- Which of the following is a necessary condition for natural selection to lead to evolutionary change in a population?
Answer: There must be heritable variation for the trait being selected.
The correct answer is D. Natural selection can only act on traits that have a genetic basis and can be passed on to offspring (heritable). Furthermore, there must be differences (variation) among individuals for selection to have an effect. Without heritable variation, any survival differences among individuals will not result in a change in the population's genetic makeup over time. A is incorrect; Hardy-Weinberg equilibrium describes a population that is not evolving. B is incorrect; variation in fitness is the very basis of natural selection. C is incorrect; environmental change is often the driving force of natural selection.
- What is the most likely immediate consequence for the finch population in the next generation?
Answer: The average beak size in the population will increase because individuals with larger beaks are more likely to survive and reproduce.
The correct answer is B. The drought created a strong selective pressure. Finches that already had larger beaks (due to existing genetic variation) were better able to crack the available hard seeds, giving them a survival and reproductive advantage. This leads to an increase in the frequency of alleles for larger beaks in the next generation, thus increasing the average beak size. This is directional selection. A describes Lamarckian evolution. C is incorrect because mutations are random, not directed by environmental needs. D is possible, but given the stated variation in beak size, adaptation is a more likely outcome than immediate extinction.
- A significant environmental change occurs, creating a new selection pressure on a population. Which of the following factors would most increase the likelihood that the population will adapt and not go extinct?
Answer: A high degree of pre-existing genetic variation.
The correct answer is C. Natural selection can only work with the raw material available. A population with high genetic variation is more likely to contain individuals who already possess alleles that are advantageous in the new environment. These individuals will survive and reproduce, allowing the population to adapt. A small population size (A) makes a population more vulnerable to extinction and genetic drift. A low mutation rate (B) reduces the input of new variation. A long generation time (D) slows down the rate at which natural selection can produce change.
- In many mammal species, males are significantly larger and have more elaborate features (e.g., antlers, manes) than females, despite these features potentially increasing predation risk. How does the theory of natural selection explain the evolution of these seemingly disadvantageous traits?
Answer: These traits increase reproductive success by improving success in male-male competition or being attractive to females.
The correct answer is C. This describes sexual selection, which is a component of natural selection. Traits that increase an individual's reproductive success can be selected for even if they slightly decrease survival. In this case, success in competing for mates (intrasexual selection) or being chosen by mates (intersexual selection) outweighs the survival cost. A is incorrect because such widespread and complex traits are unlikely to be the result of random drift. B is a possible secondary benefit, but the primary driver for these sexually dimorphic traits is typically competition for mates. D describes the inheritance of acquired characteristics, a Lamarckian idea.
- How does this process differ from natural selection? [HL]
Answer: The selective agent is human preference rather than environmental pressures.
The correct answer is B. The key difference between artificial selection (domestication) and natural selection is the agent of selection. In natural selection, environmental factors (predation, climate, resource availability) determine which individuals are more likely to survive and reproduce. In artificial selection, humans determine which individuals reproduce based on desired traits. A is incorrect; both processes require heritable traits. C is an overgeneralization; while artificial selection is often faster, natural selection can also occur rapidly (e.g., antibiotic resistance). D is incorrect; both processes depend on pre-existing variation.
- A common misconception is that an organism can adapt to its environment during its lifetime. For example, a giraffe might stretch its neck to reach higher leaves. How does the modern theory of evolution by natural selection refute this idea?
Answer: Natural selection acts on populations, not individuals, by favouring heritable traits that enhance reproductive success.
The correct answer is A. This addresses the core difference between Lamarckian evolution and Darwinian evolution. Natural selection operates on the level of the population over generations. An individual organism does not evolve. Instead, individuals with certain inherited traits are more likely to survive and reproduce than others, causing the frequencies of those traits to increase in the population over time. B is not entirely true (e.g., muscle development), but more importantly, such acquired traits are not heritable. C is incorrect; mutation is random, but selection is non-random. D is an unsupported claim and irrelevant to the mechanism.
