SAT Reading: 'Which Choice Most Logically Completes the Text?' (Mustard Glucosinolates)

Exact Passage & Question Stem

Many species of mustard plants synthesize glucosinolates, chemical compounds that deter generalist insect herbivores by releasing toxic breakdown products when plant tissues are damaged. Specialist herbivores, such as the larvae of the diamondback moth (Plutella xylostella), have evolved desulfatase enzymes that neutralize glucosinolates, allowing them to feed without ill effect. Evolutionary biologists observed that in wild mustard populations exposed exclusively to high densities of P. xylostella, the average concentration of glucosinolates decreased significantly over successive generations. Given that synthesizing glucosinolates imposes an energetic and metabolic cost on the plant, this evolutionary decline suggests that ________

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Which choice most logically completes the text?

- A) glucosinolate production provides a selective disadvantage to mustard plants when the primary herbivorous threat is unaffected by the chemical defense.

- B) the diamondback moth larvae will eventually lose the ability to produce desulfatase enzymes if glucosinolate concentrations drop below a critical threshold.

- C) wild mustard plants are unable to produce any alternative physical or chemical defenses against specialist herbivores once glucosinolate synthesis diminishes.

- D) generalist herbivores will inevitably replace specialist herbivores in habitats where wild mustard plants stop producing glucosinolates.

Logic Proof & Trap Breakdown

• Correct Answer: A
• Logical Proof: Glucosinolates cost metabolic energy. If P. xylostella neutralizes them, the plant pays energy with zero protection, reducing biological fitness (selective disadvantage).