Try a Sample Logical Reasoning Question: Strengthen

Test 73, Section 2, Question 22

Because the native salmon in Lake Clearwater had nearly disappeared, sockeye salmon were introduced in 1940. After being introduced, this genetically uniform group of sockeyes split into two distinct populations that do not interbreed, one inhabiting deep areas of the lake and the other inhabiting shallow areas. Since the two populations now differ genetically, some researchers hypothesize that each has adapted genetically to its distinct habitat.


Which of the following, if true, most strongly supports the researchers’ hypothesis?


A

Neither of the two populations of sockeyes has interbred with the native salmon.

B

When the native salmon in Lake Clearwater were numerous, they comprised two distinct populations that did not interbreed.

C

Most types of salmon that inhabit lakes spend part of the time in shallow water and part in deeper water.

D

One of the populations of sockeyes is virtually identical genetically to the sockeyes originally introduced in 1940.

E

The total number of sockeye salmon in the lake is not as large as the number of native salmon had been many years ago.

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The argument claims that, because a genetically uniform group of sockeyes introduced into Lake Clearwater in 1940 has now separated into two genetically distinct populations that don’t interbreed, the two populations must each have “adapted genetically” to its environment. We’re asked to strengthen that claim.

With these Strengthen questions, we’re generally looking for one of two things. We either want to show that the argument follows logically from the premises, or we want to defend the argument against some counterargument.

Here are my predictions for this question:

1) “When species branch into two distinct populations, it’s often/always because they have adapted to their environments.”

2) "An answer that rejects any number of potential counterarguments like, “neither of the sockeye populations was tested by scientists in ways that would alter their genetics.”

A. Great answer, right off the bat. A devastating weakener to this logic might have been, “No, dummy—they didn’t ‘adapt genetically’ to their deep-water or shallow-water environments; it’s just that some of them started mating with the native salmon in the lake, thereby becoming genetically distinct from the ones that didn’t.” I didn’t predict this attack, but when reading this answer, I’m like “oh dang—this answer defends the argument against that attack.” This is a defensive strengthened. As long as I can eliminate B–E, this will be the answer.

B. I don’t see how this explains why this new species is genetically distinct. It says the native species had two distinct populations. This native species might have been distinct because they, themselves, adapted to the environment of the lake. We just don’t know. In order to make B a strengthener at all, we have to assume that the native species were genetically distinct not because of their environment. We don’t have enough information to assume that. Even if we did assume that, we don’t care about the old native species. They could have genetically distinct for any number of reasons, and that wouldn’t say anything about the new sockeye salmon. The researchers didn’t make a case for why the native salmon were distinct, just the new sockeyes.

C. Who cares? This does nothing to change the argument. Think about it this way. If most salmon didn’t spend time in the deep and shallow ends of the lake, would that weaken the researchers’ argument? Not at all, since what we’re concerned with is these new sockeyes. We want to know why these sockeyes are genetically distinct.

D. This is the opposite of what we’re looking for. If one of the sockeye species is virtually identical to the originals from 1940, then one of the sockeyes didn’t genetically change. And if one of the sockeyes didn’t genetically change, they didn’t adapt to their environment. If anything, this weakens the argument.

E. We don’t care about the total number of salmon. Numbers of salmon don’t have anything to do with why the two populations of sockeyes are genetically distinct.

In a Strengthen question, stronger is better, but none of the other answers do anything to strengthen the researchers’ argument. A isn’t an offensive strengthener that helps the argument prove its conclusion based on the facts, but it’s still the best answer because at least it defends the argument against the attack of “how do you know some of them didn’t just get it on with the local salmon.” Since A defends the argument against one potentially devastating problem, A is our answer.

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