#25 - Scientist: A small group of islands near Australia

[Margo]

Hi,

I have read the explanations so far and they've been helpful! Please bear with me as I try to clarify a few things..

I understand that the conclusion is "iguanas floated to the islands near Australia from America," and we're trying to look for an answer that weakens this conclusion, while accepting everything in the stimulus as true.

Would you mind explaining why C is not a valid answer? If documented cases of iguanas rafting long distances between land masses are uncommon, wouldn't this weaken the conclusion that the iguanas rafted all the way to the islands near Australia from America? After all, that is quite a long distance. My understanding is that weaken questions don't have to completely destroy the argument, and this doesn't--because even though it is rare for them to raft long distances, it COULD have happened in this case. But I thought this answer made it a lot LESS likely.

Thanks!

[James Finch]

Hi Margo,

Reading quickly (almost inevitable as this is the penultimate question in the section), answer choice (C) seems to do what you describe. But if we think about the implication a little more deeply, we can see that it doesn't actually have any effect on the likelihood of the conclusion being true or not. Why not? By saying that "documented cases" are "uncommon," (C) implies that while rare, there are some cases of iguanas doing exactly what the stimulus concludes they must have done; the certainty of the stimulus's conclusion is based upon its erroneous belief that no other options exist to explain the similarities between the island iguanas and their relatives in South America. Rafting across the Pacific is argued to be the sole possible cause for the effect of having iguanas related to one another in the two distant parts of the world. So the stimulus's author would be able to simply counter (C) by responding that this is one of those "uncommon" occurrences, because it doesn't attack the underlying causal link that the argument is based upon. Instead we need an answer choice that makes the rafting from South America to the islands less likely to be the sole cause for the effect of closely related iguanas in the two places. (D) does this by offering a more likely alternate cause.

Hope this helps!

[Mark83]

Hi dtodaizzle,

Your two possibilities could be, and another possibility is that the iguanas pre-date the break up, came from Australia, and the iguanas in the Americas also are pre-break up and so both sets came from Gondwana originally. Really, the important thing is that it introduces other possibilities, other than rafting on floating debris across the Pacific, weakening the conclusion that that is what must have happened.

But it doesn't say how old the fossils are. The fossils could have been left after the species travelled there for all one knows. (B) seems to weaken it better since it shows that the species aren't even the same.

[Adam Tyson]

We aren't looking to prove that the iguanas didn't raft across the Pacific, Mark, but only raise some doubts about that conclusion. Fossils found in Australia don't prove anything, but they do raise doubts about the rafting explanation because the iguanas on the islands near Australia could have rafted from Australia, or swum, or been carried by birds, etc. D gives us an alternative explanation as a possibility, and that is enough to weaken the claim that they must have rafted from the Americas.

I liked answer B and kept it as a contender, but ultimately rejected it because despite the differences the two are, per the stimulus, closely related. Those difference could be explained by mutations and adaptations over time once the groups were separated. In any event, while B gives us a difference, it doesn't give us an alternative like D does, and that makes D the better choice here. With much less distance to cover, D gives us a much more likely explanation, weakening the proposed explanation that the author was sure of.

[Mark83]

We aren't looking to prove that the iguanas didn't raft across the Pacific, Mark, but only raise some doubts about that conclusion. Fossils found in Australia don't prove anything, but they do raise doubts about the rafting explanation because the iguanas on the islands near Australia could have rafted from Australia, or swum, or been carried by birds, etc. D gives us an alternative explanation as a possibility, and that is enough to weaken the claim that they must have rafted from the Americas.

I liked answer B and kept it as a contender, but ultimately rejected it because despite the differences the two are, per the stimulus, closely related. Those difference could be explained by mutations and adaptations over time once the groups were separated. In any event, while B gives us a difference, it doesn't give us an alternative like D does, and that makes D the better choice here. With much less distance to cover, D gives us a much more likely explanation, weakening the proposed explanation that the author was sure of.

I think I see what you're saying. I was going beyond what they were asking for.

