#14 - Must Be True, Author's Perspective

[Administrator]

Complete Question Explanation
(See the complete passage discussion here: lsat/viewtopic.php?t=14154)

The correct answer choice is (D)

As with the previous question, the concept reference here is too broad to produce a workable
prephrase. The question concerns the relationship between plants and insects, which is discussed in
the second and third paragraphs. Either can serve as a useful reference point.

Answer choice (A): At first glance, this is an attractive answer choice. Indeed, the secondary
substances that develop in plants do so as a result of certain evolutionary pressures. It seems likely,
then, that the diversity of such substances in any given species of plants would be somewhat
proportional to the number of insects with which that species has interacted. This is not, however,
necessarily true. For instance, it is possible that several insects exerted the same exact evolutionary
pressure on the plant species (e.g. the insects might prefer the same scent, or be repelled by the same
toxic substance). Likewise, it is possible that the same one insect caused several different secondary
substances to emerge (e.g. the insect managed to circumvent a particular harmful substance, as
a result of which a new secondary substance emerged). All in all, it is impossible to infer with
the requisite degree of certainty that the diversity of secondary substances in plants is directly
proportional to the number of insects with which the plant has interacted.

Answer choice (B): The passage does not limit the benefit of plants’ evolutionary interaction with
insects to a select “few” species of plants (pollinating insects, for instance, seem quite beneficial to
the survival of all plants). Furthermore, while it is possible that some species of insects interact with
plants without causing either benefit or harm, this is never mentioned in the passage.

Answer choice (C): The passage provides no information to help us infer that the number of plant
species within each family has increased over time, while the number of families of plants has
decreased. All we know from the discussion in the third paragraph is that fewer and fewer plants
have remained as suitable food sources for any one species of insect (lines 52-54).

Answer choice (D): This is the correct answer choice. In the beginning of the second paragraph,
the author states that secondary substances “undoubtedly” appear as the result of genetic mutations
in individual plants (lines 19-21). Consequently, we can infer that no particular secondary substance
has appeared in direct response to insects. The author then proceeds to describe the various ways in
which insects have influenced which particular secondary substances are present in a plant species.

Answer choice (E): The passage does not specify how many insects manage to circumvent plants’
chemical defenses (at least some do, but we cannot prove “many”). In addition, we cannot determine
whether any insects possess outright immunity to plants’ secondary substances. It is possible that
none of them do, but this is neither asserted nor alluded to in the passage.

[pmuffley]

How would you differentiate between the meaning of "major role" (1st sentence 2nd paragraph) and "in direct response to"?

If an ant destroys a plant by eating it, and that plant develops genetic mutations to combat the ant's ability to eat it, then how can we say that is not a direct response? What is it but a direct response?

How do you make that argument to yourself?

[Robert Carroll]

pmuffley,

Your way of speaking is a bit loose, which is causing you to misunderstand how the development of defenses via natural selection works. Lines 19-21 explain how the defenses actually arise - plants exhibit genetic mutations. Those genetic mutations that are useful to the plants, including, in our case, ones that provides defenses against insects, cause the individual plants having them to survive and reproduce at a higher rate than plants lacking those defenses, causing the mutations to increase in prevalence among the plant population over time.

So there is no "reaction" against insects that causes plants to develop defenses. Plants mutate constantly. Some of those mutations end up beneficial, so plants with the mutations survive better. Some of those beneficial mutations are good defenses against insects, and it is for this reason that we say that plants "develop" defenses against insects. But in fact plants develop all sorts of mutations, not all of them useful, and even among the useful ones, not all of them are useful specifically because they protect against insects. Those that end up being useful defenses against insects get preserved, and that's why we call them defenses against insects.

The insects don't make the defenses exist in the first place. The defenses arise as part of a number of mutations always arising, and selection pressures keep those defenses around. Thus, those secondary defenses never appear as a direct response to insects at all. They appear because mutations make them and many other things constantly appear.

Robert Carroll

[bonnie_a]

I have a question on answer choice E. According to line 42 through 45, if insects are to survive as the plants they eat develop defenses against them, they must switch to other food or evolve ways to circumvent the plants' defenses. Insects must take either one of these actions in order to survive, and if they have, even just one, outright immunity to them, why would they have to actually put effort into finding ways to circumvent them? For this reason, I thought E was supported.