Genetics Quiz
Practice Types Of Natural Selection in Genetics with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A plant species grows on a mainland continent and a small offshore island. On the mainland, stabilizing selection favors an intermediate flower size. On the island, a unique large-bodied pollinator exerts strong directional selection for larger flowers. There is a constant, low level of gene flow via pollen from the mainland to the island. What is the most likely consequence for the island population?
This quiz focuses on Types Of Natural Selection, giving you a quick way to practice the rules, question types, and explanations that matter most for Genetics.
Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.
A plant species grows on a mainland continent and a small offshore island. On the mainland, stabilizing selection favors an intermediate flower size. On the island, a unique large-bodied pollinator exerts strong directional selection for larger flowers. There is a constant, low level of gene flow via pollen from the mainland to the island. What is the most likely consequence for the island population?
Explanation: This scenario describes a migration-selection balance. The island population is subject to two opposing forces: local directional selection favoring large flowers and gene flow from the mainland introducing alleles for smaller, intermediate-sized flowers. The result is an equilibrium where the population is pulled away from its local optimum by the 'maladaptive' gene flow. The mean phenotype will therefore be somewhere between the mainland mean and the island optimum. Choice A ignores gene flow. Choice C describes disruptive selection. Choice D would only be true if gene flow were extremely high relative to the strength of selection.
The enzyme hexokinase is critical for glycolysis, and its kinetic properties are under strong stabilizing selection to maintain metabolic homeostasis. Most new non-synonymous mutations are deleterious. When comparing the hexokinase gene sequence between two closely related species, what pattern of nucleotide substitution would be expected?
Explanation: Stabilizing selection that acts to preserve the function of a protein is also known as purifying selection at the molecular level. It selectively removes non-synonymous mutations that alter the amino acid sequence and are likely to be deleterious. Synonymous mutations, which do not change the amino acid sequence, are largely neutral and accumulate at a rate reflecting the mutation rate. Therefore, in a gene under strong purifying/stabilizing selection, the rate of non-synonymous substitutions (dN) will be much lower than the rate of synonymous substitutions (dS), resulting in a (dN/dS) ratio significantly less than one. A (dN/dS > 1) indicates positive (directional) selection, while (dN/dS \approx 1) suggests neutral evolution.
Human birth weight is a classic example of a trait under stabilizing selection, where infants with very low or very high birth weights have lower survival rates. If modern medical interventions significantly improve the survival rates of infants at both extremes of the birth weight spectrum, what is the most probable long-term consequence for the distribution of this trait in the population?
Explanation: Stabilizing selection acts to reduce phenotypic variance by selecting against extreme phenotypes. If this selective pressure is relaxed by medical interventions that improve the survival of very small and very large infants, then alleles contributing to these extreme phenotypes will be removed from the population less efficiently. This will lead to an increase in the overall phenotypic variance for birth weight over time. Choice A suggests directional selection. Choice C is the opposite of what would happen; relaxing selection would slow or stop the purification of alleles. Choice D confuses the concepts; while the relative contribution of environment to survival increases, this doesn't automatically decrease the heritability of the trait itself, which is a measure of the proportion of phenotypic variance due to genetic variance.
For many generations, a population of cave-dwelling fish has been under strong directional selection for the loss of eyes, as they are non-functional and metabolically costly. Suppose a large number of these fish are transferred to a new, brightly-lit surface stream where vision is advantageous. Assuming some residual genetic variation for eye development exists, what is the most likely initial evolutionary response in the new environment?
Explanation: The change in environment has reversed the direction of selection. In the new, lit environment, vision provides a fitness advantage. Therefore, directional selection will now act to favor any existing alleles that promote eye development, leading to a gradual re-evolution of functional eyes. Choice A assumes fixation, which is not guaranteed; the stem specifies that some variation remains. Choice B is incorrect because the pressure is not to maintain an intermediate state but to move from a non-functional state towards a functional one. Choice D is not supported by the scenario, as there is no indication that intermediates have lower fitness than both extremes.
A population of insects is exposed to a new, long-lasting pesticide. Resistance to the pesticide is conferred by a single recessive allele, 'r', which was present at a very low frequency (p(r) = 0.01) before exposure. Assuming the pesticide imposes a strong selective pressure against non-resistant individuals, what is the expected initial trajectory of the 'r' allele's frequency?
