Logical Reasoning

PT16 · S3 · Q14 Caffeine can kill or inhibit

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Caffeine can kill or inhibit the growth of the larvae of several species of insects.

Conclusion (this is evidence for)

The presence of non-negligible quantities of caffeine in various parts of many diverse species of plants is not accidental, but evolved as a defense for those plants.

Evidence

Caffeine can kill or inhibit the growth of larvae of several species of insects.

An experiment showed that tobacco hornworm larvae die when they ingest a preparation that consists in part of tea leaves that have caffeine.

Evaluate

There are two different ways we could think about this argument.

There's a move from to "Plants must have evolved caffeine as a defense".

This seems to assume that these plants were previously threatened by the insects whose larvae are vulnerable to caffeine.

There's also the layer of this argument that deals with the experiment. The author is interpreting the results of this experiment to mean that the hornworm larvae died because of the caffeine content of the preparation they ingested. The author is assuming they didn't die because of some other ingredient in the preparation.

Goal

Let's look for a link connecting "caffeine can kill larvae of insects" to "evolved as a defense", or let's look for an answer ruling out an alternate explanation for the hornworm's death in the experiment.

But it might be neither of those, so we'll stay flexible.

14.

The argument assumes that

  1. caffeine-producing plants are an important

    Out of Scope: commercial insecticides

    The author never mentions or implies anything about how commercial insecticides work. Just because she believes that plants evolved caffeine as a defense against insects doesn't mean she assumes that human attempts to thwart insects also involve caffeine.

    0% picked this

  2. caffeine is stored in leaves

    Too Strong: roughly equal

    There's no reason the author needs to assume that the concentration of coffee in leaves is roughly equal to the caffeine concentration of the mix fed to the hornworm. Whether there's more caffeine in leaves or more caffeine in the mix wouldn't make any difference to the argument. The first sentence establishes that caffeine has a deleterious effect on many insects. The experiment might have had a much more concentrated dose than leaves do, but why would that mess up the argument?

    18% picked this

  3. caffeine-producing plants grow wherever insect

    Too Strong: wherever Reversal, if anything

    The author doesn't have to assume that in every single place where insect larvae threaten local plants or ancestors of those plants, that caffeine-producing plants grow. If there's some island where insect larvae pose a threat to local plants, and there's no caffeine-producing plants to be found on that island, would that badly weaken this argument? No, it would have no effect. It's not like the author was ever saying that the only way plants can defend themselves against insect larvae is to evolve caffeine. This answer would be closer to being apt if it were reversed. Since the author believes that caffeine is an evolutionary response to these insects, she would be closer to believing that "wherever we find indigenous caffeine-producing plants, we'll find that there are or were insect larvae posing a threat to those plants".

    11% picked this

  4. the tobacco plant is among

    Out of Scope: tobacco plant

    We might speculate that the tobacco hornworm is named that because it eats tobacco plants (and thus that tobacco plants might have evolved caffeine as a defense), but none of that needs to be true. If the tobacco plant isn't a species that produces caffeine, that wouldn't hurt the argument.

    9% picked this

  5. Correct

    caffeine-producing plants or their ancestors

    Why this is right

    This has lovably soft language "have at some time", and it's basically the reversed version of (C). In order to believe that caffeine-producing plants evolved caffeine as a defense mechanism, you have to believe that there was something in the environment of these plants that threatened the plants and that would be deterred by caffeine. i.e., in order to believe that the giraffe evolved its long neck in order to better reach food that was high up, you have to assume that within the giraffe's habitat, there were food sources that were too hard to reach without a long neck. If we negate this answer, it's saying, "Caffeine-producing plants, as well as their ancestors, have never been fed upon by creatures sensitive to caffeine." That would badly weaken the argument. How could caffeine possibly have evolved as a defense mechanism if nothing that feeds/fed on these plants is deterred by caffeine?

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

    62% picked this

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