#22 - Fish with teeth specialized for scraping algae

[Administrator]

Complete Question Explanation

Flaw in the Reasoning. The correct answer choice is (C).

We learn some information about two different groups of fish with a similar trait, teeth that are specialized for scraping algae. The author then provide a conditional premise - if the fish were closely related, then the trait evolved only once:

Closely Related Evolved Only Once

The author tells us next that the two groups of fish are not closely related, meaning that the sufficient condition has not occurred:

Closely Related

From this piece of information, the author improperly concludes that the necessary condition also did not occur:

Evolved Only Once

This is a classic error of conditional reasoning, a Mistaken Negation, and the correct answer will describe it using the language of conditionality.

Answer choice (A): This describes a causal flaw, not a conditional flaw. There was no causal aspect to the argument.

Answer choice (B): A "lack of evidence" flaw, this does not describe what occurred in the stimulus. The evidence was not that something has not been confirmed, but rather that something HAS been confirmed - the fish are NOT closely related.

Answer choice (C): This is the correct answer choice. As expected, conditional language is present in the correct answer. Do not be dissuaded by the fact that the answer seems to describe a Mistaken Reversal rather than a Mistaken Negation, because Mistaken Reversals and Mistaken Negations are the contrapositive of each other and are thus logically equivalent. In other words, they are the same thing as each other, and describing one is just as good as describing the other.

Answer choice (D): The argument had no element of likelihood to it, nor was there any claim that anything did occur. Instead, the argument concludes that something is not true.

Answer choice (E): This answer describes a mix of an improper Appeal to Authority and some form of survey flaw based on an unrepresentative sample. But the argument did not rely on the testimony of experts, as the author is the one claiming that the conditional relationship exists, and it is the author who mistakenly negates the conditions. The biologists made no such claims in the stimulus.

[mpoulson]

Hello,

Can you explain why the answer is C? I thought B seemed more valid considering the evidence didn't support the idea that the two fish were closely related, but didn't prove that they had evolved more than once. Essentially, why is this wrong and why is C correct. Thank you.

- Micah

[Clay Cooper]

Hi Micah,

Thanks for your question.

Answer choice C is correct because the argument uses mistaken conditional reasoning. The evidence given is that:

closely related evolved only once

And yet the conclusion claims:

~closely related ~evolved only once

This is, specifically, mistaken negation; and answer choice C is one of the two basic ways to describe mistaken negation.

Answer choice B names an evidence error - specifically, B states that the argument mistakenly describes a claim as false because the evidence in favor of the claim has not yet been confirmed. This description is inaccurate, though, because the basis for the conclusion is not that some evidence has not yet been confirmed, but in fact that their is evidence to the contrary (we are told that the species, despite being related, are not closely related).

Does that clarify it some?

[salmach]

Hi,

I got the answer right, I was able to pick out the incorrect SN reasoning but I've never come across an LR question where they ask you to point out the "mistaken reversal" into words, i.e. I've only seen it during parallel flaw.

So I suppose even though it's negating the sufficient condition when it really should have been negating the necessary "algae-scraping specialization evolved only once" ... is this good enough of an understanding to know why C is correct? Or rather, does this even make sense?

[James Finch]

Hi Salmach,

The wording used in LSAT answer choices to formally and abstractly identify both Mistaken Negations and Mistaken Reversals is the same, and is either written the same way as answer (C) ("takes a sufficient condition as a necessary one") or switched around ("takes a necessary condition as a sufficient one"). This works because a Mistaken Negation is essentially a Mistaken Reversal of the correct inference, the contrapositive (and vice versa). So if we have in the stimulus:

A B

and the conclusion of the stimulus is:

A B (Mistaken Negation)

the sufficient and necessary conditions are switched, just as if the conclusion of the stimulus had been:

B A (Mistaken Reversal)

As you've probably seen, though, often the answer uses the actual conditions presented in the stimulus, or in a parallel flaw question, a parallel setup in the answer choice that includes either a Mistaken Negation or Mistaken Reversal.

Hope this helps!