Explanation: This is a case of directional selection favoring a recessive allele. When the recessive allele 'r' is rare, most copies are found in heterozygotes ('Rr'). Since the allele is recessive, these heterozygotes do not express the resistant phenotype and are not favored by selection. Selection can only act on the rare 'rr' homozygotes. Therefore, the allele's frequency will increase very slowly at first. As the frequency of 'r' increases, more 'rr' individuals appear, and the rate of selection accelerates. Choice A describes the trajectory for a favored dominant allele. Choice C is an oversimplification. Choice D introduces a distractor concept (mate limitation) not supported by the premise.
A researcher is studying bristle number in a Drosophila population. The mean bristle number of the parental generation is 40. The researcher selects only the individuals with the highest bristle numbers to be parents for the next generation; this selected group has a mean bristle number of 50. The narrow-sense heritability ((h^2)) for this trait is 0.3. What is the expected mean bristle number of the offspring generation?
Explanation: This problem requires applying the breeder's equation, (R = h^2S), which predicts the response to directional selection. First, calculate the selection differential ((S)), which is the difference between the mean of the selected parents and the mean of the entire parental population: (S = 50 - 40 = 10). Next, calculate the response to selection ((R)), which is the expected change in the mean of the trait in the next generation: (R = h^2 \times S = 0.3 \times 10 = 3). Finally, the expected mean of the offspring generation is the original population mean plus the response: (40 + 3 = 43).
In a species of guppy, females preferentially mate with males that have large, brightly colored tails (sexual selection). However, these conspicuous tails also make the males more visible to predators (natural selection). In a stream with a high density of predators, what is the expected evolutionary outcome of these opposing selective pressures on male tail morphology?
Explanation: This scenario describes a trade-off between sexual selection (favoring larger, brighter tails) and natural selection (favoring smaller, duller tails for crypsis). These opposing pressures will likely lead to a form of stabilizing selection around an intermediate phenotype that balances the benefits of mating success against the costs of predation risk. The location of this optimum depends on the relative strengths of the two selective forces. Choices A and B are incorrect because they ignore one of the two selective pressures. Choice C describes disruptive selection, which is not supported by the scenario; there is no indication that intermediate tails are specifically selected against.
In a species of scale-eating cichlid fish, two mouth morphologies exist: right-twisting and left-twisting. This allows them to attack prey from a specific side. Prey fish learn to watch for attacks from the more common morph. Consequently, the rarer of the two morphs has a higher feeding success rate. This results in the frequencies of the two morphs oscillating around 0.5. This ecological scenario is a direct example of:
Explanation: This is a classic example of negative frequency-dependent selection, where a phenotype's fitness is inversely related to its frequency. The rarer morph is always more successful, which increases its frequency until it becomes the more common morph, at which point its fitness decreases. This process maintains both phenotypes in the population. While it is true that two morphs are maintained (a feature sometimes associated with disruptive selection, choice A), the underlying mechanism is based on frequency, not on a fitness disadvantage of an intermediate phenotype. While the direction of selection does shift (Choice B), the overarching pattern is one of balancing selection, not a consistent trend. It is not stabilizing selection (Choice D) because that term refers to selection favoring an intermediate phenotype, not maintaining multiple distinct phenotypes.
A population of finches exists in an environment with two primary food sources: a plant producing very small, soft seeds and a plant producing very large, hard seeds. Finches with small beaks are most efficient at handling small seeds, while finches with large beaks are best at cracking large seeds. Finches with intermediate-sized beaks are inefficient at processing either seed type and have lower survival rates. Assuming beak size is a heritable trait, which of the following outcomes is most likely after many generations of selection?
Explanation: The scenario describes disruptive (or diversifying) selection, where extreme phenotypes (small and large beaks) are favored over the intermediate phenotype. This mode of selection increases the variance of the trait in the population and can lead to a bimodal distribution, where two distinct phenotypic peaks emerge. Choice A describes directional selection. Choice C describes stabilizing selection. Choice D describes a random event (bottleneck), not a direct and predictable outcome of this type of selection, although selection can reduce diversity at specific loci over time.
A population of annual insects inhabits an environment where winters are cold and summers are hot. Small body size is favored during the hot summers for better heat dissipation, while large body size is favored during the cold winters for better heat retention. Generations are discrete and do not overlap. What is the most likely long-term outcome for the genetic variation related to body size in this population?
