Reading Comprehension

PT138 · S1 · P2 · Q13 Plant Evolution

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The characteristic smell or taste of a plant, to insects as well as to humans, depends on its chemical composition.

Topic

How plants’ distinctive smells and tastes (from secondary chemical substances) have evolved, especially through interactions with insects.

Framework

Old / New and Answer / Explain

Main Point

Plants contain unique secondary chemical substances—mainly for attracting or defending against insects—and this chemical arms race has driven the evolutionary specialization between plants and plant-eating insects. The "most valuable sentence" for the main point is near the end of P2:

P1: Basic Plant Chemistry—Primary vs. Secondary Substances

Plants all have “primary substances” (things like proteins and carbs) they need for survival, but the interesting “secondary substances”—unique to different plants and responsible for their taste and smell—don’t impact their basic function.

P2: Why Plants Have Secondary Substances—Insects and Natural Selection

These secondary substances evolved because of genetic mutations, but only stuck around if they provided an evolutionary advantage, mostly by either attracting helpful insects (like pollinators with appealing scents) or defending against harmful ones (with bad tastes, toxins, or warning smells).

P3: The Ongoing Evolutionary Battle between Plants and Insects

Plants and insects have been locked in a chemical arms race for ages: as plants develop new defenses, insects are forced to adapt or find new food. Over time, this has led to insect species specializing in eating just a few types of plants—basically, both sides have become more and more choosy!

13.

The passage most strongly suggests that which one of the following is true of secondary substances in plants?

  1. Correct

    Some of them are the

    Why this is right

    We're very attracted by the mild wording, "some". If we CTRL + F or scan for "mutations" we'll find it at the beginning of the second paragraph: Such [secondary] substances undoubtedly first appeared, and new ones continue to appear, as the result of genetic mutations in individual plants. If new secondary substances continue to appear as the result of mutations, then we can support that "some secondary substances are the results of recent mutations".

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

    81% picked this

  2. They typically contribute to a

    Too Strong: typically

    The end of the first paragraph says, "It is these secondary substances that give plants their distinctive tastes and smells". That's it. It doesn't give us any way to justify the quantitative specificity of saying "more than 50% of the time, a secondary substance does not contribute to both taste and smell". We have no idea with what frequency they contribute to taste, smell, both, or neither.

    1% picked this

  3. Some of them undergo chemical

    Out of Scope: chemical reactions

    The "some" is very attractive, and this may sound reminiscent of the discussion in the 2nd and 3rd paragraphs where we hear about secondary substances evolving to solve an insect related problem. But we never hear about a secondary substance getting mixed together with a substance from the insects, undergoing a chemical reaction, and thereby changing the plants' chemical composition going forward. We just hear about plants mutating a new substance; the new substance proves to be useful in the survival battle with insects; the substance is thus selected for by evolution; and thus the secondary substance becomes a "conserved trait". So we could say that interactions with insects, over evolutionary time, alter the plant species' chemical composition. But that's miles away from plant juice + insect juice = chemical reaction.

    10% picked this

  4. Some species of plants produce

    Too Strong: only one

    We're told that "only a few [secondary] substances occur in any one species of plant". We never hear about plants that have only one secondary substance.

    7% picked this

  5. A few of them act

    Out of Scope: regulators of primary

    We only hear about primary substances in the first couple sentences, and we're told that secondary substances have "no known role in the internal chemical processes of plants' growth or metabolism", whereas primary substances are specifically required for growth and proper functioning. So since we're told that secondary substances have no known role in doing what primary substances are doing, it's nearly contradicted to say that "some secondary substances regulate the production of primary substances".

    1% picked this

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