IB Biology Quiz: Understand Natural Selection
18 questions · exam conditions
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Understand Natural SelectionQuestion 1 of 18

What is meant by 'evolutionary fitness' in the context of natural selection?

The relative contribution an individual makes to the gene pool of the next generation.
The ability of an organism to consciously adapt to changes in its environment during its lifetime.
The physical strength and health of an individual organism compared to others in its population.
The number of years an individual survives, regardless of its reproductive output.
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IB Biology Quiz

IB Biology Quiz: Understand Natural Selection

Practice Understand Natural Selection in IB Biology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Understand Natural Selection, giving you a quick way to practice the rules, question types, and explanations that matter most for IB Biology.

How to use this quiz

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.

All questions

Question 1

What is meant by 'evolutionary fitness' in the context of natural selection?

  1. The relative contribution an individual makes to the gene pool of the next generation. (correct answer)
  2. The ability of an organism to consciously adapt to changes in its environment during its lifetime.
  3. The physical strength and health of an individual organism compared to others in its population.
  4. The number of years an individual survives, regardless of its reproductive output.
Explanation: Evolutionary fitness is a measure of reproductive success. It is not about being the strongest, fastest, or longest-lived, but about how many viable, fertile offspring an individual produces relative to other individuals in the population. An organism that lives a short life but produces many offspring has higher fitness than a long-lived organism that produces none.

Question 2

In a population of rock pocket mice, coat colour is controlled by a single gene. Most mice have a light-coloured coat that camouflages them in their sandy habitat. A volcanic eruption covers part of the habitat with dark lava rock. Over many generations, the frequency of mice with dark coats increases significantly in the lava rock area, while remaining low in the sandy areas.

What is the most accurate description of the evolutionary process occurring in the mice on the lava rock?

  1. Genetic drift caused a random increase in the frequency of the dark coat allele.
  2. Directional selection favoured the dark coat phenotype due to increased predation on light-coloured mice. (correct answer)
  3. Stabilizing selection eliminated both light and dark coat phenotypes, favouring an intermediate colour.
  4. Gene flow from a neighbouring dark-coloured mouse population caused the change in allele frequency.
Explanation: This is a classic example of directional selection. The change in the environment (dark lava rock) created a new selection pressure (predation) that favoured one extreme phenotype (dark coat) over the other (light coat). This caused a shift in the population's average phenotype. Genetic drift is random and less likely to cause such a strong, environment-specific change. Stabilizing selection would favour the intermediate, and the scenario does not mention gene flow as the primary cause.

Question 3

The use of insecticides on a population of crop pests often results in the pests evolving resistance. What is the initial source of the insecticide resistance trait?

  1. The need to survive the insecticide causes the pests to develop resistance.
  2. Pests from other resistant species interbreeding with the target pest population.
  3. The insecticide itself, which chemically alters the pest's DNA to create resistance.
  4. Random mutation or pre-existing genetic variation within the pest population. (correct answer)
Explanation: Natural selection can only act on variation that is already present in a population. This variation arises from random mutations or is maintained in the gene pool from previous mutations. The insecticide does not create the resistance allele; it only selects for individuals that already happen to possess it. This is a crucial distinction from Lamarckian ideas, which suggest that the need for a trait can cause it to arise.

Question 4

A species of bird feeds on seeds that are either very small or very large. Birds with small beaks are efficient at cracking small seeds, and birds with large beaks are efficient at cracking large seeds. Birds with intermediate-sized beaks are inefficient at cracking either type of seed and have lower survival rates.

[HL only] Which type of selection is most likely acting on beak size in this bird population, and what would be the expected long-term outcome?

  1. Directional selection, leading to a gradual increase in the average beak size of the population.
  2. Stabilizing selection, leading to a narrower distribution of beak sizes around the intermediate mean.
  3. Disruptive selection, leading to a bimodal distribution of beak sizes with few intermediates. (correct answer)
  4. Sexual selection, leading to exaggerated beak sizes in males due to female preference.
Explanation: This scenario describes disruptive (or diversifying) selection, where extreme phenotypes (small and large beaks) are favoured over the intermediate phenotype. This type of selection can lead to a bimodal distribution of the trait, where two distinct peaks of frequency emerge. Over a very long time, it can even contribute to sympatric speciation.

