LSAT 121 – Section 4 – Question 26

LSAT 121 - Section 4 - Question 26

December 2005

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Question
QuickView		 Type Tags Answer
Choices		 Curve Question
Difficulty		 Psg/Game/S
Difficulty		 Explanation
PT121 S4 Q26
+LR
Weaken +Weak
Link Assumption +LinkA
A
11%
160
B
67%
166
C
8%
159
D
11%
160
E
3%
158
149
158
167
 +Harder 146.544 +SubsectionMedium

Researcher: It is commonly believed that species belonging to the same biological order, such as rodents, descended from a single common ancestor. However, I compared the genetic pattern in 3 rodent species—guinea pigs, rats, and mice—as well as in 13 nonrodent mammals, and found that while rats and mice are genetically quite similar, the genetic differences between guinea pigs and mice are as great as those between mice and some nonrodent species. Thus, despite their similar physical form, guinea pigs stem from a separate ancestor.

Summarize Argument: Phenomenon-Hypothesis
The researcher hypothesizes that guinea pigs do not share a common ancestor with mice. Although guinea pigs and mice are both rodents, the researcher found that they have very different genetic patterns—as different as between mice and some nonrodent species.

Notable Assumptions
The researcher assumes that dissimilar genetic patterns are sufficient to conclude that two species have separate ancestors.

A
The researcher examined the genetic material of only 3 of over 2,000 species of rodents.
This does not affect the argument. The researcher hypothesizes that guinea pigs and mice stem from separate ancestors based on her finding that they have dissimilar genetic patterns—the researcher does not make an argument about rodents broadly.
B
Some pairs of species not having a common ancestor are genetically more similar to each other than are some pairs that do have a common ancestor.
This weakens the argument. It attacks the researcher’s assumption that dissimilarity in genetic patterns is enough to conclude two species lack a single common ancestor by noting that some species who do share an ancestor also lack similar genetic patterns.
C
The researcher selected nonrodent species that have the specific cell structures she wanted to analyze genetically, though many nonrodent mammals lack these cell structures.
This does not affect the argument. The researcher only claims that the genetic difference between mice and guinea pigs is as great as between mice and “some” nonrodent species.
D
For some genuine biological orders, the most recent common ancestor dates from later epochs than does the most recent common ancestor of other biological orders.
This does not affect the argument, which is that given their dissimilar genetic patterns, mice and guinea pigs do not share a single common ancestor at all. The recency of a common ancestor is irrelevant.
E
Peculiarities of body structure, such as distinctive teeth and olfactory structures, are shared by all rodents, including guinea pigs.
This does not affect the argument, which already notes that guinea pigs and mice share similar physical forms.

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LSAT PrepTest 121 Explanations

Section 1 - Logical Reasoning

Section 2 - Reading Comprehension

Section 3 - Reading Comprehension

Section 4 - Logical Reasoning

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