SAT Reading & Writing · Command of Evidence (textual)
Populations of Mexican cavefish living in separate, isolated caves have independently evolved reduced eyes and pigmentation. A textbook…
Populations of Mexican cavefish living in separate, isolated caves have independently evolved reduced eyes and pigmentation. A textbook states that because all cavefish populations share this same outward trait, they must all carry an identical mutation affecting the same gene. Which finding would most directly undermine the textbook's claim?
Answer and explanation
Answer: B: Hybrid crosses between fish from different caves can sometimes partially restore vision, because different cave populations carry mutations affecting different genes, meaning the same eye-loss trait…
The textbook's claim requires one single, shared mutation across all populations, and B directly refutes that by showing hybrid crosses can restore vision, a result only possible if different populations carry mutations in different genes instead of one identical shared mutation. A and C are general background facts about cave environments and cave-dwelling species that don't address the specific genetic-uniformity claim. D is background about relatedness that doesn't speak to the number of distinct mutations involved.
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