Question 3·Hard·Inferences
In 2003 wildlife managers attempting to curb an overabundant European rabbit population on Isla Verde released what they believed were locally sourced European rabbit fleas (Spilopsyllus cuniculi) that parasitize only rabbits. In 2016, geneticists compared DNA from those fleas to samples of an Australian flea strain that had been used decades earlier for a similar purpose on that continent, as well as to fleas collected in several ports of western Europe closest to Isla Verde. The Isla Verde fleas shared six rare mutations with the Australian strain, mutations completely absent from all contemporary European samples. Port authority archives for 2000–2005 contain no record of live animal or insect shipments from Australia directly to Isla Verde. Thus, the researchers concluded that...
Complete the sentence.
Which choice most logically completes the text?
For SAT "complete the text logically" questions, especially with words like "Thus," "Therefore," or "Consequently," first summarize in your own words what conclusion the author is building toward using the last few sentences. Identify the key pieces of evidence (here, the genetic match and the shipping records), then choose the option that (1) uses all of that evidence, (2) stays within what the passage makes reasonable, and (3) does not introduce new, unsupported explanations. Quickly eliminate choices that require extra assumptions or ignore one of the main facts just stated.
Hints
Focus on what the DNA comparison shows
Look at the sentence about the six rare mutations and which flea populations do and do not have them. What does that suggest about where the Isla Verde fleas came from?
Pay attention to the word "Thus"
The word "Thus" signals that the conclusion must follow from the two main facts just given: the pattern of mutations and the lack of direct shipping records. Ask yourself: what conclusion uses both pieces of information?
Check for new, unsupported ideas
Eliminate any option that introduces a process or event (like hybridization or past mutations) that the passage does not mention or support. The correct ending should be a reasonable inference, not speculation.
Step-by-step Explanation
Identify the key evidence about the fleas
First, restate the important facts:
- In 2003, managers released fleas they thought were local (European) onto Isla Verde.
- In 2016, geneticists compared DNA from Isla Verde fleas, an Australian flea strain, and fleas from nearby European ports.
- Isla Verde fleas shared six rare mutations with the Australian strain.
- These six mutations were completely absent from all current European flea samples.
Understand what the genetics imply
If the Isla Verde fleas share six rare mutations with the Australian fleas, and none of the nearby European fleas have them, the strong implication is that the Isla Verde fleas are closely related to, or descended from, the Australian strain rather than from local European fleas. Six rare shared mutations happening independently in different places would be extremely unlikely.
Use the port records to refine the conclusion
The port archives say there was no record of direct live animal or insect shipments from Australia to Isla Verde during the relevant years. That does not mean fleas never traveled from Australia; it only means they did not do so directly in recorded shipments. A reasonable scientific conclusion is that the fleas still got from Australia to Isla Verde, but in an unrecorded or indirect way, such as passing through other ports or traveling unnoticed.
Match the logical conclusion to the choices
Now pick the option that:
- Explains why Isla Verde fleas have the same rare mutations as the Australian strain.
- Fits the lack of recorded direct shipments.
- Does not add new, unsupported ideas.
Only choice B) fleas carrying the rare mutations were unknowingly transported from Australia to Isla Verde through one or more intermediary ports uses both the genetic evidence and the port records to give a logical, evidence-based explanation.