Logical ReasoningDifficulty: Easy

PT16 S3 Q21 Explanation

The similarity between ichthyosaurs and fish

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsMust be True

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Stimulus

The similarity between ichthyosaurs and fish is an example of convergence, a process by which different classes of organisms adapt to the same environment by independently developing one or more similar external body features. Ichthyosaurs were marine reptiles and thus do not belong to the same class of organisms as fish. However, similar to those of fish. Most strikingly, ichthyosaurs, like fish, had fins.

What this question is testing

Must be True

Your task

Break the argument into its conclusion and evidence, then do exactly what the question stem asks with that structure.

Common trap

Answers that sound relevant to the topic but don't connect to the argument's actual reasoning.

Winning move

Predict what a right answer must do, then test each choice against the conclusion-evidence gap.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
21.

If the statements above are true, which one of the following is an inference that can be properly drawn on

Answer choices

  1. Trap1% picked this

    The members of a single class of organisms that inhabit the same environment must be identical in all

  2. Trap4% picked this

    The members of a single class of organisms must exhibit one or more similar external body features that differentiate that class from

  3. Trap11% picked this

    It is only as a result of adaptation to similar environments that one class of organisms develops external body features similar to

  4. Correct83% picked this

    An organism does not necessarily belong to a class simply because the organism has one or more external body features similar to

    Why this is right

    Answer D is correct.

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

  5. Trap1% picked this

    Whenever two classes of organisms share the same environment, members of one class will differ from members of the other class

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