- In the context of evolution, what is the most accurate definition of 'biological fitness'?
Answer: The number of fertile offspring an individual produces that survive to reproduce.
The correct answer is C. Biological or Darwinian fitness is a measure of reproductive success. It is not just about survival (A) or physical prowess (B), but about the contribution an individual makes to the gene pool of the next generation. An individual that survives a long time but has no offspring has a fitness of zero. An individual that produces many offspring that themselves go on to reproduce is considered highly fit. Complexity (D) is not a measure of fitness.
- A large, genetically diverse population of snails lives on an island where the terrain consists of either very light-coloured rocks or very dark, volcanic soil, with no intermediate shades. Birds that prey on the snails hunt by sight. Which pattern of selection is most likely to act on the shell colour of these snails over time? [HL]
Answer: Disruptive selection, favouring both very light and very dark shells while selecting against intermediate shades.
The correct answer is C. The environment has two distinct niches where different extreme phenotypes are favoured. Snails with very light shells are camouflaged on the rocks, and snails with very dark shells are camouflaged on the soil. Intermediate-coloured snails would be poorly camouflaged in both environments and thus be preyed upon more heavily. This selection against the mean and for the extremes is the definition of disruptive selection. A is incorrect because both extremes are favored. B is incorrect because the intermediate phenotype is selected against. D is incorrect as the birds' hunting is a natural, not an artificial, selection pressure.
- Which statement accurately describes the role of the new plant toxin in the evolution of the insect population?
Answer: The toxin acted as a selective pressure, increasing the frequency of the pre-existing resistance allele.
The correct answer is B. Natural selection acts on existing variation. The plant toxin did not create the resistance allele; it only provided an environmental pressure that favored individuals already possessing it. This led to an increase in the allele's frequency over generations. A is incorrect because selection pressures do not cause specific mutations; mutations arise randomly. C describes a Lamarckian concept of inheritance of acquired characteristics, which is not the mechanism of natural selection. D is incorrect because while the toxin initially decreases fitness for many, the population is clearly adapting, as shown by the survival of resistant individuals.
- A new viral disease emerges and wipes out 99% of a cheetah population. The remaining 1% of cheetahs survive because they possess a rare, heritable trait that confers immunity. What is the most likely long-term consequence for the cheetah population if it recovers?
Answer: The recovered population will have less genetic diversity than the original population.
The correct answer is A. This event is a population bottleneck, a type of genetic drift. Even though natural selection determined which individuals survived (the immune ones), the drastic reduction in population size means that a vast amount of genetic variation (alleles unrelated to immunity) is lost from the gene pool. The recovered population will be descended from a small number of founders, resulting in lower overall genetic diversity. B is incorrect because the allele frequencies (especially for immunity) will be drastically different. C is incorrect as the immunity is a heritable trait. D describes inheritance of acquired characteristics.
- This situation is an example of which type of natural selection? [HL]
Answer: Stabilizing selection, as it favours the intermediate phenotype.
The correct answer is C. Stabilizing selection occurs when intermediate phenotypes have higher fitness than extreme phenotypes. In this case, babies with an average birth weight have the highest survival rates, so the selection pressures act to keep the population mean stable and select against the extremes (very small and very large babies). A is the opposite of the scenario. B would occur if, for example, only larger babies survived best. D is incorrect as mate choice is not the primary selective pressure on birth weight described.
- What is the expected outcome of the second antibiotic application compared to the first?
Answer: A higher percentage of the bacterial population will survive the second application.
The correct answer is A. The first application of the antibiotic selected for resistant individuals. These individuals reproduced, passing the resistance allele to their offspring. Therefore, the second generation starts with a much higher frequency of the resistance allele than the original population, leading to a higher survival rate when the same antibiotic is reapplied. B is incorrect because antibiotics do not cause mutations. C is incorrect; the population is not 'weakened' but is instead 'stronger' in terms of resistance. D is the opposite of what natural selection would predict; the resistance allele is beneficial and will be selected for, not lost.
