LSAT Explanation PT 43, S3, Q17: If one of the effects of

LSAT Question Stem

If the statements above are true, each of the following could be true EXCEPT: 

Logical Reasoning Question Type

This is a Must Be False question. 

Correct Answer

The correct answer to this question is E. 

LSAT Question Complete Explanation

This LSAT problem presents a passage that describes the process of natural selection, specifically focusing on genetic mutations and their effects on the survival of a species. The question type is MBF (Must Be False), which asks us to identify the answer choice that cannot be true if the statements in the passage are true.

Let's first analyze the passage and understand the main points. The passage states that a genetic mutation will be favored in natural selection if and only if it makes a substantial contribution to the survival of the species. It also mentions a proviso that any traits not favored but carried along by a favored trait must not be so negative as to cancel out the benefits of the new, favored trait.

We can represent this relationship using formal logic notation:

SC = substantial contribution to the survival of a species

MF = mutation favored in natural selection

SC <-> MF

Now, let's evaluate each answer choice in light of the passage:

a) A species possesses a trait whose effects are all neutral for the survival of that species.

This answer choice is possible, as the passage does not place any restrictions on the types of traits a species may already possess. However, such a trait would not be favored in natural selection but could be carried along by a trait that was favored.

b) All the effects of some genetic mutations contribute substantially to the survival of a species.

This answer choice is also possible, and such mutations would likely be favored in natural selection, as they meet the condition of making a substantial contribution to the survival of the species.

c) A species possesses a trait that reduces the species' survival potential.

This answer choice is possible, as the passage does not preclude the existence of traits that reduce the survival potential of a species. The passage only states that such traits will not be favored in natural selection.

d) A genetic mutation that carries along several negative traits is favored in natural selection.

This answer choice is possible, as long as the associated benefit of the mutation in question is greater than all of the negative traits that it carries. In this case, the mutation would still be favored in natural selection.

e) A genetic mutation whose effects are all neutral to a species is favored in natural selection.

This is the correct answer choice, as it contradicts the passage's main point. According to the passage, for a mutation to be favored in natural selection, it must make a substantial contribution to the survival of the species (SC <-> MF). A mutation with neutral effects does not make a substantial contribution, so it cannot be true that such a mutation is favored in natural selection.

In conclusion, answer choice (e) is the one that cannot be true if the statements in the passage are true, making it the correct answer for this LSAT problem.