#22 - Because the native salmon in Lake Clearwater had

[Administrator]

Complete Question Explanation

Strengthen—CE. The correct answer choice is (A)

The author of this stimulus discusses the genetic adaptation of sockeye salmon introduced into Lake Clearwater in 1940 after the native salmon population nearly disappeared. When introduced, the sockeye salmon were a genetically uniform group. After their introduction, the group split into two, with one of the new subgroups inhabiting deep areas of the lake while the other inhabited the lake’s shallow areas. The two groups did not interbreed. Now the two subgroups of sockeye salmon differ genetically, and some researchers attribute this difference to each group having adapted genetically to its respective habitat.

This is a causal argument, in which the researchers conclude that genetic adaptation to habitat is what caused the salmon populations to differ genetically, as opposed to some other cause. This argument is flawed, because other than the fact that the sockeye salmon split into groups that have distinct habitats, we have no reason to think that adaptation to these habitats actually caused the genetic differences. We do not even know from the stimulus whether such adaptation is possible. Further, the stimulus contains a hint of an alternate cause: the native salmon, which had nearly but not completely disappeared from the lake. Perhaps it was interbreeding with these native salmon, rather than adaptation to habitat, that caused the genetic differences in the sockeye salmon populations.

The question stem identifies this as a Strengthen question. Our prephrase is that the correct answer choice will support the argument’s causal conclusion that the cause of the genetic difference is adaption to the distinct habitats, and not interbreeding with the native salmon remaining in the lake or some other cause.

Answer choice (A): This is the correct answer choice because it tells us that none of the sockeye salmon interbred with the native salmon, meaning the alternate cause did not occur.

Answer choice (B): This information about the native salmon refers to a time prior to the introduction of the sockeye salmon into the lake, a time period irrelevant to the conclusion.

Answer choice (C): Here, the answer choice indicates that the sockeye behavior is not unique, but does nothing to explain the genetic differences between the two groups of sockeye salmon.

Answer choice (D): This answer choice is more tempting than answer choices (B) and (C) because it addresses genetic differences. However, it is incorrect because it does not address what caused the genetic differences between the two groups of sockeye salmon to occur.

Answer choice (E): In this case, the answer choice does not address the genetic differences between the two groups of sockeye salmon, and so is irrelevant to the conclusion. The total number of sockeye salmon, and the comparison of that number to the number of native salmon many years ago, has nothing to do with the researchers’ conclusion regarding the genetic differences between the two groups of sockeye salmon.

[Frank]

Hey guys,

This one had me really confused, and even looking back over the exam I can't understand how A is the right answer. After reading the stimulus and then the question stem I approach the answer choices with a broad pre phrase: "I'm looking for something that will support the conclusion that each distinct population of sockeye salmon has adapted to its own distinct habitat (one in the deep areas and one in shallow areas)." I went with C based on the argument that if most lake salmon move in and out of both deep water and shallow water, then, based on the statement that one breed of sockeye now inhabits the deep part of the lake and the other inhabits the shallow part, this behaviour is different from regular lake salmon in a way that would suggest they are more restricted and perhaps even genetically adapted to their distinct habitat.

I feel like these inferences are somewhat stretched but I'm only trying to strengthen the hypothesis not guarantee it. To be honest all the answer choices didn't really seem to fit well, C was just the one I thought I could make work best.

[Lucas Moreau]

Hello, Frank,

I can see why answer choice A wouldn't intuitively make sense right away - it does require a little bit of thinking!

Answer choice C wouldn't really support the argument. It doesn't help defend the claim that the salmon have adapted genetically, it just talks about the behavior of salmon. We're looking for something that specifically supports the idea that the salmon have changed genetically, not just decided to hang out in different parts of the lake.

Answer choice A, however, strengthens that claim by way of removing an alternative possibility. If either of the native salmon had interbred with the new salmon, then changes in the new salmon wouldn't be genetic adaptation strictly speaking, it would be genetic mixing. Genetic adaptation would have to occur within the species over time. If answer choice A is correct, then there was no mixing and the claim that their behavior is the result of adaptation is therefore that much stronger.

Hope that helps,
Lucas Moreau

[Frank]

Hey Lucas,

Thanks a lot. That makes more sense.

[mokkyukkyu]

Hi

I was not sure between A and D.
For A, I was not sure the part "adapted genetically."
Isn't adapt genetically including interbreeding with other species? They develop by doing so isn't it? How can they just develop within one generation or within the same species?

For D, I thought it is correct because it made the startline the same...
So they were the same at first, but by being put in different environment, they started to show differences=adapted genetically.
In other words, this answer eliminates other causes...
Why is D wrong?

[Adam Tyson]

I have to disagree with you about the impact of answer D. I think it weakens the argument. If one of the two populations is still almost identical to the 1940 population of introduced sockeyes, that suggests that at least that population has NOT adapted genetically to the environment. We want to strengthen the claim that both populations have adapted.

Answer A eliminates a possible alternate cause for their genetic differences. If the ones in shallow water, for example, had interbred with the native salmon, one would expect that population to now be genetically distinct from the deep water sockeyes simply because of the mixing of two gene pools in the one group. That would be an alternate cause for genetic differences between the shallow and deep water salmon. If we take the natives out of the equation, it strengthens the idea that the differences are due to adaptation by both. It doesn't prove it, of course (maybe only one group adapted?), but it does strengthen it, and that's all we need.

[mN2mmvf]

Why is the prior time period irrelevant to the conclusion in choice (B)? If salmon previously comprised two distinct populations that did not interbreed, might that not make it more likely that those two populations were also separated by depth and adapted to their habitats? Thus making it more likely that newly introduced salmon would follow the same behavior?

[Alexandra Ruby]

Hi mN2mmvf,

I think I see where you are going with your question in that it might show that since the situation happened before with the native salmon it is more likely that it happened again with the sockeye salmon.

The problem here, however, is that the researcher's hypothesis relates to the new sockeye salmon only and that these salmon genetically adapted to the new habitat i.e. deep v. shallow. Answer choice B does not mention that the native salmon genetically adapted to the environment just that they had two distinct populations that did not interbreed. Or, more specifically, we do not know that the native salmon populations had a relationship with depth/shallowness as do the sockeye salmon now as stated in the stimulus. So, it does not help the researcher's hypothesis as written.

I hope I understood your question and that this helps clarify!

[mN2mmvf]

Yes, thank you! Perhaps in B they occupied two different areas and didn't interbreed but nonetheless didn't differ genetically.

[swong1267]

My hesitation with A came from the thought that if both groups bread with the native populations, they'd have pretty similar genetic changes. I know there are a couple weaknesses here -- needing both groups to breed with the native population, and having /similar/ genetic changes still means differences in genetics. Are the weaknesses in this thinking the only thing that rule it out?