- What type of selection best explains this long-term trend in the horse lineage? [HL]
Answer: Directional selection
The correct answer is A. Directional selection occurs when conditions favour individuals exhibiting one extreme of a phenotypic range, thereby shifting a population's frequency curve for the phenotypic character in one direction or the other. The consistent trend towards larger size and fewer toes in response to the changing environment (from forest to grassland) is a classic example of directional selection operating over a long evolutionary timescale. B, C, and D do not fit the described pattern of a consistent shift in one direction.
- This scenario, where the fitness of a phenotype depends on its frequency in the population, is an example of what evolutionary mechanism? [HL]
Answer: Negative frequency-dependent selection
The correct answer is C. Negative frequency-dependent selection occurs when the fitness of a phenotype decreases as it becomes more common. In this case, both the dominant and sneaker male strategies are successful when they are rare and less successful when they are common. This type of selection maintains genetic variation in the population by preventing any single strategy from becoming fixed. A is incorrect as this is a selective process, not a random one. B is incorrect as it favors an intermediate, not oscillating frequencies of different strategies. D is the opposite; it occurs when fitness increases as a phenotype becomes more common, which would lead to the fixation of one strategy.
- What is the most accepted evolutionary explanation for the loss of eye function in these cavefish?
Answer: Maintaining functional eyes requires metabolic energy, so individuals with mutations causing eye reduction had higher fitness.
The correct answer is C. This explanation relies on the principle of energy trade-offs. Developing and maintaining complex structures like eyes is metabolically expensive. In a completely dark environment where eyes provide no benefit, any random mutation that reduces eye development would free up energy for other functions like growth or reproduction. These individuals would have a slight fitness advantage and be selected for over time. A is a Lamarckian explanation. B is incorrect because mutations are random and are not directed by environmental needs. D is speculative and not supported by the principles of selection; evolution is not goal-directed.
- Why is the concept of 'overproduction of offspring' a critical component of the theory of natural selection?
Answer: It creates competition for limited resources, leading to differential survival and reproduction.
The correct answer is C. Thomas Malthus's principle, which Darwin applied to nature, states that organisms produce more offspring than can possibly survive given limited resources (food, space, etc.). This leads to a 'struggle for existence'. In this struggle, individuals with advantageous heritable traits are more likely to survive and reproduce, driving natural selection. A is incorrect; sexual reproduction ensures offspring are not identical. B is the opposite of what happens; competition limits population growth. D is incorrect; overproduction and sexual reproduction maintain or increase variation.
- What is the best evolutionary explanation for this observation?
Answer: The pesticide has acted as a selective agent, favouring the reproduction of pre-existing resistant pests.
The correct answer is C. This is a classic example of natural selection for pesticide resistance. The original pest population had variation in susceptibility. The pesticide killed the susceptible individuals, leaving the resistant ones to survive and reproduce. Over generations, the frequency of resistance alleles in the population increased. A is incorrect because acclimatization is a non-heritable change within an individual's life, not an evolutionary change in a population. B incorrectly states that the pesticide caused the mutations for resistance. D describes a behavioural change, which is possible, but the need for a higher pesticide concentration points directly to increased physiological resistance in the pest population that still feeds on the crop.
- What is the most plausible explanation for the difference in coloration between the two fish populations?
Answer: The different predation pressures in each environment have selected for different optimal phenotypes.
The correct answer is C. This is a classic example of divergent selection. In the stream, predation from above selects for camouflage (drab colours). In the lake, the absence of this pressure and the presence of a different pressure (detection by movement) may allow sexual selection for bright colours (which can signal fitness to mates) to become the dominant selective force. A is incorrect as environmental conditions do not cause specific mutations. B suggests genetic drift is the sole cause, which is less likely than selection given the strong correlation between environment and phenotype. D incorrectly suggests that individuals can change their traits in response to a need and that migration is directed towards this goal.