Question 5

A population of island birds shows variation in wing length. A series of unusually strong, persistent winds sweeps the island for several years. After this period, scientists observe that the bird population consists mainly of individuals with very short wings and individuals with very long wings.

Which conclusion is most consistent with the principles of natural selection?

  1. The winds acted as a selective pressure, removing birds with intermediate wing lengths from the population. (correct answer)
  2. Individual birds changed their wing length during their lifetime to better cope with the wind.
  3. The wind caused mutations for very short and very long wings to appear in the bird population.
  4. The bird population experienced stabilizing selection, favouring an average wing length.
Explanation: The observation of two extreme phenotypes becoming more common while the intermediate phenotype becomes rare is characteristic of disruptive selection. The selective pressure (wind) likely disfavoured intermediate wings, perhaps because they were not effective for either staying grounded (short wings) or soaring efficiently (long wings). This non-random survival and reproduction led to the observed shift in phenotype distribution.

Question 6

Why is genetic variation within a population essential for natural selection to occur?

  1. Variation ensures that the population size is large enough to avoid genetic drift.
  2. Variation allows individuals to choose their traits based on the selective pressures they encounter.
  3. High variation directly causes environmental changes, which then drive the process of selection.
  4. Without variation, all individuals would have the same fitness, and selection could not favour any particular trait. (correct answer)
Explanation: Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. If there is no variation, all individuals are genetically and phenotypically identical for a given trait. Therefore, no individual can have a survival or reproductive advantage over another, and natural selection cannot operate on that trait. Variation is the raw material for selection.

Question 7

Industrial melanism in the peppered moth (Biston betularia) is a classic example of natural selection. In polluted industrial areas, the frequency of the dark (melanic) form increased, while in clean rural areas, the light (typical) form remained common.

For this change in morph frequency to occur through natural selection, which condition was absolutely necessary?

  1. The pollution directly caused the mutation for dark colouration in the moths.
  2. The moths must have been able to choose their resting places to match their colour.
  3. Individual light-coloured moths could change their colour to dark in polluted areas.
  4. The colour variation was heritable, being passed from parents to offspring. (correct answer)
Explanation: For natural selection to lead to evolutionary change, the trait under selection must be heritable. If the variation in colour was not due to genetic differences that could be passed on, then the differential survival of dark moths would not lead to an increase in their frequency in the next generation. Heritability is one of the three core requirements for natural selection, along with variation and differential fitness.

Question 8

Which statement correctly distinguishes between the roles of mutation and natural selection in evolution?

  1. Mutation is a directed process that creates beneficial alleles, while natural selection sorts them.
  2. Natural selection is a random process, while mutation provides the non-random direction for evolution.
  3. Mutation introduces genetic variation randomly, while natural selection acts non-randomly on this variation. (correct answer)
  4. Natural selection is the ultimate source of all genetic novelty, which is then refined by mutation.
Explanation: This question tests a fundamental concept. Mutation, the ultimate source of new alleles, is a random process with respect to fitness; a mutation's effect can be beneficial, neutral, or harmful. In contrast, natural selection is the non-random process of differential survival and reproduction based on the fitness of those alleles in a given environment. Selection 'sorts' the random variation created by mutation.

Question 9

A bacterial population exhibits variation in a gene that provides resistance to the antibiotic streptomycin. The population is cultured in a nutrient-rich medium. Subsequently, a high concentration of streptomycin is added to the medium, and the culture is allowed to grow for several generations.

What is the primary role of streptomycin in the evolution of resistance in this bacterial population?

  1. It causes new mutations in the resistance gene, allowing bacteria to adapt.
  2. It acts as a selective pressure, favouring the reproduction of pre-existing resistant bacteria. (correct answer)
  3. It induces individual bacteria to develop resistance, which is then passed to their offspring.
  4. It directly modifies the bacterial genome to create the resistance allele in all survivors.
Explanation: Natural selection acts on existing variation. The streptomycin does not cause the resistance mutation; the mutation already exists in the population. The antibiotic acts as a selective pressure or agent, eliminating non-resistant individuals and allowing those that already possess the resistance allele to survive and reproduce at a higher rate. This leads to an increase in the frequency of the resistance allele in the population over time.

