LR Drill Set 8, Question 4
For millions of years the spider Latrodectus mactans(53 more words not shown)
Which one of the following(11 more words not shown)
- AIf a trait(23 more words not shown)
- BA trait that(19 more words not shown)
- CAll traits that(14 more words not shown)
- DA trait will(22 more words not shown)
- EOnly those traits(14 more words not shown)
Reading the argument
What if the extra-strong venom protects the spider from something trying to eat it?
The author says weaker venom would still kill all the spider's prey, so the extra toxicity provides no evolutionary advantage. I'd question that step. Killing prey isn't the only possible use of venom. Deterring a predator is another possibility. We don't need to know that this species actually uses its venom that way to see what the argument hasn't considered.
But the question asks us to support the inference with a principle. We have to change jobs: instead of pursuing that objection, work out what would help the author's explanation.
The proposed explanation is that the extreme toxicity has lasted for millions of years because it is genetically linked to some advantageous trait. Imagine a trait coming along as part of a package: the package persists because another feature is useful, even if this feature isn't doing useful work itself.
What we’re looking for
I'd want a rule saying that a trait probably won't last this long unless either it helps the species or something genetically linked to it does. Then, taking the author's claim that the extra toxicity has no advantage, its long survival points toward an advantageous linked trait.
Working the answers
- A
This rule applies to a disadvantageous trait. The author says the extreme toxicity supplies no advantage, which is different from saying it harms the spider. A feature might be neutral. Since the passage hasn't established a disadvantage, we can't use A's rule to make the inference about this venom.
- B
This tells us what probably happens when a neutral trait is linked to harmful traits: it disappears. But that doesn't leave advantageous linked traits as the only explanation for survival. What if both the extreme toxicity and the traits linked to it are neutral? B says nothing about that possibility. The argument needs help ruling it out, not just a rule about a harmful package.
- C
This points toward the long-lasting trait itself being advantageous. Yet the author's explanation starts from the claim that the extreme toxicity provides no advantage and concludes that something linked to it must help. C pulls against that starting point instead of supporting the proposed explanation through a linked trait.
- DCorrect
If neither the trait nor anything genetically linked to it were advantageous, the trait probably would not survive for long. Yet this extreme toxicity has survived for millions of years. Under D's rule, we'd expect an advantage somewhere in the package. The author has already denied an advantage in the extreme toxicity itself, which makes a linked advantageous trait the supported explanation. Notice that the rule says “probably.” It strengthens the inference without making the proposed explanation absolutely certain.
- E
This says a trait is likely to be advantageous only if it has survived for a long time. It doesn't say that every long-lasting trait is advantageous, and it doesn't say anything about advantageous linked traits. Knowing that this venom has lasted a long time therefore doesn't let us reach the author's explanation through E.
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