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Biology Help: Use Probability For Trait Frequency

Review real example questions for Use Probability For Trait Frequency in Biology.

Question 1 / 10

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A rabbit population has two coat colors: white and brown. In a snowy habitat, white rabbits have a 75% chance of surviving to reproduce, while brown rabbits have a 25% chance. If the habitat stays snowy, which outcome is most likely after many generations?

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Question 1

Brown coat color becomes more common because it is currently less common
Coat color frequencies do not change because survival is not related to reproduction
White coat color becomes more common because white rabbits are more likely to survive and reproduce (correct answer)
Individual brown rabbits will turn white during winter, increasing white frequency without selection

Explanation: This question tests your ability to use probability and differential survival/reproduction data to predict how trait and allele frequencies change in populations over time through natural selection. Probability reasoning for evolution: when different variants have different survival or reproduction probabilities, this creates PREDICTABLE changes in allele frequencies: if individuals with allele A have 90% survival probability while individuals with allele a have 30% survival probability (large probability difference), then A individuals contribute disproportionately more offspring to next generation, causing A allele frequency to INCREASE and a allele frequency to DECREASE. The DIRECTION of change is predictable (higher survival/reproduction → increase frequency, lower survival/reproduction → decrease frequency), and the RATE depends on probability differences (larger differences = stronger selection = faster change, smaller differences = weaker selection = slower change). In this snowy habitat, white rabbits have 75% survival probability while brown rabbits have only 25% survival probability—this 50 percentage point difference means white rabbits contribute three times as many offspring to future generations, causing white coat color frequency to increase over time. Choice C correctly predicts that white coat color becomes more common because white rabbits are more likely to survive and reproduce in the snowy environment. Choice A incorrectly assumes rare traits increase (selection favors beneficial traits regardless of current frequency), B wrongly claims survival doesn't affect reproduction (survivors are the ones who reproduce!), and D misunderstands evolution (individuals can't change their genes—populations evolve through differential survival). Predicting frequency changes from probabilities: (1) IDENTIFY survival probabilities: White coat: 75% survive to reproduce. Brown coat: 25% survive to reproduce. (2) COMPARE probabilities: White rabbits have 3× higher survival (75% vs 25%). (3) PREDICT direction: Higher probability variant (white) → frequency INCREASES. Lower probability variant (brown) → frequency DECREASES. (4) ASSESS magnitude: LARGE probability difference (50 percentage points) → RAPID frequency change. Real-world example: snowshoe hares change from brown in summer to white in winter, but with climate change reducing snow cover, brown hares now survive better during shorter winters, causing evolutionary change in molt timing—probability of survival drives evolution!

Question 2

A population of insects has genotypes $BB$ , $Bb$ , and $bb$ for a gene affecting toxin resistance. In an environment with the toxin, survival probabilities are: $BB$ : 85%, $Bb$ : 40%, $bb$ : 10%. Which statement best predicts what happens to allele $B$ ?

Allele $B$ decreases because $Bb$ has lower survival than $BB$
Allele $B$ increases because genotypes with $B$ (especially $BB$ ) survive much more often than $bb$ (correct answer)
Allele $B$ stays the same because selection affects individuals, not allele frequencies
Allele $B$ increases only if the toxin causes new mutations to create more $B$ alleles