Question 10

[HL only] A population of flowering plants is in Hardy-Weinberg equilibrium for a gene controlling flower colour, where the allele for red flowers (R) is dominant to the allele for white flowers (r). A new herbivore is introduced that preferentially eats red-flowered plants.

What is the expected immediate effect on the allele frequencies in the next generation?

  1. The frequency of the R allele will decrease, and the frequency of the r allele will increase. (correct answer)
  2. Both R and r allele frequencies will remain constant as the population was in equilibrium.
  3. The frequency of the R allele will increase as plants evolve a defense mechanism.
  4. The frequency of the r allele will decrease because homozygous recessive individuals are rare.
Explanation: The introduction of the herbivore represents a new selective pressure against the dominant phenotype (red flowers). Individuals with the RR and Rr genotypes will be eaten more frequently, reducing their ability to reproduce. This means the r allele, present in surviving rr (white) plants and heterozygous Rr plants, will have a higher relative success rate. Consequently, the frequency of the R allele (p) will decrease, and the frequency of the r allele (q) will increase, disrupting the Hardy-Weinberg equilibrium.

Question 11

In a certain species of fish, males exhibit bright colouration. Females preferentially mate with the most brightly coloured males. However, these bright colours also make the males more visible to predators.

Which statement best describes the evolutionary trade-off in this scenario?

  1. The population is undergoing disruptive selection, leading to two distinct male phenotypes.
  2. The trait for bright colouration is influenced by both sexual selection and natural selection. (correct answer)
  3. The bright colour is a neutral trait that has become fixed in the population due to genetic drift.
  4. The male fish can change their colour based on the presence of females or predators.
Explanation: This scenario illustrates a classic evolutionary trade-off. Sexual selection (female preference) drives the evolution of brighter colours because it increases mating success. At the same time, natural selection (predation) acts against bright colours because it decreases survival. The resulting phenotype is a balance between these two opposing selective pressures. It is not necessarily disruptive selection, nor is it a neutral trait. Individual colour change (D) is a form of phenotypic plasticity, not the evolutionary trade-off described.

Question 12

In a forest ecosystem, a fungal disease spreads and kills most of the trees of a dominant species. This significantly changes the light conditions on the forest floor, making it much sunnier.

How would this environmental change most likely affect a population of understory plants that has existing variation in light tolerance?

  1. All individual plants would immediately adapt by increasing their photosynthetic rate.
  2. The environmental change would cause new mutations for sun tolerance to arise in the plants.
  3. Individuals with pre-existing traits for higher light tolerance would have a reproductive advantage. (correct answer)
  4. The population would go extinct as it is only adapted to shady conditions and cannot evolve.
Explanation: Natural selection acts on pre-existing variation. The change in the environment does not cause adaptation in individuals (A) or cause the necessary mutations to appear (B). Instead, it changes the selective pressures. Individuals that already possess alleles conferring higher light tolerance will now be more successful (higher fitness) and will contribute more offspring to the next generation, leading to an increase in the frequency of those alleles.

Question 13

Which of the following is a key distinction between artificial selection and natural selection?

  1. Artificial selection acts on existing genetic variation, whereas natural selection creates new variation.
  2. In artificial selection, the selective pressure is human preference, while in natural selection, it is environmental conditions. (correct answer)
  3. Natural selection leads to adaptation, whereas artificial selection does not result in evolutionary change.
  4. Artificial selection only occurs in domesticated animals, while natural selection only occurs in wild populations.
Explanation: The fundamental difference between the two processes is the source of the selective pressure. In natural selection, environmental factors (e.g., climate, predators, resource availability) determine which traits are advantageous. In artificial selection, humans intentionally choose which individuals will reproduce based on traits that are desirable to them, not necessarily traits that enhance survival in the wild. Both processes act on existing variation and lead to evolutionary change.

