Logical reasoning PrepTest 141 · Section 2 · Question 22

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen

Stimulus Summary:
Genetically uniform salmon were introduced into a lake that was losing native salmon, and they split into deep salmon and shallow salmon that don't interbreed and are genetically different.
Hypothesis: Each evolved to its distinct habitat.

Answer Anticipation:
The hypothesis here is designed to explain a phenomenon—a salmon population that split in a lake and is now genetically distinct. Whenever there's a hypothesis explaining a phenomenon, your first thought should always be towards alternatives—if there are other viable explanations to explain the phenomenon, then the hypothesis could be wrong. To strengthen this hypothesis, therefore, an answer will likely rule out another explanation for how these salmon populations became genetically distinct that's unrelated to their habitat (deep vs. shallow).

Answer Explanation:
Another potential way for these populations to have become distinct is that some of the new salmon interbred with the old salmon (which are noted as nearly disappearing, not completely disappearing). This answer rules out that possibility, thus strengthening the hypothesis.

Key Takeaway:
Hypotheses are generally meant to explain a certain phenomenon. When one is introduced on the LSAT, it usually doesn't rule out all alternatives. In order to strengthen a hypothesis, therefore, a correct answer can eliminate one of these alternative explanations; in order to weaken one, a correct answer can therefore raise or provide evidence for an alternative.

Incorrect. First, this answer is about the native salmon, not the genetically uniform salmon. Second, this answer doesn't state whether these two populations developed in response to different habitats.

Incorrect. If anything, this answer suggests that the salmon aren't evolving to fit the shallow or deep habitats since they spend time in both locations.

Incorrect. Since the hypothesis is that "each" group of salmon has adapted to its environment, one of the groups being nearly identical to the fish as they were introduced would undercut it at least partially.

Incorrect. The size of the population doesn't speak to genetic adaptations to habitats, so this answer is wholly out of scope.