Explanation: This question tests your ability to use probability and differential survival/reproduction data to predict how trait and allele frequencies change in populations over time through natural selection. Probability reasoning for evolution: when different variants have different survival or reproduction probabilities, this creates PREDICTABLE changes in allele frequencies: if individuals with allele A have 90% survival probability while individuals with allele a have 30% survival probability (large probability difference), then A individuals contribute disproportionately more offspring to next generation, causing A allele frequency to INCREASE and a allele frequency to DECREASE. The DIRECTION of change is predictable (higher survival/reproduction → increase frequency, lower survival/reproduction → decrease frequency), and the RATE depends on probability differences (larger differences = stronger selection = faster change, smaller differences = weaker selection = slower change). In this insect population exposed to toxin, survival probabilities are BB: 85%, Bb: 40%, bb: 10%—while Bb has intermediate survival, BB has much higher survival than both other genotypes, and overall, B-containing genotypes survive better than bb, so B allele frequency will increase over time. Choice B correctly predicts that allele B increases because genotypes with B (especially BB at 85%) survive much more often than bb (only 10%), even though Bb is intermediate. Choice A incorrectly focuses only on Bb being lower than BB (missing that both are higher than bb), C wrongly claims selection doesn't affect allele frequencies, and D incorrectly requires new mutations rather than selection on existing variation. Predicting frequency changes from probabilities: (1) IDENTIFY survival probabilities: BB: 85%, Bb: 40%, bb: 10%. (2) ANALYZE allele representation: B appears in BB (85% survival) and Bb (40% survival). b appears in Bb (40% survival) and bb (10% survival). (3) CALCULATE weighted average: B allele "sees" average survival of ~62.5% (averaging BB and Bb). b allele "sees" average survival of ~25% (averaging Bb and bb). (4) PREDICT: B has higher average survival → B frequency INCREASES. This shows even with heterozygote disadvantage (Bb lower than both homozygotes), B still increases because BB survival is so high! Real-world example: sickle cell and malaria—even though sickle cell heterozygotes have some health issues, the allele persists in malaria regions because homozygotes for normal hemoglobin die from malaria at high rates!

Question 3

Two genotypes of a plant differ in drought survival. In a dry year, genotype $AA$ has a 92% survival probability and genotype $aa$ has an 88% survival probability. Compared with a situation where $AA$ survives at 92% and $aa$ survives at 12%, which statement best describes how fast allele frequencies will change in the dry year?

Allele frequencies will change faster in the dry year because both genotypes have high survival
Allele frequencies will change at the same rate in both situations because selection is always equally strong
Allele frequencies will change more slowly in the dry year because the survival difference (92% vs 88%) is small (weak selection) (correct answer)
Allele frequencies will not change at all in the dry year because any difference under 10% cannot cause evolution

Explanation: This question tests your ability to use probability and differential survival/reproduction data to predict how trait and allele frequencies change in populations over time through natural selection. Probability reasoning for evolution: when different variants have different survival or reproduction probabilities, this creates PREDICTABLE changes in allele frequencies: if individuals with allele A have 90% survival probability while individuals with allele a have 30% survival probability (large probability difference), then A individuals contribute disproportionately more offspring to next generation, causing A allele frequency to INCREASE and a allele frequency to DECREASE. The DIRECTION of change is predictable (higher survival/reproduction → increase frequency, lower survival/reproduction → decrease frequency), and the RATE depends on probability differences (larger differences = stronger selection = faster change, smaller differences = weaker selection = slower change). This question compares two selection scenarios: in the dry year, AA survives at 92% and aa at 88% (only 4 percentage point difference), while the comparison scenario has AA at 92% and aa at 12% (80 percentage point difference)—the tiny 4-point difference in the dry year means WEAK selection and SLOW frequency change, while the huge 80-point difference means STRONG selection and RAPID change. Choice C correctly recognizes that allele frequencies will change more slowly in the dry year because the survival difference (92% vs 88%) is small, indicating weak selection. Choice A gets it backwards (small differences = slow change, not fast), B incorrectly claims selection is always equally strong, and D wrongly suggests no evolution occurs with small differences (evolution still happens, just slowly). Predicting frequency changes from probabilities: (1) IDENTIFY survival differences: Dry year: 92% - 88% = 4 percentage points. Comparison: 92% - 12% = 80 percentage points. (2) COMPARE selection strength: 4-point difference = WEAK selection. 80-point difference = STRONG selection. (3) PREDICT rate: Weak selection → SLOW frequency change (many generations for noticeable change). Strong selection → RAPID frequency change (big changes in few generations). (4) Both cause evolution, but at very different speeds! Real-world example: antibiotic resistance with different drugs—if Drug A kills 99% of susceptible bacteria but 1% of resistant (98-point difference), resistance evolves FAST. If Drug B kills 55% of susceptible but 50% of resistant (5-point difference), resistance still evolves but MUCH more slowly!

