Some types of organisms originated through endosymbiosis, the engulfin

OFFICIAL EXPLANATION

The stimulus can be intimidating because it contains several formidable scientific terms, but as usual these terms are explained as the stimulus progresses. Let us take a moment to recap the information in this problem. The stimulus begins by defining endosymbiosis, which is when one organism engulfs another such that the “conquered” organism still functions. Next, we are told that a nucleomorph—a DNA-containing, nucleus-like structure—has been discovered in a plant known as a chlorarachniophyte. As the last sentence begins we are told that this nucleomorph contains two versions of a particular gene. The remainder of the final sentence is the key to the problem and it illustrates how the test makers force students to maintain an awareness of conditional indicators. The conditional relationship in the final sentence is contained in the following section of text: “one would expect to find only a single version of this gene if the nucleomorph were not the remains of an engulfed organism’s nucleus.” In the middle of this section of text the word “if” appears, and produces the following conditional relationship: R = nucleomorph is the remains of an engulfed organism’s nucleus, 1 = find only a single version of this gene ~R—>1. However, as we know from our discussion of the last sentence, there was not a single version of the gene but two versions. This is equivalent to the necessary condition not occurring: ~1. Combining the two above diagrams, if the necessary condition does not occur, then the contrapositive is enacted and we can conclude that the sufficient condition does not occur: R. We can see the contrapositive in action more clearly if we link the two conditions in an arrow diagram: ~1—>R. In GMAT problems, the contrapositive is often introduced in this fashion, where a premise is given that contains both a sufficient and necessary condition, and then the necessary condition is denied. Adding those two pieces together produces a conclusion via the contrapositive that the sufficient condition cannot occur, as the following summary reveals: Premise 1: ~R—>1, Premise 2: 1. Conclusion: R. Premise 2 and the conclusion link together in the traditional contrapositive relationship we have seen previously: ~1—>R. Thus, the contrapositive produces the inference that the nucleomorph is the remains of an engulfed organism’s nucleus. With this information in hand, we can combine some of the other elements in the stimulus. Given that the nucleomorph is part of the chlorarachniophyte and is the remains of an engulfed organism’s nucleus, we have strong evidence that the chlorarachniophyte came about as the result of endosymbiosis (the engulfing
of one organism by another). This is most clearly stated in answer choice (E), the correct answer.

Answer choice (A): This is an exaggerated answer: the word “only” at the beginning of the answer choice is too strong, and there is no evidence in the stimulus to suggest that a conditional relationship exists between endosymbiosis and nucleomorphs.

Answer choice (B): This is also an exaggerated answer: the word “all” in the middle of the answer choice is too strong.

Answer choice (C): The stimulus discusses one unusual nucleomorph whereas the answer choice attempts to make a general statement about all nucleomorphs. As such, the answer choice exaggerates the situation and is incorrect. Note that the first three answer choices in this problem all exaggerate some aspect of the stimulus. This is not surprising: in a stimulus with complex terminology, the test makers wisely attempt to prey upon that difficulty by presenting answers that have a degree of truth but go just a bit too far. These answer choices are attractive to a test taker who fails to lock down the facts of the stimulus.

Answer choice (D): This answer choice is similar to answer choice (A), and is incorrect for similar reasons: no conditional relationship is established where nucleomorphs are necessary for endosymbiosis.