Question 181·200 Super-Hard SAT Reading Questions·Information and Ideas

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To reconstruct how wind-pollinated silverleaf shrubs colonized Windlass Cay, ecologist Priya Nanduri and colleagues compared the shrubs’ DNA with that of mainland populations. In this species, chloroplast DNA is transmitted through seeds but not through pollen, whereas nuclear DNA can arrive by either route. Every cay shrub carries a chloroplast variant otherwise found only at Cape Orin. Moreover, the oldest cay shrubs’ nuclear DNA most closely resembles that of Cape Orin shrubs, but progressively younger shrubs contain larger proportions of nuclear variants common in two other mainland regions. Ocean-current simulations show that floating fruits can travel from Cape Orin to the cay, while currents carry fruits from the other regions away from it. Parentage analysis of seeds now forming on the cay identifies pollen donors in all three mainland regions. Taken together, these findings indicate that ______

Which choice most logically completes the text?

For inference questions involving several forms of evidence, assign a specific role to each detail before evaluating the choices. Here, distinguish evidence about seed movement from evidence about pollen movement, then use the shrubs’ ages to determine the sequence of events. Reject options that invent an unsupported process or conflict with how the passage says each kind of DNA is transmitted.

Hints

Hint 1
Separate the two kinds of DNA
Consider which kind of DNA can reveal the geographic source of seeds and which kind can also be transmitted through pollen.
Hint 2
Compare shrub ages
Notice how the nuclear DNA of the oldest shrubs differs from that of progressively younger shrubs.
Hint 3
Use the transport evidence
Compare the ocean-current simulations with the parentage analysis to determine which material can plausibly travel from each mainland region.

Step-by-step

Step 1
Identify the likely seed source
Because chloroplast DNA is transmitted only through seeds, the Cape Orin chloroplast variant in every cay shrub points to Cape Orin as the founding seed source. The ocean-current simulations reinforce this inference.
Step 2
Interpret the age pattern
The oldest shrubs are genetically closest to Cape Orin, while younger shrubs contain progressively more nuclear variants associated with other regions. This pattern suggests that non-Cape Orin genetic material accumulated after colonization.
Step 3
Determine how the later variation arrived
Nuclear DNA can be transmitted through pollen, and the parentage analysis directly identifies pollen donors from all three mainland regions. Thus, pollen provides the best-supported explanation for the additional nuclear variation in younger shrubs.

Question 181·200 Super-Hard SAT Reading Questions·Information and Ideas

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