LR Question 22 Explanation
QUESTION TEXT: Because the native salmon in Lake Clearwater had…
QUESTION TYPE: Strengthen
CONCLUSION: Hypothesis: each separate sockeye salmon group adapted genetically to its habitat.
REASONING: The two groups of sockeye live in different parts of the lake and don’t interbreed. The two groups also have genetic differences.
ANALYSIS: This is a difficult question; there’s a lot of information to keep track of. Here are some alternative hypotheses:
- The genetic differences are just random genetic drift between populations that don’t interbreed.
- One or both groups of sockeye bred with the native salmon, leading to genetic differences.
You can strengthen the answer by eliminating an alternate hypothesis.
You might wonder how I generated those hypotheses. Well, the LSAT expects you to have basic scientific knowledge, including an understanding of natural selection. If you know how natural selection works, those two hypotheses are the obvious alternate possibilities.
___________
- CORRECT. It’s common scientific knowledge that interbreeding leads to genetic differences. So it’s an alternate reason for genetic differences that doesn’t involve adaptation. Eliminating a plausible alternative theory strengthens the researchers’ hypothesis.
You’re allowed, and expected, to use common scientific knowledge of facts to answer questions. - This doesn’t tell us if the native salmon had different populations because they adapted to environments, or because of randomness.
- This doesn’t tell us anything about genetic differences.
- This weakens the hypothesis. The hypothesis is that each population of sockeye adapted to its environment. If one sockeye has stayed the same genetically, then they likely didn’t adapt.
- The number of salmon doesn’t matter. Adaptation to environment doesn’t depend on total population numbers.
More Resources for Strengthen Questions
- Intro Course lesson: This intro course lesson covers Strengthen questions.
- Mastery Seminar lesson: This LR Mastery seminar lesson covers strengthen questions.
I believe that A is incorrect. The definition of adaptation encompasses the possibility of using available genes in the interbreeding pool. Adaptation doesn’t require de novo mutation. I wouldn’t expect the test taker to know this ‘outside knowledge’, but your conclusion seems to actually require incorrect outside knowledge. And there is no information in the text which circumscribes the definition to the one you conceived.
I’ll start off by saying that my formal education in science only goes as far as high school, so I’m definitely no expert. However, I don’t believe there is an issue with answer A nor Graeme’s explanation.
The stimulus says that the two populations of sockeyes have adapted genetically to their distinct HABITATS. For this to be true, the genetic differences would need to result from adaptations to their environments, rather than other causes. Answer A strengthens this hypothesis by ruling out interbreeding with native salmon as a potential source of these genetic differences. This eliminates an alternative explanation and thus strengthens the hypothesis that adaptation (specifically to the environment) caused the differences.
Regarding the use of outside knowledge, Graeme’s explanation doesn’t require test takers to understand adaptation solely as requiring de novo mutation. Rather, it relies on the commonly understood scientific idea that interbreeding can introduce genetic differences independently of environmental adaptation. By ruling out interbreeding, A helps focus the cause of the genetic differences on adaptation to the distinct habitats, thereby supporting the researchers’ hypothesis.
It’s true that adaptation can involve selecting from existing genetic variation, but the concern here isn’t whether adaptation requires de novo mutation. It’s about eliminating interbreeding (as a confounding variable or the actual causal factor) as a competing explanation for why the populations differ genetically. Without ruling out interbreeding, the researchers’ hypothesis of adapting to differing environments would be weaker. So, A is correct because it strengthens the causal link between environmental adaptation and genetic differences by ruling out an alternate explanation.
Let me know if you still disagree or would like to discuss further!
As a side note, would adding “So it’s an alternate reason for genetic differences that doesn’t involve adaptation [to their distinct habitats]” clarify the explanation? If so, I’ll make sure to amend it. Thanks for your comment!
A is not a correct answer. breading with the native salmon is irrelevant in this context as its outside of the scope of this passage and requires and inference that is not at all supported by the passage. B is the correct answer as it gives a exact same occurrence with a different species of salmon and is an inference that is supported within the scope of the passage via an analogy.
Answer A is deeply flawed and is inconsistent with the science of evolution. Breeding and adapting genetically to a habitat are NOT mutually exclusive. The sockeyes don’t develop mutations in response to the environment. Rather, the environment exerts a pressure (or natural selection) on a genetically diverse population. Over time this selection pressure results in genetically diverse populations. However, the pressure can only act on genetic differences that already exist. In fact, breeding between the sockeye and the native salmon would increase the pool of genetic diversity available and increase the likelihood of divergence.
Breeding between the sockeye and the native salmon alone (without natural selection) can NOT result in the creation of two distinct populations as described by the prompt (without selection pressure you’d get a new but unified fish). Thus, breeding with a native species can be complementary (expanded gene pool = more potentially advantageous characteristics), and even if it wasn’t complementary cannot serve as an alternative explanation for the phenomenon described in the passage.
For LSAT questions, it’s best to take the information at face value, so it could be that the stimulus is inconsistent with evolution theory. If we assume what’s contained in the stimulus is true, then what would strengthen the scientists’ hypothesis is that it’s only genetic adaption that’s caused the split between the salmon population, and not that genes from the original native population have intermingled with the introduced salmon. By ruling out this alternative explanation, the scientists’ hypothesis is strengthened.
I was directly responding to the answer explanation above which explicitly says “it’s common scientific knowledge,” because the correct answer is not based on the commonly held assumptions surrounding genetic change. I totally understand that the LSAT is not a science-based exam, but there is nothing in the question stem itself that would prompt me to ignore the basic rules/logic of evolution. Thanks for the response and I understand your point & advice! Thank you.
Totally agree. Answer B is an equally insufficient-but-arguably-closest to the right answer. As someone with a background in bio I find this question incredibly frustrating.
The rationale behind answer B being that if two distinct populations of an entirely different salmon species in the same close physical proximity which also did not interbreed occurred, then it is likely there is some common stimulus accounting for the parallel divergences: e.g. the respective environments each exerting pressure resulting in adaptation of the two species.
It’s not a perfect answer by any means but I picked it because A seemed just as flawed, so six one half dozen the other. Sigh.
Hi Graeme,
I believe you meant to type out: It’s common scientific knowledge that interbreeding leads to genetic SIMILARITIES*, not differences, in your explanation of why (A) is the correct answer choice.
Ah, I meant differences between the original population and the portion of the population that interbred with another population. I can see how that’s confusing though. I’m keeping the differences phrasing since the argument’s emphasis is between differences between the two salmon populations that were originally a single similar group.
Note: This is an old comment but I wanted to clarify the point.
I disagree that the LSAT requires you to have “basic scientific knowledge”. The LSAT does not assume any test taker has knowledge of a certain academic subject, and this is confirmed by many subjects. I believe this is even confirmed by LSAC, but I can’t find where. However, the LSAT does require that one has common sense. I think you also stated to one of my replies that it was assumed by LSAC that a test taker would know an asteroid is a meteor. I don’t know that, and I highly doubt LSAC assumes that the average test taker should know that. Could you point to any sources that state we should know “basic scientific knowledge” or any knowledge from another discipline? And what level is “basic”?
I say that based on having looked at thousands of questions. There are questions that are easier to answer with scientific knowledge, and some that are impossible to answer without it.
In the other reply, I wrote that LSAC expected you to know that asteroids and meteors are basically the same thing: https://lsathacks.com/explanations/lsat-68/logical-reasoning-2/q-17/