Explanation: When you encounter questions about populations facing alternating selective pressures, think about how different types of selection affect genetic variation over time. This scenario describes fluctuating selection, where the optimal trait value changes predictably between seasons. In this insect population, small body size is advantageous during hot summers (better heat dissipation), while large body size is favored during cold winters (better heat retention). Because these opposing selective pressures alternate each generation, neither extreme body size can be permanently eliminated from the population. Individuals with genes for small body size will be favored during summer reproduction, while those with genes for large body size will be favored during winter reproduction. This back-and-forth selection maintains both sets of alleles in the gene pool. Answer A correctly identifies that fluctuating selection preserves genetic variation rather than reducing it. Answer B is wrong because the population won't evolve a fixed intermediate size—the alternating pressures prevent this stabilizing selection outcome. Answer C incorrectly suggests speciation, but these insects experience the same environment and can interbreed; temporal variation in selection doesn't cause reproductive isolation. Answer D misunderstands the scenario by suggesting one direction of selection will dominate, ignoring that both seasonal pressures are consistent and ongoing. Remember that fluctuating selection is a key mechanism for maintaining genetic diversity in natural populations. When you see alternating environmental pressures, expect the maintenance of variation rather than fixation of a single optimal trait.
In a diploid plant species, a rare recessive allele confers complete resistance to a potent herbicide. After several years of continuous herbicide application, researchers note that the frequency of the resistance allele has increased much more slowly than their models predicted. Which of the following provides the best genetic explanation for this observation?
Explanation: When you encounter questions about allele frequency changes under selection, consider how dominance relationships affect which individuals express the trait and experience selection pressure. In this scenario, the resistance allele is recessive, meaning only homozygous recessive individuals (rr) show resistance. Since the allele is rare, most resistance alleles exist in heterozygous individuals (Rr) who appear susceptible and die when exposed to herbicide. This creates a "hidden" reservoir of resistance alleles that aren't subject to positive selection because they don't confer protection in heterozygotes. The frequency increases slowly because selection can only act on the small fraction of resistance alleles present in homozygous recessive individuals. Answer A correctly identifies this masking effect. The resistance alleles in heterozygotes can't be "seen" by selection, dramatically slowing the response to selection pressure. Answer B describes pleiotropy (one gene affecting multiple traits), but this would cause the frequency to increase even more slowly or decrease, not just slower than predicted models that likely assumed simple directional selection. Answer C focuses on mutation rate, but the question states the allele already exists and is increasing—the issue isn't generating new mutations but the rate of frequency change of existing alleles. Answer D mentions stabilizing selection, which maintains intermediate phenotypes. However, herbicide resistance is typically an all-or-nothing trait, and stabilizing selection wouldn't specifically target the resistance allele. Remember: recessive alleles respond slowly to selection when rare because most copies are "hidden" in heterozygotes where they can't be selected for or against.
A plant species grows on a mainland continent and a small offshore island. On the mainland, stabilizing selection favors an intermediate flower size. On the island, a unique large-bodied pollinator exerts strong directional selection for larger flowers. There is a constant, low level of gene flow via pollen from the mainland to the island. What is the most likely consequence for the island population?
Explanation: This scenario describes a migration-selection balance. The island population is subject to two opposing forces: local directional selection favoring large flowers and gene flow from the mainland introducing alleles for smaller, intermediate-sized flowers. The result is an equilibrium where the population is pulled away from its local optimum by the 'maladaptive' gene flow. The mean phenotype will therefore be somewhere between the mainland mean and the island optimum. Choice A ignores gene flow. Choice C describes disruptive selection. Choice D would only be true if gene flow were extremely high relative to the strength of selection.
The fitness of a lizard species is strongly dependent on its adult body size. Individuals that are too small are poor competitors for territory, while individuals that are too large are more easily spotted by aerial predators. The optimal body size is 150g. Which of the following describes the most likely long-term effect of this selection regime on the genetic architecture for body size?
Explanation: This scenario describes stabilizing selection, which favors an intermediate phenotype and selects against extremes. This type of selection acts to remove deleterious alleles that cause deviations from the optimum. However, new mutations constantly reintroduce such alleles. The result is a mutation-selection balance, where these alleles are maintained at a low but stable frequency. Choice B describes directional selection. Choice C describes disruptive selection. Choice D is incorrect because gene flow from populations with different optima would counteract, not reinforce, local stabilizing selection.
