Reading Comprehension

PT138 · S1 · P2 · Q11 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!

11.

The passage provides the most support for inferring which one of the following?

Correct
Some chemicals that are not
Why this is right
This is lovably soft, because it's saying "some", so this is worth investigating. Did we talk about chemicals "not known to be directly involved in the growth or metabolism of any species of plant"? "Growth and metabolism" is discussed in the middle of paragraph 1, where it says: the secondary substances are a diverse and multitudinous array of chemicals that have no known role in the internal chemical processes of plants' growth or metabolism. Okay, so "secondary substances" apply to this description. Thus, this answer is saying "some secondary substances play a vital role in the lives of various kinds of plants". Can we support that some secondary substances play a vital role? We know secondary substances give plants their distinctive tastes and smells, but does that qualify as "playing a vital role"? The 2nd paragraph is saying that secondary substances would not have been passed to so many subsequent generations if they weren't "increasing the likelihood of the organism's surviving and reproducing". That line seems like enough support for the idea that some secondary substances play a vital role. We also hear stuff about secondary substances like - attracts pollinators (that's vital) - biochemical defense against enemies (that's vital)
Skill tested: Inference · how this choice captures the passage's function is the move to repeat next time.
75% picked this
Most plants that have evolved
Too Strong: most
The only "most" fact about plants we get in the passage is the 2nd sentence: most plants contain two categories of chemical substances: primary and secondary This answer is about "most plants w/ chemical defense systems". We definitely don't have any generalization about that subgroup that would allow us to say what is true of most of them. In the final paragraph we hear that when a plant develops a chemical defense system, "the insects [that eat that plant] must switch to other foods or evolve ways to circumvent the plants' defenses." This answer choice is saying more than 50% take the 2nd option, but we can quantify that. Maybe most of the time insects just switch to other foods.
13% picked this
Most insects that feed exclusively
Too Strong: most
Here we have another Unknown Most Group. The passage never said what was true of "most insects that feed exclusively on a botanically restricted group of plants". The final sentence of the passage is saying, "Most insect species (tended to) become associated with botanically restricted groups of plants". But we have no evidence that most insects who are constrained to a certain group of plants can identify them by means other than taste / smell.
4% picked this
Many secondary substances that are
Out of Scope: evolved independently
The passage is suggesting the opposite of this. It's saying (P2) that "Insects appear to have played a major role in many plants' having the secondary substances (like toxic chemicals) they have today." The passage is saying that secondary substances first appear as random mutations, but the survival advantage conferred by some of these mutations (because of the insects that otherwise eat these plants) made these traits naturally selected for, i.e. this mutation got passed onto subsequent generations because it helped the plant survive. Nothing in the passage is saying that many of these secondary substances evolved independently in various species. The final paragraph is stressing how "for hundreds of millions of years there has been an evolutionary competition for advantage between plants and plant-eating insects". The emergence of secondary substances that eventually become a trait of the plant species is dynamically related to the insects that are preying on that plant.
3% picked this
Some toxic substances that are
Out of Scope: non-secondary toxins
This answer is lovably soft, because of "some", but we can't find anything in the passage that talks about some toxins that are not themselves secondary substances. The only toxic substances ever talked about were all secondary substances.
4% picked this

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