Logical Reasoning

PT142 · S4 · Q22 Fish with teeth specialized for scraping

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Fish with teeth specialized for scraping algae occur in both Flower Lake and Blue Lake.

Conclusion (thus)

The algae-scraping specialization evolved more than once.

Evidence

There are fish with an algae-scraping specialization in both Flower Lake and Blue Lake.

If they are closely related, then the algae-scraping specialization evolved only once.

But genetic tests show that they're not closely related.

Evaluate

Whenever we're doing Flaw and we come across a Conditional premise (in this case, signified by If), we know to be on red alert for the #1 Famous Flaw, confusing Necessary with Sufficient.

This flaw is when the author presents a conditional premise, like "If X, then Y" and then either argues via an illegal reversal (Pam is Y. Thus Pam is X) or via an illegal negation (Pam is not X. Thus Pam is not Y).

This was our conditional premise:

the two fish are → , algae-scraping closely related evolved only once

An illegal reversal would sound like,

An illegal negation would sound like, .

That second argument, the illegal negation, is exactly what this author did. She established the two fish are not closely related (the trigger isn't true) and then concluded from that that algae-scraping evolved more than once (the outcome isn't true).

Goal

Since this is such a famous flaw, we'd head to the answers hoping to find the classic Necessary vs. Sufficient answer choice.

Sometimes they give us a more conversational objection to these arguments, rather than naming their formal error, which would sound like

But we only worry about that if we have to.

22.

The reasoning in the argument is flawed in that it

  1. infers a cause merely from

    Not Correlation vs. Causality

    This refers to one of the other 10 famous flaws, Causal, in which the author's conclusion assumes or states a causal explanation for some curious fact in the evidence that could potentially be explained a number of ways. Any Flaw answer structured like infers X merely from Y means that X is the conclusion and Y is the only premise. Does the conclusion state or assume a causal relationship between two things? No. It actually is claiming that the algae-scraping of the fish in Flower Lake evolved separately from the algae-scraping of the fish in Blue Lake.

    6% picked this

  2. infers that just because the

    Not Unproven vs. Proven False

    This refers to one of the other 10 famous flaws, Unproven vs. Proven False, in which an author assumes that since someone has failed to prove X or doesn't have any evidence in favor of X, that X is therefore false. This answer says that the conclusion is "inferring that a claim is false", but our conclusion isn't disputing a claim anyone else made. And the evidence doesn't sound anything like, "the evidence that algae-scraping evolved only once hasn't been confirmed yet". The evidence was "these two species are not closely related".

    10% picked this

  3. Correct

    takes a sufficient condition as

    Why this is right

    This is what we were looking for, because the argument presented a conditional premise and then argued via an illegal negation of that conditional. (illegal reversals and illegal negations are technically the same thing, so they are both Necessary vs. Sufficient errors). "Being closely related" is a sufficient condition, an If idea. If the two fish are closely related, then the algae-scraping evolved only once. "Being closely related" guarantees that the scraping evolved only once. However, our author acts like "being closely related" is necessary in order to believe that the scraping evolved only once. She acts like "being closely related" is the only way that scraping evolved only once, because she argues that if the two fish aren't closely related, then scraping didn't evolve only once.

    Skill tested: Flaw · how this choice captures the argument's function is the move to repeat next time.

    70% picked this

  4. infers merely because something was

    Not Possible vs. Certain

    This refers to a semi-famous flaw (top 15, but not top 10) called Possible vs. Certain. We can also just look at the structure, infers merely because X that Y and then ask ourselves whether X matches the evidence and Y matches the conclusion. Is the conclusion saying that "something did occur"? Yes, it's saying that algae-scraping specialization evolved more than once. So then did the evidence say "it was likely that algae-scraping evolved more than once"? Nope. Since this doesn't match the argument, it can't be right.

    11% picked this

  5. appeals to the authority of

    Not Inappropriate Appeal

    This refers to one of the other 10 famous flaws, Inappropriate Appeals, in which the author's argument relies on the opinion of someone whose expertise hasn't been established (or, rarely, when the author's argument relies on an appeal to the listener's fear or emotion). Appealing to the authority of the biologists would mean arguing, "X must be true. After all, these biologists believe that X is true." This argument doesn't try to convince us that algae-scraping evolved more than once by saying that "biologists believe that it evolved more than once"

    3% picked this

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