Human birth weight is a classic example of a trait under stabilizing selection, where infants with very low or very high birth weights have lower survival rates. If modern medical interventions significantly improve the survival rates of infants at both extremes of the birth weight spectrum, what is the most probable long-term consequence for the distribution of this trait in the population?
Explanation: Stabilizing selection acts to reduce phenotypic variance by selecting against extreme phenotypes. If this selective pressure is relaxed by medical interventions that improve the survival of very small and very large infants, then alleles contributing to these extreme phenotypes will be removed from the population less efficiently. This will lead to an increase in the overall phenotypic variance for birth weight over time. Choice A suggests directional selection. Choice C is the opposite of what would happen; relaxing selection would slow or stop the purification of alleles. Choice D confuses the concepts; while the relative contribution of environment to survival increases, this doesn't automatically decrease the heritability of the trait itself, which is a measure of the proportion of phenotypic variance due to genetic variance.
In a species of scale-eating cichlid fish, two mouth morphologies exist: right-twisting and left-twisting. This allows them to attack prey from a specific side. Prey fish learn to watch for attacks from the more common morph. Consequently, the rarer of the two morphs has a higher feeding success rate. This results in the frequencies of the two morphs oscillating around 0.5. This ecological scenario is a direct example of:
Explanation: This is a classic example of negative frequency-dependent selection, where a phenotype's fitness is inversely related to its frequency. The rarer morph is always more successful, which increases its frequency until it becomes the more common morph, at which point its fitness decreases. This process maintains both phenotypes in the population. While it is true that two morphs are maintained (a feature sometimes associated with disruptive selection, choice A), the underlying mechanism is based on frequency, not on a fitness disadvantage of an intermediate phenotype. While the direction of selection does shift (Choice B), the overarching pattern is one of balancing selection, not a consistent trend. It is not stabilizing selection (Choice D) because that term refers to selection favoring an intermediate phenotype, not maintaining multiple distinct phenotypes.
In a diploid plant species, a rare recessive allele confers complete resistance to a potent herbicide. After several years of continuous herbicide application, researchers note that the frequency of the resistance allele has increased much more slowly than their models predicted. Which of the following provides the best genetic explanation for this observation?
Explanation: When you encounter questions about allele frequency changes under selection, consider how dominance relationships affect which individuals express the trait and experience selection pressure. In this scenario, the resistance allele is recessive, meaning only homozygous recessive individuals (rr) show resistance. Since the allele is rare, most resistance alleles exist in heterozygous individuals (Rr) who appear susceptible and die when exposed to herbicide. This creates a "hidden" reservoir of resistance alleles that aren't subject to positive selection because they don't confer protection in heterozygotes. The frequency increases slowly because selection can only act on the small fraction of resistance alleles present in homozygous recessive individuals. Answer A correctly identifies this masking effect. The resistance alleles in heterozygotes can't be "seen" by selection, dramatically slowing the response to selection pressure. Answer B describes pleiotropy (one gene affecting multiple traits), but this would cause the frequency to increase even more slowly or decrease, not just slower than predicted models that likely assumed simple directional selection. Answer C focuses on mutation rate, but the question states the allele already exists and is increasing—the issue isn't generating new mutations but the rate of frequency change of existing alleles. Answer D mentions stabilizing selection, which maintains intermediate phenotypes. However, herbicide resistance is typically an all-or-nothing trait, and stabilizing selection wouldn't specifically target the resistance allele. Remember: recessive alleles respond slowly to selection when rare because most copies are "hidden" in heterozygotes where they can't be selected for or against.
A population of annual insects inhabits an environment where winters are cold and summers are hot. Small body size is favored during the hot summers for better heat dissipation, while large body size is favored during the cold winters for better heat retention. Generations are discrete and do not overlap. What is the most likely long-term outcome for the genetic variation related to body size in this population?
