Logical Reasoning

PT152 · S4 · Q12 The Amazon River flows eastward

A free, expert breakdown of this official LSAT Logical Reasoning question.

  • Save & drill this skill — build targeted practice sets from questions like this one

  • Video walkthroughs — watch every question solved step by step

  • 81 official LSATs — as questions, timed sections & full-length tests

Start practicing free

The Amazon River flows eastward into the Atlantic Ocean from its source in the western part of South America.

Conclusion / Hypothesis

The Amazon River once flowed into the Pacific Ocean.

Evidence

The Amazon is in the western part of South America (the Pacific Ocean is to the west of South America), but it currently flows eastward into the Atlantic Ocean.

The Andes Mountains now lie in between the Amazon river and the Pacific Ocean. But there is a freshwater fish in the Amazon that descended from a salt-water fish that was found only in the Pacific.

Evaluate

These scientists are thinking that at some point the Amazon was not cut-off from the Pacific by the Andes. During that period a saltwater fish found only in the Pacific ocean evolved into a freshwater fish that lived in the Amazon and continues to, to this day.

If we were going to play devil's advocate to the hypothesis and say "no, the Amazon never flowed into the Pacific", then we would sort of need to explain how there's a fish in the Amazon that descended from a now-extinct fish that only inhabited Pacific waters.

We could also just bring up anything that would make this hypothesis implausible, for other reasons. For example, we could say,

Goal

Look for an answer that rules out a different way of explaining how a pacific saltwater fish ended up evolving into an Amazonian freshwater fish, or an answer that creates a possibility in the past for the Amazon to have flowed into the Pacific, or shows some more evidence suggesting that it did.

12.

Which one of the following, if true, provides additional evidence in support of the hypothesis that the Amazon River once flowed into the Pacific Ocean?

  1. In many cases, species of

    No Impact / Weak

    I'm happy that's the case, in many cases. Unfortunately, it's not the case in this case. Here, we're talking about a freshwater species that has a saltwater ancestor. So this answer isn't relevant to the conversation. (It's also very weakly worded and it sounds like it has the tone of pointing out an objection)

    2% picked this

  2. Correct

    Most of the fossilized remains

    Why this is right

    This is suggesting that when this Pacific saltwater ancestor lived, the Andes Mountains had not yet formed. That increases the plausibility of the author's causal story -- she thinks that the freshwater fish in the Amazon whose ancestor is this Pacific saltwater fish got where it is because at some point the Pacific Ocean and the Amazon river were connected. In order for this storyline to work, it needs to be true that when the saltwater fish was alive, there was some connection between the Pacific and the Amazon. Since those two things are currently disconnected by the Andes Mountains, if the Andes Mountains didn't exist yet, then the two bodies of water could commingle.

    Skill tested: Strengthen · how this choice captures the argument's function is the move to repeat next time.

    85% picked this

  3. Many species of fish that

    Irrelevant Distinction / Weak

    Our conversation has a fish that inhabits the Amazon River, related to a fish that only inhabits the Pacific ocean, and that's a bit of a Curious Fact, since the Amazon and the Pacific are currently cut off from one another. This weak idea that "at least 5-10 species of fish in the Atlantic Ocean are related to something in the Pacific" has no impact on that conversation. The Atlantic and Pacific Ocean are not cut off from one another.

    10% picked this

  4. The Andes Mountains extend from

    No Impact

    This is just fleshing out what the concept of "cut off from the Pacific Ocean" means. Whether the mountains cut off the Amazon river by a straight vertical line or a diagonal line, or an L-shape, doesn't matter. They're currently cut off. This answer is focused on the present, whereas our hypothesis should have us wanting to get more information about the past.

    2% picked this

  5. There are very few fish

    No Impact

    This the third answer choice now that is just saying some generality about fish in the world. We need some specific information about the Amazonian freshwater fish that are descended from the Pacific-only saltwater fish, or more information about the Amazon River / Andes Mountains / Pacific Ocean.

    2% picked this

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill
LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free