PrepTest 121, Section 4, Question 26
Researcher: It is commonly believed that species belonging(77 more words not shown)
Which one of the following,(8 more words not shown)
- AThe researcher examined(12 more words not shown)
- BSome pairs of(23 more words not shown)
- CThe researcher selected(21 more words not shown)
- DFor some genuine(22 more words not shown)
- EPeculiarities of body(16 more words not shown)
Reading the argument
Could two species share an ancestor and still end up genetically quite different? That's the possibility the researcher needs to address. We learn that guinea pigs and mice differ genetically as much as mice and some nonrodents do. But the conclusion concerns where those species came from. Perhaps descendants of the same ancestor changed in different ways as they adapted to different circumstances. If so, their present differences wouldn't establish separate origins.
What we’re looking for
For a weakener, I'd look for a reason the genetic comparison doesn't settle ancestry. We don't have to claim genetics tells us nothing or establish the actual history of guinea pigs. We just need to undermine the assumption that the observed degree of difference rules out the shared ancestor.
Working the answers
- A
Studying more rodent species might provide more information. But pointing to the number left unstudied doesn't explain what is wrong with the inference about the species that were examined. The conclusion concerns guinea pigs' ancestry, not a claim that the three sampled species represent every rodent. A larger possible research project doesn't itself resolve our concern about what genetic differences establish.
- BCorrect
Some species with separate ancestors are genetically closer than some species with a shared ancestor. That means the degree of genetic similarity doesn't sort the pairs neatly into common-ancestor and separate-ancestor groups.
A pair can share an ancestor despite being more genetically different than another pair that doesn't. So the researcher's comparison could be accurate without establishing the claimed separate origin of guinea pigs. B doesn't prove that guinea pigs and mice share an ancestor; it shows why this comparison is not enough to rule that possibility out.
- C
Selecting species with the cell structures needed for the analysis doesn't by itself show that the genetic comparisons are misleading. We'd need a reason that selection distorts the connection to ancestry. C tells us why certain species were available for comparison, but leaves the researcher's inference about genetic difference and origins unaddressed.
- D
Some biological orders have a more recent common ancestor than others. That tells us about differences in the age of shared ancestry, not what degree of genetic difference can occur among species sharing an ancestor. Without that connection, the dates don't undermine the researcher's use of the particular genetic comparison.
- E
The researcher already acknowledges similar physical form and argues that the genetic evidence nevertheless indicates separate ancestry. Listing shared teeth and olfactory structures adds detail to that acknowledged resemblance. It doesn't explain why the genetic difference could coexist with a shared ancestor, which is the inference we need to challenge.
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