Question 4

A bacterial population contains 10% antibiotic-resistant cells and 90% susceptible cells. When an antibiotic is applied, 95% of susceptible cells die, while only 5% of resistant cells die. After treatment, what happens to the frequency of resistance in the surviving population?

Resistance frequency increases because resistant cells have a much higher survival probability (correct answer)
Resistance frequency decreases because most bacteria were susceptible to begin with
Resistance frequency stays the same because antibiotics do not affect allele frequencies
Resistance frequency changes randomly with no predictable direction

Explanation: This question tests your ability to use probability and differential survival/reproduction data to predict how trait and allele frequencies change in populations over time through natural selection. Probability reasoning for evolution: when different variants have different survival or reproduction probabilities, this creates predictable changes in allele frequencies: if resistant bacteria have 95% survival while susceptible have 5% (huge probability difference), then resistant individuals contribute disproportionately more to the next generation, causing resistance frequency to increase dramatically. The direction of change is predictable (higher survival/reproduction → increase frequency, lower survival/reproduction → decrease frequency), and the rate depends on probability differences (larger differences = stronger selection = faster change, smaller differences = weaker selection = slower change). In this bacterial population, starting with 10% resistant, the antibiotic creates a massive survival difference (95% resistant survive vs 5% susceptible), so the surviving population will have a much higher proportion of resistant cells, increasing the resistance frequency. Choice A correctly predicts that resistance frequency increases because resistant cells have a much higher survival probability, directly linking the probability gap to the frequency shift. Choice B fails by suggesting resistance decreases due to initial majority of susceptible cells, overlooking how differential survival disproportionately favors resistant ones. Predicting frequency changes from probabilities: (1) Identify survival probabilities: resistant 95%, susceptible 5%. (2) Compare: resistant much higher. (3) Predict direction: resistance increases. (4) Assess magnitude: Huge difference (90 percentage points) means rapid change, like in the example where resistance jumps from 10% to about 68% in one generation! This is why antibiotic resistance evolves so quickly in real-world bacterial populations under treatment.

Question 5

In a population of mice, fur thickness is controlled by alleles $F$ and $f$ . In a colder climate, survival probabilities are: $FF$ = 55%, $Ff$ = 50%, $ff$ = 45%. Compared with a situation where survival is equal for all genotypes, what best describes the expected change in allele frequencies over time?

No change, because the survival differences are small so selection cannot act.
A rapid increase in $F$ frequency, because any survival advantage causes immediate fixation.
A slow increase in $F$ frequency, because $F$ -containing genotypes survive slightly more often. (correct answer)
A slow increase in $f$ frequency, because weaker selection favors the recessive allele.

Question 6

In a lizard population, a gene has alleles $H$ and $h$ . Under normal conditions (no new predators, no drought, no disease), survival and reproduction are similar for all genotypes, so allele frequencies are stable. If conditions remain the same, what does Hardy-Weinberg equilibrium predict about allele frequencies?

They will stay approximately constant because no selection (or other forces) is acting (correct answer)
They will always shift toward whichever allele is rarer
They will change because evolution happens every generation no matter what
They will change only if individual lizards choose mates with the same genotype

