PrepTest 155, Section 2, Question 26
The three-spine stickleback is a small fish that(55 more words not shown)
Which one of the following,(5 more words not shown)
- ASticklebacks with armor(13 more words not shown)
- BHaving a larger(16 more words not shown)
- CUnlike ocean stickleback,(15 more words not shown)
- DBoth ocean stickleback(11 more words not shown)
- ESticklebacks originated in(14 more words not shown)
Reading the argument
Why assume the lake fish lost their armor to defend themselves better?
Ocean stickleback have armor, while lake stickleback have virtually none. Armor slows growth, so giving it up could let the lake fish become larger faster. The author treats that trade-off as evidence that size is a better defense against the lake predators.
My first reaction is to question whether the difference in armor has to be explained by defense at all. Perhaps the lake environment creates a different need. We shouldn't simply declare that there are no predators—the argument itself refers to lake predators—but we can question whether avoiding predators is the reason faster growth matters.
What we’re looking for
For the weaken question, I'd look for something making the defense explanation less convincing. Another reason that growing larger would help lake fish could do that. The evidence might then fit a benefit unrelated to which defense is better, leaving the author's comparison unproven.
Working the answers
- A
Armor makes fast swimming harder, which could matter when escaping a fast predator. But we don't know whether those are the relevant lake predators. More importantly, A doesn't show that larger size is an inferior defense or give a different reason the lake fish favor faster growth. It supplies another possible cost of armor without clearly challenging the author's comparison.
- BCorrect
Here is a different benefit of size: surviving cold winters. Larger size matters to winter survival for lake stickleback but not ocean stickleback. That helps explain why the lake fish would benefit from giving up growth-slowing armor, even if size weren't a better defense against predators. The pressure could come from winter conditions instead. B doesn't prove that size offers no protection; it gives another explanation for the difference the author treats as evidence about protection.
- C
This identifies a difference in what eats the fish. It doesn't tell us whether size or armor works better against predatory insects. We shouldn't assume that an insect can't attack a larger fish, or that armor would necessarily be better against insects than against fish. Without that missing comparison, C doesn't undermine the claim about the relative defenses.
- D
What the stickleback eat isn't the same as what eats them. Similar diets don't establish whether larger size or armor offers better protection against predators. The answer gives us a common feature of their feeding without explaining the different armor or growth trade-offs.
- E
This tells us how the lake populations originated. It doesn't explain why lake fish would fare better with little armor, or whether the benefit involves predators rather than something else. A history of their movement from ocean to freshwater isn't itself an explanation of this particular difference.
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