15. A small group of dark-eyed juncos was geographically isolated from their ancestral mountain population and established a new colony on the California coast. Genetic analysis shows that coastal males have significantly less white in their tail feathers (36%) than the ancestral group (45%). If this difference appeared immediately upon the establishment of the new colony, what is the most likely cause? (A) A high rate of de novo mutations occurring after the move to the coast. (B) Gene flow between the mountain and coastal populations increasing tail diversity. (C) A bottleneck effect caused by a sudden environmental disaster on the coast. (D) A founder effect, where the small initial group of birds possessed less white in their tails than the average of the source population. (E) Sexual selection by coastal females shifted the trait toward less white in less than one generation.

16. The common edible frog (Rana esculenta) is a hybrid resulting from the cross between Rana lessonae and Rana ridibunda. While these hybrids are widely distributed and physically robust, they exhibit "hybrid breakdown" or sterility when mating with other hybrids. Which of the following is the most accurate conclusion regarding the evolutionary relationship between the two parent species? (A) They are distinct species maintained by postzygotic isolation, as the hybrid offspring fail to produce a viable F₂ generation. (B) They are the same species because the production of a widely distributed F₁ generation proves a lack of reproductive barriers. (C) They are separated by prezygotic isolation, preventing the initial formation of zygotes in the wild. (D) The hybrids represent a new species under the Biological Species Concept because they can survive across diverse climates. (E) The two species are undergoing "reinforcement," where hybrid sterility eventually leads to the evolution of better mating calls to prevent hybridization.

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