Explanation: This question tests your ability to use probability and differential survival/reproduction data to predict how trait and allele frequencies change in populations over time through natural selection. Probability reasoning for evolution: when different variants have different survival or reproduction probabilities, this creates PREDICTABLE changes in allele frequencies: if individuals with allele A have 90% survival probability while individuals with allele a have 30% survival probability (large probability difference), then A individuals contribute disproportionately more offspring to next generation, causing A allele frequency to INCREASE and a allele frequency to DECREASE. The DIRECTION of change is predictable (higher survival/reproduction → increase frequency, lower survival/reproduction → decrease frequency), and the RATE depends on probability differences (larger differences = stronger selection = faster change, smaller differences = weaker selection = slower change). In this lizard population under normal conditions with no selection pressures, all genotypes have similar survival and reproduction—this means NO differential probabilities, so no genotype contributes disproportionately to the next generation, and Hardy-Weinberg equilibrium predicts allele frequencies remain constant over time. Choice A correctly predicts that allele frequencies will stay approximately constant because no selection (or other evolutionary forces) is acting—this is the fundamental prediction of Hardy-Weinberg equilibrium. Choice B incorrectly suggests rare alleles are favored (no mechanism for this without selection), C wrongly claims evolution always happens (it requires differential survival/reproduction or other forces), and D misunderstands that mate choice patterns alone don't change allele frequencies without differential reproduction. Predicting frequency changes from probabilities: (1) IDENTIFY survival/reproduction probabilities: All genotypes have SIMILAR survival and reproduction. (2) COMPARE probabilities: NO significant differences between genotypes. (3) PREDICT direction: No probability differences → NO directional change in frequencies. (4) This is Hardy-Weinberg equilibrium: when survival/reproduction probabilities are equal, allele frequencies stay constant! The five conditions for Hardy-Weinberg: no selection (equal survival/reproduction), no mutation, no migration, large population (no drift), random mating. When these hold, allele frequencies remain stable generation after generation—evolution requires breaking at least one condition!

Question 7

A bacterial population contains two types: antibiotic-resistant (R) and susceptible (S). Before treatment, only 10% are resistant. During antibiotic treatment, 95% of susceptible bacteria die, but only 5% of resistant bacteria die. After treatment, what happens to the frequency of resistant bacteria among the survivors?

It increases because resistant bacteria have a much higher survival probability (correct answer)
It decreases because most bacteria die, so resistance becomes less common
It stays the same because antibiotics do not affect allele frequencies
It changes randomly with no predictable direction because survival is unrelated to resistance

Explanation: This question tests your ability to use probability and differential survival/reproduction data to predict how trait and allele frequencies change in populations over time through natural selection. Probability reasoning for evolution: when different variants have different survival or reproduction probabilities, this creates PREDICTABLE changes in allele frequencies: if individuals with allele A have 90% survival probability while individuals with allele a have 30% survival probability (large probability difference), then A individuals contribute disproportionately more offspring to next generation, causing A allele frequency to INCREASE and a allele frequency to DECREASE. The DIRECTION of change is predictable (higher survival/reproduction → increase frequency, lower survival/reproduction → decrease frequency), and the RATE depends on probability differences (larger differences = stronger selection = faster change, smaller differences = weaker selection = slower change). In this bacterial population, we start with 10% resistant and 90% susceptible, but antibiotic treatment creates HUGE survival differences: resistant bacteria have 95% survival (only 5% die) while susceptible bacteria have only 5% survival (95% die)—this massive 90 percentage point difference means resistant bacteria dominate among survivors, causing resistant frequency to skyrocket! Choice A correctly predicts that resistance frequency increases because resistant bacteria have much higher survival probability during antibiotic treatment. Choice B incorrectly assumes resistance decreases just because many bacteria die (but resistant ones survive much better!), while C and D wrongly claim antibiotics don't affect frequencies or that changes are random. Predicting frequency changes from probabilities: (1) IDENTIFY survival probabilities: Resistant: 95% survive (5% die). Susceptible: 5% survive (95% die). (2) COMPARE probabilities: Resistant bacteria have 19× higher survival rate! (95% vs 5%). (3) PREDICT direction: Higher probability variant (resistant) → frequency INCREASES dramatically. Lower probability variant (susceptible) → frequency DECREASES dramatically. (4) ASSESS magnitude: EXTREME probability difference (90 percentage points) → VERY RAPID frequency change. Example calculation: if we start with 100 bacteria (10 resistant, 90 susceptible), after treatment we have ~9.5 resistant survivors and ~4.5 susceptible survivors, so resistance jumps from 10% to ~68% (9.5/14) in just one generation! This is why antibiotic resistance spreads so fast—extreme selection pressure creates rapid evolution!

