓒ. Gene flow in Q acts exactly like founder effect
ⓓ. Isolation in P prevents any allele-frequency change
Correct Answer: drift can diverge P, while repeated gene flow limits divergence in Q
Explanation: Population P is small and isolated, two conditions that make random genetic drift especially important. Chance differences in transmission can shift allele frequencies and cause P to diverge from other populations even without a selective advantage. Q experiences the contrasting process: reproducing migrants repeatedly introduce alleles from a neighbouring gene pool. When frequencies are broadly similar, this continuing exchange tends to keep the populations genetically connected and limits independent divergence. Gene flow and drift can have opposing effects on differences among populations. Isolation does not freeze a gene pool; it removes migration while leaving mutation, selection and especially drift free to alter frequencies. The comparison highlights how population connectivity influences evolutionary divergence.
418. Every individual in a population is genetically \(AABB\) at two loci. Recombination occurs normally during gamete formation. Which conclusion is most accurate for these loci?
ⓐ. Recombination will create new alleles a and b
ⓑ. Recombination must generate AB, Ab, aB and ab gametes
ⓒ. recombination cannot create new combinations without alternative alleles to reshuffle
ⓓ. Recombination necessarily changes allele frequencies even in a genetically uniform population
Correct Answer: recombination cannot create new combinations without alternative alleles to reshuffle
Explanation: Recombination rearranges genetic variants that already exist; it does not create an alternative allele simply because gamete formation occurs. Every individual here is \(AABB\), so all chromosomes carry the same allelic state at both specified loci. There is nothing different to reshuffle into a new combination, and the relevant gametes remain AB. Mutation would be required to introduce a or b as genuinely new alleles. This changed condition reveals an important limitation of recombination as a source of variation: its ability to generate different combinations depends on genetic variation already being present. Recombination can greatly increase combinational diversity in variable populations, but it cannot manufacture diversity from a completely uniform set of alleles.
419. Assertion: A beneficial allele present as a single copy in a very small population can sometimes be lost before it becomes common.
Reason: Genetic drift always moves allele frequencies in the direction opposite to natural selection.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Assertion is true, but Reason is false
Explanation: The Assertion is true since a newly arisen beneficial allele can begin at extremely low frequency. In a small population, chance sampling is strong, and the carrier may fail to reproduce or fail to transmit the allele before its selective advantage produces a substantial population-level effect. The Reason is false since genetic drift has no fixed direction relative to selection. Random sampling can oppose a selective trend, reinforce it or produce little net effect in a particular generation. Selection is directional with respect to fitness differences, whereas drift is stochastic. Their interaction is especially important when an allele is rare and population size is small, since chance can influence whether a potentially advantageous variant survives long enough for selection to increase it.
420. Arrange the following events into the most appropriate sequence for founder effect.
P. A source population contains a particular set of allele frequencies.
Q. A small, non-representative group colonises a new habitat.
R. The founders begin the new population with allele frequencies different from the source.
S. Descendants increase in number from this altered starting gene pool.
ⓐ. \(Q\rightarrow P\rightarrow S\rightarrow R\)
ⓑ. \(P\rightarrow Q\rightarrow R\rightarrow S\)
ⓒ. \(P\rightarrow R\rightarrow Q\rightarrow S\)
ⓓ. \(R\rightarrow P\rightarrow Q\rightarrow S\)
Correct Answer: \(P\rightarrow Q\rightarrow R\rightarrow S\)
Explanation: Founder effect begins with genetic variation already present in a source population. A small group leaves or becomes isolated and establishes a new population. As that founding group is only a sample of the source population, its allele frequencies can differ substantially by chance. Those altered frequencies become the genetic starting point from which the new population expands. The key shift occurs before extensive reproduction in the new habitat, distinguishing founder effect from a later selective change caused by local fitness differences. Population growth can preserve or further modify the initial founder pattern, but the defining event is the non-representative sampling that occurs when the colony is first established.
