Unit 3: Evolution and Diversity

Answer: C — The correct answer is A. Molecular clock theory suggests that the rate of genetic change accumulates relatively predictably over time. Higher DNA sequence similarity between two species indicates less evolutionary time has passed since they shared a common ancestor, meaning they diverged more recently. Since Species X and Y share 94% similarity (the highest percentage), they have experienced the least genetic divergence and therefore share the most recent common ancestor. Species X and Z (87%) and Y and Z (85%) show greater divergence, indicating these pairs diverged earlier. This data supports a branching pattern where X and Y are sister species that diverged from Z earlier. Option B is incorrect because Species X and Z share only 87% similarity, lower than X and Y. Option C is incorrect because the different percentages clearly indicate different divergence times, not simultaneous divergence. Option D is incorrect because Species Y cannot be ancestral to both X and Z while also sharing 85% similarity with Z; an ancestral species would show greater similarity to derived species, and the bidirectional similarity percentages indicate a sister-species relationship rather than an ancestor-descendant relationship.

Answer: D — The correct answer is A. This scenario describes natural selection, where organisms with traits better suited to their environment are more likely to survive and reproduce. The darker beetles likely had a survival advantage because they were better camouflaged from the new predator, allowing them to survive and pass on their genes more frequently. Option B is incorrect because evolution does not involve intentional adaptation; organisms do not consciously change their traits in response to environmental pressures. Option C is wrong because predators do not cause specific mutations, and mutations are random events unrelated to environmental needs. Option D is incorrect because while some migration might occur, this explanation does not account for why darker coloration would increase in frequency within the population itself; the question indicates a shift in the existing population's composition over generations, which is characteristic of natural selection acting on existing genetic variation.

Answer: C — The correct answer is C because natural selection is the process by which a population adapts to its environment through the survival and reproduction of individuals with favorable traits. In this scenario, the birds with larger beaks were more likely to survive and reproduce, passing on their trait to the next generation. A is incorrect because genetic drift is a random change in the frequency of a trait, which is not the case here. B is incorrect because gene flow would involve the introduction of new genes from a neighboring population, which is not mentioned. D is incorrect because mutation would involve a random change in the DNA sequence, which is not the primary mechanism driving the change in beak size in this scenario.

Answer: B — The correct answer is A. This question requires students to synthesize molecular data (genetic distances from mtDNA sequencing) with reproductive isolation data to infer evolutionary relationships and speciation mechanisms. The 94% sequence identity between A and B indicates a more recent divergence time compared to B and C (88% identity), which aligns with the molecular clock concept. Crucially, the ability of A and B to produce viable (though sterile) hybrids indicates they have undergone incomplete reproductive isolation, while the inability of B and C to produce viable embryos demonstrates more advanced reproductive isolation—consistent with a longer separation time. This combination supports that A and B diverged more recently. Answer B is incorrect because producing sterile hybrids actually indicates they are separate biological species under the biological species concept, and recent common ancestry alone doesn't determine current species status. Answer C confuses adaptive similarity with genetic distance; sequence identity reflects evolutionary time, not environmental adaptation, and environmental similarity would not explain the specific pattern of reproductive isolation observed. Answer D is incorrect because convergent evolution at mitochondrial loci would increase sequence similarity, not decrease it, and the data more parsimoniously explains the differences through divergence time rather than convergent evolution.

Answer: B — The correct answer is C because the scenario describes the intermediate stage of allopatric speciation. Key evidence supporting this: (1) The populations are geographically isolated by a river, which is the defining characteristic of allopatric speciation. (2) They show morphological divergence due to adaptation to different environmental conditions (sandy vs. clay soil, sunlight exposure), demonstrating that natural selection is driving evolutionary change. (3) They retain 98% genetic similarity and can still produce viable, fertile offspring, indicating that reproductive isolation is NOT yet complete—this is crucial. If complete speciation had occurred (option B), they would be reproductively isolated. The 50,000-year timeframe is consistent with early-stage divergence in many plant species. Why other options are incorrect: A) Sympatric speciation occurs without geographic isolation (populations diverge while in the same location). This scenario explicitly involves geographic separation by a river, making it allopatric, not sympatric. B) While the populations show morphological differences, the ability to produce viable, fertile offspring demonstrates they are still the same biological species. Reproductive isolation is the key criterion for speciation, and it hasn't occurred here. D) Convergent evolution produces similar traits in different species due to similar selective pressures, but here we see morphological DIVERGENCE (different traits), not convergence. Additionally, convergent evolution involves unrelated species, not populations of the same species.