- PowerScore Staff
- Posts: 8950
- Joined: Feb 02, 2011
- Tue Jun 21, 2011 11:00 pm
#36680
Complete Question Explanation
Cannot Be True—SN. The correct answer choice is (E)
This stimulus is a simplified description of the process of natural selection. That is, any traits, and only
those traits, that convey a significant advantage to a species will be favored in natural selection. To
evaluate this stimulus, it is critical to note that any given genetic mutation may have numerous effects.
Some of these may help a species survive, some may harm the species, and others may have no effect
at all. A species may have any number of genetic mutations, but only those whose cumulative effect is
beneficial to the species will be favored in natural selection.
For example, suppose that a genetic mutation led to the birth of the first rhinoceros with a horn. Some
effects of this mutation – such as the rhinoceros’ unusual appearance – would probably be neutral to
the animal’s survival. Other effects, such as increased weight and drag of the horn, may have made this
rhinoceros somewhat slower and easier to catch than other rhinoceroses. This is obviously negative.
Of course, this rhino’s ability to maim and gouge predators with its horn would make a substantial
contribution to its survival. As long as the advantage of being able to wound predators outweighed the
disadvantage of running slower, then the trait of possessing a horn would be favored in natural selection
(unless the horn made the rhinoceros so odd or undesirable that it was unable to attract a mate).
The stimulus also contains an interesting feature, in that it uses the phrase “then, and only then” to relate
the ideas of a trait making a substantial contribution to the survival of a species and the favorability of
that trait. “Then, and only then” yields the following diagram in this instance:
SC = substantial contribution to the survival of a species
MF = mutation favored in natural selection
SC 
MF
The double-arrow is due to the fact that “then, and only then” indicates that the idea of a genetic
mutation making a substantial contribution to the survival of a species is both a sufficient and a
necessary condition with respect to that mutation being favored in natural selection. This is a crucial
inference and will be tested in the correct answer.
A quick scan of the answer choices reveals that only (D) and (E) address natural selection, which is
clearly a critical aspect of the stimulus. Although each answer choice will be discussed sequentially
below, test takers will save time and increase accuracy during the test by accelerating to the most likely
answer choices as quickly as possible.
Answer choice (A): The stimulus places no restrictions upon the kinds of traits which a species may
already possess. Having two tails would probably be completely neutral for the survival of a rhinoceros,
but it could be possible. Such a trait could certainly exist, however, if the stimulus is true, such a trait
could not be favored in natural selection (but could be, as the stimulus mentions, a trait that was carried
along by a trait favored in natural selection).
Answer choice (B): It is certainly possible that all the effects of some genetic mutations contribute
substantially to the survival of a species. Furthermore, such mutations, if present, would almost certainly
be favored in natural selection.
Answer choice (C): The stimulus does not preclude the existence of traits which reduce the survival
potential of a species. It only states that such traits will not be favored in natural selection. Therefore,
this answer is possible and is not the correct choice.
Answer choice (D): So long as the associated benefit of the mutation in question is greater than all of
the negative traits that it carries, that mutation would still be favored in natural selection. Thus, answer
choice (D) could also be true.
Answer choice (E): This is the correct answer choice. According to the stimulus, for a mutation to
be favored in natural selection it must make a substantial contribution to the survival of the species
(remember the “then, and only then” diagram: SC MF). So it cannot be true that a mutation
who effects are neutral (not substantially positive) is favored in natural selection. Therefore, the scenario
in answer choice (E) could not be true.
Cannot Be True—SN. The correct answer choice is (E)
This stimulus is a simplified description of the process of natural selection. That is, any traits, and only
those traits, that convey a significant advantage to a species will be favored in natural selection. To
evaluate this stimulus, it is critical to note that any given genetic mutation may have numerous effects.
Some of these may help a species survive, some may harm the species, and others may have no effect
at all. A species may have any number of genetic mutations, but only those whose cumulative effect is
beneficial to the species will be favored in natural selection.
For example, suppose that a genetic mutation led to the birth of the first rhinoceros with a horn. Some
effects of this mutation – such as the rhinoceros’ unusual appearance – would probably be neutral to
the animal’s survival. Other effects, such as increased weight and drag of the horn, may have made this
rhinoceros somewhat slower and easier to catch than other rhinoceroses. This is obviously negative.
Of course, this rhino’s ability to maim and gouge predators with its horn would make a substantial
contribution to its survival. As long as the advantage of being able to wound predators outweighed the
disadvantage of running slower, then the trait of possessing a horn would be favored in natural selection
(unless the horn made the rhinoceros so odd or undesirable that it was unable to attract a mate).
The stimulus also contains an interesting feature, in that it uses the phrase “then, and only then” to relate
the ideas of a trait making a substantial contribution to the survival of a species and the favorability of
that trait. “Then, and only then” yields the following diagram in this instance:
SC = substantial contribution to the survival of a species MF = mutation favored in natural selection SC MFThe double-arrow is due to the fact that “then, and only then” indicates that the idea of a genetic
mutation making a substantial contribution to the survival of a species is both a sufficient and a
necessary condition with respect to that mutation being favored in natural selection. This is a crucial
inference and will be tested in the correct answer.
A quick scan of the answer choices reveals that only (D) and (E) address natural selection, which is
clearly a critical aspect of the stimulus. Although each answer choice will be discussed sequentially
below, test takers will save time and increase accuracy during the test by accelerating to the most likely
answer choices as quickly as possible.
Answer choice (A): The stimulus places no restrictions upon the kinds of traits which a species may
already possess. Having two tails would probably be completely neutral for the survival of a rhinoceros,
but it could be possible. Such a trait could certainly exist, however, if the stimulus is true, such a trait
could not be favored in natural selection (but could be, as the stimulus mentions, a trait that was carried
along by a trait favored in natural selection).
Answer choice (B): It is certainly possible that all the effects of some genetic mutations contribute
substantially to the survival of a species. Furthermore, such mutations, if present, would almost certainly
be favored in natural selection.
Answer choice (C): The stimulus does not preclude the existence of traits which reduce the survival
potential of a species. It only states that such traits will not be favored in natural selection. Therefore,
this answer is possible and is not the correct choice.
Answer choice (D): So long as the associated benefit of the mutation in question is greater than all of
the negative traits that it carries, that mutation would still be favored in natural selection. Thus, answer
choice (D) could also be true.
Answer choice (E): This is the correct answer choice. According to the stimulus, for a mutation to
be favored in natural selection it must make a substantial contribution to the survival of the species
(remember the “then, and only then” diagram: SC
MF). So it cannot be true that a mutationwho effects are neutral (not substantially positive) is favored in natural selection. Therefore, the scenario
in answer choice (E) could not be true.
