19. Which of the following scenarios best illustrates top-down control in an ecosystem? (A) Nutrient enrichment in a lake increases phytoplankton growth, which then supports higher zooplankton and fish populations. (B) Reintroducing wolves to Yellowstone reduces elk grazing pressure, triggering a trophic cascade that allows woody vegetation and associated animal populations to recover. (C) Increased sunlight due to forest clearing boosts primary production, leading to higher herbivore biomass and more predators. (D) Fertilization of a grassland stimulates plant growth, resulting in larger populations of insects and insectivorous birds. (E) Drought reduces plant productivity, causing declines in herbivore numbers and subsequently fewer carnivores.

4. Which of the following descriptions is incorrect for a recognized type of natural selection? (A) Directional selection: A mode of natural selection in which individuals with phenotypes at one extreme of a trait distribution have higher fitness than those with intermediate or opposite extreme phenotypes. (B) Stabilizing selection: A mode of natural selection that favors both phenotypic extremes over intermediates, potentially increasing variation and leading to a bimodal distribution or speciation. (C) Balancing selection: A mode of natural selection that maintains genetic diversity in a population by favoring multiple alleles, often through mechanisms like heterozygote advantage or varying selection pressures across environments. (D) Purifying selection: A mode of selection that removes deleterious mutations or alleles, preserving functional genetic sequences and reducing genetic variation at conserved loci. (E) Frequency-dependent selection: A mode of selection where a phenotype's fitness depends on its frequency in the population—often, rare phenotypes have higher fitness.

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