- Which statement provides a Darwinian explanation for the evolution of a complex adaptation, such as the vertebrate eye?
Answer: A series of small, advantageous mutations accumulated over generations, with each step providing a survival advantage.
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).
- What is the expected immediate effect on the allele frequencies in the next generation?
Answer: The frequency of the R allele will decrease, and the frequency of the r allele will increase.
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.
- 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?
Answer: Random mutation or pre-existing genetic variation within the pest population.
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.
- A common misconception about natural selection is that it leads to 'perfect' organisms. Which statement best refutes this idea?
Answer: Natural selection can only act on existing genetic variation, which may not include the 'perfect' allele.
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.
- [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?
Answer: Disruptive selection, leading to a bimodal distribution of beak sizes with few intermediates.
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.
- For this change in morph frequency to occur through natural selection, which condition was absolutely necessary?
Answer: The colour variation was heritable, being passed from parents to offspring.
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.
- Which statement correctly distinguishes between the roles of mutation and natural selection in evolution?
Answer: Mutation introduces genetic variation randomly, while natural selection acts non-randomly on this variation.
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.
- 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?
Answer: Individuals with phenotypes that confer higher fitness are more likely to survive and pass their underlying alleles to the next generation.
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.
- 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?
Answer: It leads to a struggle for existence due to competition for limited resources.
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).
- What is meant by 'evolutionary fitness' in the context of natural selection?
Answer: The relative contribution an individual makes to the gene pool of the next generation.
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.
- Why is genetic variation within a population essential for natural selection to occur?
Answer: Without variation, all individuals would have the same fitness, and selection could not favour any particular trait.
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.
- Which statement best describes the evolutionary trade-off in this scenario?
Answer: The trait for bright colouration is influenced by both sexual selection and natural selection.
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.
- Which of the following is a key distinction between artificial selection and natural selection?
Answer: In artificial selection, the selective pressure is human preference, while in natural selection, it is environmental conditions.
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.
- How would this environmental change most likely affect a population of understory plants that has existing variation in light tolerance?
Answer: Individuals with pre-existing traits for higher light tolerance would have a reproductive advantage.
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.
- What is the most accurate description of the evolutionary process occurring in the mice on the lava rock?
Answer: Directional selection favoured the dark coat phenotype due to increased predation on light-coloured mice.
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.
- Which conclusion is most consistent with the principles of natural selection?
Answer: The winds acted as a selective pressure, removing birds with intermediate wing lengths from the population.
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.
- Which of the following describes a situation where natural selection is LEAST likely to be the primary agent of evolutionary change?
Answer: A small, isolated population of island flowers experiences a rapid shift in petal colour after a cyclone.
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.
- What is the primary role of streptomycin in the evolution of resistance in this bacterial population?
Answer: It acts as a selective pressure, favouring the reproduction of pre-existing resistant bacteria.
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.