[Pragmatism]

So, I was initially looking for an alternative explanation, but I found D insufficient to provide me that alternative. I chose E as the closest answer choice to provide me with an alternative, though an unbelievably weak one.

Here is my reasoning:

D— you found fossils in Australia, okay, so what? The fact that event took place millions of years ago doesn’t mean that they didn’t travel from the Americas to Australia floating on debris. The fact that this was a “fossil” they could have still done what the stimulus concludes and died there and become fossilized. Thus, its compellingness dwindled as a correct answer choice.

E— since “numerous plant and animals species” doesn’t explicitly mean to include the species in the stimulus nor does it exclude the species, I took this ambiguity to provide a hope of assistance to the alternative explanation.

Can you please help clarify how D suffices to weaken the stimulus over E given my reasoning above.

Thanks

[Pragmatism]

Could it be, that the stimulus says, “islands near Australia.. also exist in the Americas, but nowhere else,” to mean near Australia isn’t Austrailia. Thus, if we find something in Austrailia, it (answer choice D) not only weakens the premise by calling into question the only locations these species were found, but also weakens its conclusion?

[Adam Tyson]

I think you've got it now, Prag. Islands near Australia are not islands IN Australia, so they are two different places. It looks like you were perhaps trying to DISPROVE the conclusion, rather than merely weaken it, and you rejected answer D because it didn't disprove anything. As you said, the iguanas still could have rafted over from the Americas. But that is the wrong standard to apply to a weaken question - we aren't looking for proof against the conclusion, but just an element of doubt about it. If the iguanas on the islands appear to be closely related to fossils found in Australia, then they could have gotten to the islands from Australia instead of from the Americas. That seems much more likely, doesn't it? Imagine, for example, that the islands formed by breaking off from the Australian continent, and the iguanas were just there all along - no rafting required. Gondwana broke apart and there were iguanas on both the Australian and American segments, and then later Australia broke up further and the iguanas were on the mainland and on the islands - boom, easy answer.

Answer E is more than weak - it is completely irrelevant. So what that some species are that old, dating back to when Gondwana was still all one piece? Even if the iguanas are that old, meaning they existed when Gondwana was still together, how does that undermine the claim that they must have rafted across the ocean to get to those much younger islands? Without making further assumptions, like that they were also in Australia after the breakup, it really does nothing at all. It's that latter part that hurts the argument, which is what answer C suggests.

[BWACHS]

Hi! Thank you for the explanations. I'm still struggling with the fact that the Iguanas could not have originated from the same ancient landmass.

By saying the "island formed long after the landmass" are we supposed to assume that those islands did not break off from the landmass?

E is tempting because it could weaken the argument by showing that these iguanas were in fact both from the same landmass that then broke up. I see why D is right but keep struggling to not pick E when I revisit this problem.

Thank you!

[Beth Hayden]

Hi Bwachs,

You don't need to disprove the conclusion here, it's still totally possible that the Iguanas did all originate from the same location and got to the islands by raft. The correct answer choice just needs to make that explanation less likely, and (D) does that by providing an alternative and much more reasonable explanation.

Yes, when they say the island formed long after Gondwana broke apart, that means the island was not part of Gondwana originally and formed later. If it had been part of Gondwana, we could just assume that the iguanas never moved and their home just drifted away from South America. But because the island formed later, we can't make that assumption (hence why the author comes up with the rafting hypothesis). Though as Adam points out, maybe (it's possible) the island broke off from Australia long after Australia broke apart from South America. If that's true, (D) is an even better weakener, but (D) still works even if it's not the case; maybe the iguanas did hitch a ride on rafts, but they took the much shorter journey from Australia to the islands (not from South America).

(E) isn't really relevant here. Of course it makes sense that since Australia and South America used to be one landmass, there are some plants and animals in both places with common ancestors. But we have no reason to believe that any of those animals are the iguanas. Maybe they are, but without making that (unsupported) inference, (E) has no real affect on the conclusion.

Hope that helps!