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PrepTest 73, Logical Reasoning 1, Question 22


Question 22, when a question asks you for something in the answer choices that most strongly supports the argument you've been given, that's a strengthened question. Strengthen questions require us to support the arguments unstated assumptions. The things that the argument doesn't say but have to be true in order for the evidence the argument gives to be able to prove its conclusion. This arguments conclusion, this found the very end.

It's what some researchers hypothesize that each and spoiler alert they're talking about salmon in Lake Clearwater. Each species of salmon has adapted genetically to its distinct habitat. The evidence begins by just telling us what happened. First off, the native salmon and Lake Clearwater had almost disappeared. So they introduced sockeye salmon in 1940.

After that, the salmon that they introduced split into two distinct populations that didn't interbreed. One in the deep parts of the lake and the other in the shallow parts of the lake, the only piece of evidence that's used directly to prove their conclusion. Follows the evidence keyword, since the two populations now differ genetically. So they introduced some salmon, the salmon split up into two different parts of the lake, and they differ genetically because of that.

They say that each of those populations must have adapted genetically to the distinct habitat that they're in. Ultimately, this is a causal argument. They're saying that the reason or the cause for the different genetics is because they adapted to the habitats. As a causal argument, it has the same assumptions that all causal arguments have.

First, there's nothing else that's responsible for this. That is, there's no alternate cause that's responsible for the difference in genetics. The relationship doesn't go the other way around, or the effect isn't actually the cause. So a difference in genetics isn't what caused them to adapt.

And finally, the change wasn't coincidental. It wasn't just coincidence that their genes are different. The middle one of those is a normal causal assumption but probably not gonna be the assumption here or not gonna be relevant here. Because they told us that when the salmon got introduced, they were genetically uniform, so it would be impossible for the genetic difference to cause them to split up.

So, probably it's gonna be one of those other two assumptions that the answer choice supports. So let's see what answers we have to work with. Answer choice A, neither of the two populations of sockeyes is interbred with a native salmon. This is subtle, but it does support the first of those assumptions, nothing else is responsible for the differences.

Well, this is something else, that is interbreeding with the native salmon that could be why their genes are different. Genes are different because they adapted it's because they interbred with another group of salmon. So this answer choice says that didn't happen and by eliminating an alternate cause, it strengthens the causal argument.

So then why are the other answers wrong? B tells us about the native salmon and what happened to them when they were around. So they also had two distinct populations that didn't interbreed. What happened to the salmon before these new salmon were introduced can't possibly affect what caused anything about them.

So answer choice B is not our answer. Answer choice C introduces a new type of salmon. Apparently the majority type of salmon most salmon that haven't lakes been part of the time in shallow water and part in deeper water, all salmon don't do that. So the fact that there are some other type of salmon out there that do something different, I can't really make this work better.

Answer choice D, this says that one of the populations of sockeyes is virtually identical genetically to the sockeyes introduced in 1940. So this answer almost destroys the argument. If one of the populations is genetically identical to the original species, then they didn't adapt, and we're trying to prove that they did. So answer choice D is not the answer and then answer choice E.

The total number of salmon lake is not as big as the number of native salmon. What the number would have to do with it. I can't say, we're talking about why they differ genetically not why there are more or less of them and having more of them. Why would that change the genetics? So answer choice E can't affect anything either.

As choice A was our answer.

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