Q12 - The Amazon River flows eastward into

[ohthatpatrick]

Question Type:
Strengthen (hypothesis)

Stimulus Breakdown:
Hypothesis: The Amazon once flowed into the Pacific Ocean.
Evidence: some freshwater fish that now inhabit the Amazon are descendants of extinct saltwater fish that were in the Pacific but not the Atlantic.

Answer Anticipation:
We use our two-pronged causal prephrase:
1. Is there some OTHER WAY to explain the curious fact?
(How else could we explain how this descendant of a Pacific Ocean fish now inhabits the Amazon river?)
2. How plausible is the AUTHOR'S WAY?
(Was there a time that the Andes Mountains WEREN'T cutting off the Amazon from the Pacific? Maybe some tectonic activity has raised the Andes into a cut off position, but previously they would allow the river to flow west?)

Since this is strengthen, the correct answer will rule out some OTHER WAY to explain or it will (more likely) bolster the plausibility of the AUTHOR'S WAY

Correct Answer:
B

Answer Choice Analysis:
(A) This is irrelevant. We know that there are freshwater fish in the Amazon that have a now-extinct saltwater ancestor.

(B) This fossil evidence gives us a timeline that the Pacific Ocean saltwater fish existed BEFORE the Andes were formed. Since the Andes are what blocks the Pacific from the Amazon, it looks like this P.O. saltwater fish would have theoretically had access to the Amazon. This bolsters the plausibility that the freshwater fish we see today evolved from saltwater fish that may have gone from the Pacific Ocean into the Amazon.

(C) We can get rid of this answer since "many species" is a pretty weak quantity, and nothing about this answer relates specifically to South America, the Andes, or the Amazon. So this answer is very weak and very vague, thus not very helpful.

(D) If anything, this weakens, since it shows that there is currently no way for the Amazon to sneak by the Andes mountains in order to flow into the Pacific.

(E) This is irrelevant. The fish we're looking at do not survive in both. The present day fish survives in fresh water and the extinct ancestor survived in salt water.

Takeaway/Pattern: The correct answer helped corroborate the plausibility of the hypothesis by showing that there was a time when the Andes mountains didn't exist. That alone increases the plausibility that the Amazon might have once flowed into the Pacific. In addition, we learn that the ancestor of the Amazon freshwater fish was alive when "the gate was open", when there were no Andes Mountains to block the flow. This doesn't prove the hypothesis is true, but it establishes some minimum preconditions. If you were trying to prove the hypothesis that "Greg was the arsonist, since he is holding an empty gas can near the burning building", it strengthens the plausibility to establish that "Greg was at the building, before it was burning, with a non-empty gas can."

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