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

[Administrator]

Please post below with any questions!

[dtodaizzle]

Why is (D) correct? If there were fossils of the iguana found on Australia, are we to assume that

(1) The iguanas actually sailed from Americas to Australia first, and then to the islands?

(2) These fossils predate the fragmentation of Gondwana, and perhaps indicative that the iguanas migrated back and forth between present day Americas and the islands that are now off the coast of Australia. (On the other hand, the islands off the coast Australia could be formed after the fragmentation.)


Thanks!

[Emily Haney-Caron]

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.

[jax]

Is the flaw that the scientist assumes that iguanas travelled from Am to Aus (instead of maybe, for example, iguanas travelled from Aus to Am) there for [D] shows that the origins of the iguana is actually in Aus? But couldn't be that the fossils mentioned in [D] is the remains from the iguanas that travelled from Am (since Gondwana happened a long time ago).

I am bit confused with the whole premise/argument
I know that we are not looking for answer choices that will close all the gaps/is a perfect fit but still not sure how we can choose [D] (but also understanding that all the rest of the answer choices doesn't really seem to work either)

Would love some guidance and clarifications. Thank you!

[Jonathan Evans]

Hi, Jax,

Good question! Let's summarize this argument:

• P1: A species of iguana inhabits islands off Australia and nowhere else.
  P2: These iguanas are related to American iguanas.
  P3: The islands formed long after the fragmentation of the supercontinent that included both Australia and South America.
  C: The iguanas floated to these islands from the Americas.

The task is to weaken this argument. In other words, we want to find a statement that would provide evidence that maybe the iguanas did not come to the islands from the Americas.

For prephrasing, let's consider some possibilities. Could these iguanas have predated the breakup of the supercontinent and just remained on these islands afterward? No, this doesn't work because the islands formed long after the continent's breakup.

Thus, the iguanas must have come there from somewhere else. The author contends that the iguanas came from the Americas. We need to find evidence that perhaps the iguanas didn't come from the Americas.

This is what answer choice (D) suggests might be possible. In finding evidence of extinct related Australian iguanas, the author raises the possibility that the erstwhile species of iguana migrated over not from the Americas but from Australia.

I hope this helps!

[LSAT1802017]

Is this a causal argument?
Is the author providing a hypothesis for WHY there is a small group of iguanas living in the islands near Australia?
I didn't realize this was a phenomenon-causal explanation argument until an instructor told me it is. I simply thought the author is picking a possibility over others...

[James Finch]

Hi LSAT1802017,

Yes, the stimulus includes causal reasoning. The effect is that we now have several species of iguanas that live on islands off Australia and closely related species in South America, but nowhere else (meaning not in Australia). The stimulus then eliminates one possible explanation/cause, that the iguanas walked from the landmass that became these South America to the landmass that became these islands, or vice versa, during the period when they were connected as Gondwana, and concludes that the only other possible cause would be for the iguanas to have rafted across the ocean from South America.

If, as (D) states, there were, relatively recently, Australian species of iguanas closely related to the ones on the islands, then those now-extinct species could potentially be the ancestors of the species on the islands, and both iguanas in both South America and the islands could share common ancestors during the Gondwana era, thus giving us a more plausible alternate cause than the cause contained in the stimulus's conclusion.

Hope this clears things up!

[nlittle]

This was helpful. Thanks!

[rabedelahad]

I'm confused to why B is not right. If they are different in several genetic aspects, is it not possible that they are not actually closely related and therefore are two different types of iguanas?

[James Finch]

Hi Rabed,

When we have Weaken or Strengthen questions, we have to to accept the premises given in the stimulus as true. Here, we are given that the iguanas on the islands are closely related to species in the Americas, so we have to accept that to be true. But there is another problem with (B): being "different in several respects" does not necessarily mean that the species aren't closely related, nor does it make the conclusion (the species originated in the Americas and rafted across the ocean) less likely to be true. For a Weaken question, always attack the likelihood that the conclusion is true; for a Strengthen question, the correct answer choice will always make the conclusion more likely to be true.

Hope this helps!