Explanation: When you encounter questions about populations facing alternating selective pressures, think about how different types of selection affect genetic variation over time. This scenario describes fluctuating selection, where the optimal trait value changes predictably between seasons. In this insect population, small body size is advantageous during hot summers (better heat dissipation), while large body size is favored during cold winters (better heat retention). Because these opposing selective pressures alternate each generation, neither extreme body size can be permanently eliminated from the population. Individuals with genes for small body size will be favored during summer reproduction, while those with genes for large body size will be favored during winter reproduction. This back-and-forth selection maintains both sets of alleles in the gene pool. Answer A correctly identifies that fluctuating selection preserves genetic variation rather than reducing it. Answer B is wrong because the population won't evolve a fixed intermediate size—the alternating pressures prevent this stabilizing selection outcome. Answer C incorrectly suggests speciation, but these insects experience the same environment and can interbreed; temporal variation in selection doesn't cause reproductive isolation. Answer D misunderstands the scenario by suggesting one direction of selection will dominate, ignoring that both seasonal pressures are consistent and ongoing. Remember that fluctuating selection is a key mechanism for maintaining genetic diversity in natural populations. When you see alternating environmental pressures, expect the maintenance of variation rather than fixation of a single optimal trait.
A population of insects is exposed to a new, long-lasting pesticide. Resistance to the pesticide is conferred by a single recessive allele, 'r', which was present at a very low frequency (p(r) = 0.01) before exposure. Assuming the pesticide imposes a strong selective pressure against non-resistant individuals, what is the expected initial trajectory of the 'r' allele's frequency?
Explanation: This is a case of directional selection favoring a recessive allele. When the recessive allele 'r' is rare, most copies are found in heterozygotes ('Rr'). Since the allele is recessive, these heterozygotes do not express the resistant phenotype and are not favored by selection. Selection can only act on the rare 'rr' homozygotes. Therefore, the allele's frequency will increase very slowly at first. As the frequency of 'r' increases, more 'rr' individuals appear, and the rate of selection accelerates. Choice A describes the trajectory for a favored dominant allele. Choice C is an oversimplification. Choice D introduces a distractor concept (mate limitation) not supported by the premise.
For many generations, a population of cave-dwelling fish has been under strong directional selection for the loss of eyes, as they are non-functional and metabolically costly. Suppose a large number of these fish are transferred to a new, brightly-lit surface stream where vision is advantageous. Assuming some residual genetic variation for eye development exists, what is the most likely initial evolutionary response in the new environment?
Explanation: The change in environment has reversed the direction of selection. In the new, lit environment, vision provides a fitness advantage. Therefore, directional selection will now act to favor any existing alleles that promote eye development, leading to a gradual re-evolution of functional eyes. Choice A assumes fixation, which is not guaranteed; the stem specifies that some variation remains. Choice B is incorrect because the pressure is not to maintain an intermediate state but to move from a non-functional state towards a functional one. Choice D is not supported by the scenario, as there is no indication that intermediates have lower fitness than both extremes.
Following the extinction of the dinosaurs, early mammals underwent a rapid diversification to fill a wide variety of newly available ecological niches (e.g., terrestrial predators, herbivores, aerial insectivores). Which mode of selection was likely a key initial driver of this large-scale adaptive radiation?
Explanation: When you encounter questions about adaptive radiation and rapid diversification into new ecological niches, think about which type of selection would promote the evolution of multiple distinct traits from a single ancestral population. Disruptive selection (answer D) was the key driver here because it simultaneously favors extreme phenotypes while selecting against intermediate forms. As early mammals faced newly available niches after the dinosaur extinction, individuals with traits suited for very different lifestyles—like those adapted for flight versus burrowing versus aquatic environments—had significant advantages over generalists. This created selective pressure that "pulled" the population in multiple directions, promoting the rapid divergence we see in mammalian adaptive radiation. Let's examine why the other options don't fit: Stabilizing selection (A) maintains the status quo by favoring intermediate traits, which would prevent rather than promote diversification. Directional selection (B) pushes an entire population toward one extreme, like larger body size, but this couldn't explain the simultaneous evolution of tiny bats, massive whales, and everything in between. Purifying selection (C) simply removes harmful mutations and doesn't drive adaptive change toward new phenotypes. The key study tip for adaptive radiation questions: look for the selection type that can simultaneously favor multiple different extreme phenotypes. Disruptive selection is the only mechanism that actively promotes divergence within a population, making it the driving force behind most adaptive radiations. Remember that rapid diversification into distinct niches requires selection that "disrupts" the ancestral form into multiple new directions.