PT155 · S2 · Q26 The three-spine stickleback

We don't know whether the ocean or the lake has faster predators, so there's no way to apply this new information to the distinction we're trying to explain.

The author's explanation assumes a difference between the lake stickleback's predators and the ocean stickleback's predators. This answer establishes that there is a difference. In that sense, it strengthens the plausibility of the author's hypothesis. But the author wasn't just assuming any ol' difference. She's assuming that armor works better against ocean predators and size works better against lake predators. This answer suggests that ocean predators tend to be larger fish, whereas lake predators tend to be predatory insects. Does our common sense say that armor works better when you're being attacked by larger fish, but size works better if you're being attacked by predatory insects? People's answers to that question are pretty divergent. I've heard students (and other LSAT teachers) make the case that you'd rather have armor against predatory insects, and I've heard students/teachers make the case that you'd rather have size against predatory insects. The fact that we test takers don't agree about what impact it has to be bigger vs. possess armor, in regards to predatory insects, suggests that there isn't a strong common sense take on the impact of this answer. Meanwhile, the correct answer unequivocally weakens by showing that there's an alternate explanation for why the lake stickleback would "want to evolve" to be larger.

This would rule out any alternate story that relates to different eating needs. It's possible that lake stickleback evolved to be larger because that helped them eat lake-food. But this rules out that possibility. Since the author thinks that "different predators" are the causal difference-maker, when it comes to why the lake stickleback lacks armor and the ocean stickleback has armor, her argument is strengthened by controlling for any other potential causal difference-maker. If you want to argue that "different predators" is why these two species are different, you want to believe that the difference is not due to different food supply, different mating habits, different habitat demands, etc.

Since the author is positing an evolutionary adaptation to the lake, the author needs to assume that the stickleback has been in the lake long enough for natural selection to take its slow effect. So, if anything, establishing that the stickleback diverged into ocean and lake varieties a long, long time ago might strengthen the argument by making it seem like the two stickleback populations have been apart for long enough that they plausibly could have evolved different traits.