Question 8

In a population of beetles, color is controlled by a gene with two alleles: $G$ (green) and $g$ (brown). A pesticide is applied. After spraying, the survival probabilities are: $GG$ : 90%, $Gg$ : 80%, $gg$ : 20%. Assuming survivors are the ones that reproduce, which allele is expected to increase in frequency in the next generation?

Allele $g$ increases because rare alleles are always favored by selection
Allele $G$ increases because genotypes with $G$ have higher survival probabilities (correct answer)
Neither allele changes because survival is based on chance, not genetics
Both alleles increase because more beetles survive overall

Explanation: This question tests your ability to use probability and differential survival/reproduction data to predict how trait and allele frequencies change in populations over time through natural selection. Probability reasoning for evolution: when different variants have different survival or reproduction probabilities, this creates PREDICTABLE changes in allele frequencies: if individuals with allele A have 90% survival probability while individuals with allele a have 30% survival probability (large probability difference), then A individuals contribute disproportionately more offspring to next generation, causing A allele frequency to INCREASE and a allele frequency to DECREASE. The DIRECTION of change is predictable (higher survival/reproduction → increase frequency, lower survival/reproduction → decrease frequency), and the RATE depends on probability differences (larger differences = stronger selection = faster change, smaller differences = weaker selection = slower change). In this beetle population, we see survival probabilities of GG: 90%, Gg: 80%, gg: 20%—notice that genotypes containing allele G have much higher survival (90% and 80%) compared to gg (only 20%), so G-containing beetles contribute more offspring to the next generation, causing allele G frequency to increase. Choice B correctly predicts frequency changes by recognizing that genotypes with G have higher survival probabilities, leading to G increasing in frequency. Choice A incorrectly claims rare alleles are favored (not true—selection favors beneficial alleles regardless of rarity), while C wrongly suggests genetics doesn't matter, and D misunderstands that both alleles can't increase (frequencies must sum to 100%). Predicting frequency changes from probabilities: (1) IDENTIFY survival probabilities for each genotype: GG: 90%, Gg: 80%, gg: 20%. (2) COMPARE probabilities: Which genotypes survive better? (GG and Gg both have much higher survival than gg). (3) PREDICT direction: Since G appears in the high-survival genotypes (GG and Gg), allele G frequency INCREASES. Since gg has low survival, allele g frequency DECREASES. (4) ASSESS magnitude: LARGE probability difference (90%/80% vs 20% = 60-70 percentage point difference) → RAPID frequency change (strong selection). Real-world example: insecticide resistance works exactly like this—if RR insects have 95% survival, Rr have 85% survival, and rr have 5% survival when sprayed, the R allele rapidly increases because R-containing genotypes survive at much higher rates!

Question 9

In a bacterial population, 10% of cells carry a resistance allele $R$ and 90% are susceptible ( $r$ ). After an antibiotic treatment, 95% of susceptible cells die, but only 5% of resistant cells die. Without doing exact calculations, what will happen to the frequency of the resistance allele $R$ among the surviving bacteria?

It will decrease because most bacteria were susceptible before treatment.
It will stay the same because antibiotics do not affect allele frequencies, only population size.
It will increase because resistant cells have a much higher probability of surviving the antibiotic. (correct answer)
It will change randomly because survival is not related to genotype in this scenario.

Question 10

In a plant population, individuals with a waxy-leaf trait produce on average 2 times as many seeds as individuals without waxy leaves. If the environment stays the same for many generations, what is the most likely change in the waxy-leaf trait frequency?

The waxy-leaf trait becomes more common because those plants contribute more offspring (correct answer)
The waxy-leaf trait becomes less common because producing more seeds uses too much energy
The waxy-leaf trait stays the same because reproduction differences do not affect frequencies
The waxy-leaf trait appears in all plants within one generation because selection changes individuals