If one of the effects of a genetic mutation makes a

have the same doubt but as pointed out by metallicafan it is a 'could be true' q, tricky one.
any idea we get such questiosn on GMAT?

A has zero contradictions because of the word "possesses" a trait. Its untouched since the arg is based on "genetic mutations".

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

between D and E, D also can be possible since the negative effect may not be so much as to annul the positive effect of a mutation.

E

'D' is wrong
because

'traits must not be so negative as to annul the benefits of having the new,favored trait"

"A genetic mutation that carries along several
negative traits is favored in natural selection."
In option 'B' negative traits are favored in natural selection
but they should not be so negative

Interesting question. Took me just under 2 minutes since it's a could be question, so I had to entertain all possible scenarios.

A: Nowhere does it say that a species cannot possess a trait whose effects are all neutral. The argument deals with mutations being favored in natural selection, not traits per se.
B: Sort of tricky. While a negative trait can be carried along as part of a mutation if it's not detrimental to the species' survival, it is certainly possible that there exists mutations whose effects are entirely positive.
C: Yes, a species can definitely possess a negative trait. However it will not be favored in natural selection as part of a mutation, but the stimulus does not rule out the possibility of a species possessing such a trait.
D: As long as the negative trait is not detrimental, it can be favored during natural selection as part of a mutation.
E: Correct answer. The first sentence explicitly states that one of the effets of a mutation has to contribute substantially to the survival of a species in order for it to be favored in natural selection. Thus, if its effects are all neutral, it cannot be favored.

Some LSAT questions are very similar to GMAT questions - but not this one. This one is almost pure formal logic. Your chances of seeing "if and only if" (here, "then and only then") in a GMAT question are extremely slim. If you really want to analyze this question (which I do NOT recommend if the GMAT is your goal), the key points are: (1) "A if and only if B" means BOTH "If A is true, then B is true" AND "If B is true, then A is true"; and (2) "Could be true EXCEPT" means that the right answer MUST be false if the paragraph is true, while any answer which MUST be true or CAN be true is WRONG. Applying point (1) to the paragraph tells us that if a mutation is favoured, then it must contribute substantially to survival. If this is true, then E has to be false; it says that a certain mutation was favoured which did NOT contribute to survival.

A - could happen since the stimulus states nothing about having no such trait...
B - could happen...
C - could be true.... Some negative to annul the positive was mentioned.
D - could happen as long as it doesn't annul the benefits.. Or it is to so negative...
E - False. The stimulus states that the trait favored is the one with subs. effect...

If one of the effects of a genetic mutation makes a
substantial contribution to the survival of the species,
then, and only then, will that mutation be favored in
natural selection. This process is subject to one proviso,
namely that the traits that were not favored, yet were
carried along by a trait that was favored, must not be
so negative as to annul the benefits of having the new,
favored trait.
If the statements above are true, each of the following
could be true EXCEPT:
We basically need to understand that a genetic mutation is made up of one or more traits or effects creating a "NET EFFECT"
(A) A species possesses a trait whose effects are all
neutral for the survival of that species. A trait whose effects are neutral may be carried forward by a positive trait creating net positive effect. Hence, could be true.
(B) All the effects of some genetic mutations
contribute substantially to the survival of a
species. A mutation with several traits all of which contribute substantially will have a net positive effect. Hence, could be true.
(C) A species possesses a trait that reduces the
species’ survival potential.One of the several traits could have a negative effect but it could be outweighed and the net effect could still be positive.Hence, could be true.
(D) A genetic mutation that carries along several
negative traits is favored in natural selection.Again, the net effect could still be positive despite many negative traits. Hence, could be true
(E) A genetic mutation whose effects are all neutral
to a species is favored in natural selection. If all the traits(or "effects") in a mutation are neutral the net effect is neutral meaning it doesn't contribute to survival so it cannot be favoured.

If one of the effects of a genetic mutation makes a substantial contribution to the survival of the species, then, and only then, will that mutation be favored in natural selection. This process is subject to one proviso, namely that the traits that were not favored, yet were carried along by a trait that was favored, must not be so negative as to annul the benefits of having the new, favored trait.

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

We need to take care that the question stem says COULD BE TRUE and NOT must be true.

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

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

(C) A species possesses a trait that reduces the species’ survival potential.

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

(E) A genetic mutation whose effects are all neutral to a species is favored in natural selection.
----> this is the only option that is not in line with what is stated in the argument. As the argument states that a genetic mutation will only be favored in natural selection when it makes a significant contribution. If effects are all neutral then it will not be favored in natural selection.

For could be true except questions the wrong answer will be that which directly opposes the stimulus. Any answer that is not supported by the stimulus in at all could be true and is thus wrong. The correct answer will directly oppose the stimulus making it impossible that it could be true

Only E fits this

1. genetic mutation must have positive effect to carry forward
2. one trait in it can have negative effect if doesn't mess with the positive trait and makes overall effect negative
E) is correct as it is the only one that can never happen- a genetic mutation must have a positive effect to carry forward