Question 14

A common misconception about natural selection is that it leads to 'perfect' organisms. Which statement best refutes this idea?

  1. Natural selection can only act on existing genetic variation, which may not include the 'perfect' allele. (correct answer)
  2. The environment is constant, so once an organism is perfectly adapted, it remains that way.
  3. Evolution is a goal-oriented process that is continuously striving for perfection.
  4. Favourable traits are always simple, while perfect adaptations would require complex structures.
Explanation: Evolution is constrained by the raw material available. Natural selection cannot create new alleles on demand; it can only favour the best variants that already exist within a population's gene pool due to past mutations. Furthermore, evolutionary trade-offs and constantly changing environments mean that what is 'optimal' is a moving target. Therefore, 'perfect' organisms are not an expected outcome of natural selection.

Question 15

Which of the following describes a situation where natural selection is LEAST likely to be the primary agent of evolutionary change?

  1. A bacterial population develops antibiotic resistance after repeated exposure to antibiotics.
  2. The allele for a fatal genetic disease that manifests after reproductive age persists in a human population.
  3. The average fur thickness of a wolf population increases over generations in a cooling climate.
  4. A small, isolated population of island flowers experiences a rapid shift in petal colour after a cyclone. (correct answer)
Explanation: In a small, isolated population, random events can have a disproportionately large effect on allele frequencies. A cyclone could wipe out a large portion of the population randomly, irrespective of petal colour. The subsequent change in allele frequency would be due to genetic drift (a bottleneck effect), not natural selection. The other options describe clear cases where a selective pressure (antibiotics, late-onset disease, climate) leads to differential survival and reproduction.

Question 16

A key component of Darwin's theory of evolution by natural selection is the overproduction of offspring. What is the direct consequence of this overproduction?

  1. It guarantees that a sufficient number of individuals will survive to reproductive age.
  2. It increases the rate of mutation, generating more genetic variation for selection to act upon.
  3. It leads to a struggle for existence due to competition for limited resources. (correct answer)
  4. It ensures that mating is random among all members of the population.
Explanation: The production of more offspring than can possibly survive leads directly to competition for finite resources like food, water, territory, and mates. This 'struggle for existence' is the context in which individuals with more favourable traits are more likely to survive and reproduce. While overproduction contributes to population persistence (A), it does not directly cause mutations (B) or guarantee random mating (D).

Question 17

Natural selection acts directly on the phenotype of an organism, but its long-term evolutionary consequences are seen in the population's genotype frequencies. How does this occur?

  1. The environment directly alters the genotype of individuals to produce a more favourable phenotype.
  2. Phenotypes are not genetically determined, so selection on phenotypes does not affect genotype frequencies.
  3. Individuals with phenotypes that confer higher fitness are more likely to survive and pass their underlying alleles to the next generation. (correct answer)
  4. All phenotypes have an equal chance of survival, but some genotypes are more likely to be passed on.
Explanation: This question addresses the link between phenotype, genotype, and selection. The environment selects for or against phenotypes (the observable traits). However, because phenotypes have a genetic basis, this selection indirectly affects which genotypes (and thus which alleles) are passed on to the next generation. Individuals with successful phenotypes reproduce more, increasing the frequency of the alleles responsible for those phenotypes in the population's gene pool.

Question 18

Which statement provides a Darwinian explanation for the evolution of a complex adaptation, such as the vertebrate eye?

  1. The ancestral species anticipated the need for vision and gradually developed an eye.
  2. A series of small, advantageous mutations accumulated over generations, with each step providing a survival advantage. (correct answer)
  3. A single, large-scale mutation produced a fully formed eye, which was immediately beneficial.
  4. Constant use of light-sensing organs by an ancestor caused them to become more complex, and this trait was inherited.
Explanation: Darwinian evolution explains complex adaptations as the result of the gradual accumulation of many small, intermediate mutations over a long period. Each step, such as a simple light-sensitive spot, must have conferred some selective advantage, however slight. Distractor A is teleological (goal-directed). Distractor C describes a saltational mutation, which is not the standard model for complex adaptations. Distractor D represents a Lamarckian explanation (inheritance of acquired characteristics).