Scientist: A small group of islands near Australia is inhabited

Question

Scientist: A small group of islands near Australia is inhabited by several species of iguana; closely related species also exist in the Americas, but nowhere else. The islands in question formed long after the fragmentation of Gondwana, the ancient supercontinent that included present-day South America and Australia. Thus, these species' progenitors must have rafted on floating debris across the Pacific Ocean from the Americas.

Which one of the following, if true, most weakens the scientist's argument?

(A) A number of animal species that inhabit the islands are not found in the Americas.

(B) Genetic analysis indicates that the iguana species on the islands are different in several respects from those found in the Americas.

(C) Documented cases of iguanas rafting long distances between land masses are uncommon.

(D) Fossils of iguana species closely related to those that inhabit the islands have been found in Australia.

(E) The lineages of numerous plant and animal species found in Australia or in South America date back to a period prior to the fragmentation of Gondwana.

Source: LSAT PrepTest 80 - December 2016

nks2611 · Accepted Answer

this is quite interesting to go through this argument . 
see we have said that species of iguana fish inhabit near the islands of australia ,
conclusion : if we re-frame says that these species which currently inhabit near the islands of australia have moved from America that means they earlier inhabited in America , so we need to tell that they had not inhabited in America but in australia .

now go through the options 
A. we do not care about other species or animals , wrong 
B. A trap, but does not directly address the issue , out 
C. irrelevant 
D. yes, if fossils of iguana found in australia then these species which live near the islands of Austrlia must have come from australia not from somewhere else like America . so correct 
E. totally out of scope talking about other plant and animals .

hence D 
hit kudos if you liked my reasoning .

chesstitans · Answer

E talks about lineages => out of scope with A and C.
B does not help.
Only D is left.

munchkin123 · Answer

Iguana species exist both in South America and a few islands near Australia. So, these species must have rafted from South America to these islands.

Weaken this statement. What if we can show that these species may have come from nearby Australia and not all the way from South America on boats?

A) A number of animal species that inhabit the islands are not found in the Americas.

So? Pfffttt.. Who cares about other animal species? Tell me about iguana. Irrelevant.

(B) Genetic analysis indicates that the iguana species on the islands are different in several respects from those found in the Americas.

Cool.. DNA is different from South American iguana. But, in these millions of years evolution, the DNA may have changed a bit. So, cannot really confirm. Let's keep this option open.

(C) Documented cases of iguanas rafting long distances between land masses are uncommon.

Documented by humans? Sure. But what if something like this happened millions of years ago? Cant really say. Ignore.

(D) Fossils of iguana species closely related to those that inhabit the islands have been found in Australia.

Fossils are related to the ones in Australia. Wait a minute, what if they came from Australia and not South America? :D

(E) The lineages of numerous plant and animal species found in Australia or in South America date back to a period prior to the fragmentation of Gondwana.

OKay, so? They may have common origins? But tell me about iguana please. Reject.

So, it is either B or D.

D shows an alternate place of origin. B is a bit vague and half confirmed.

So, answer is D.

Nightmare007 · Answer

Scientist: A small group of islands near Australia is inhabited by several species of iguana; closely related species also exist in the Americas, but nowhere else. The islands in question formed Jong after the fragmentation of Gondwana, the ancient supercontinent that included present-day South America and Australia. Thus, these species'  progenitors  (their ancestor's origin) must have rafted on floating debris across the Pacific Ocean from the Americas.

Which one of the following, if true, most weakens the scientist's argument?

(A) A number of animal species that inhabit the islands are not found in the Americas. - Out of the box.

(B) Genetic analysis indicates that the iguana species on the islands are different in several respects from those found in the Americas. - Different in several respects- Yes might the time changed it. This actually strengthens a bit.

(C) Documented cases of iguanas rafting long distances between land masses are uncommon. - Does nothing, documented cases might be uncommon but might be found.
 
(D) Fossils of iguana species closely related to those that inhabit the islands have been found in Australia. - So they might come from Australia rather than Americas. - ANSWER

(E) The lineages of numerous plant and animal species found in Australia or in South America date back to a period prior to the fragmentation of Gondwana. - Out of the box.

kanthaliya · Answer

Can we assume that DNA can change after few years? As far as I know, DNA retains its property.

Please help me with this question, these type of problems are most important for GMAT

slayer1983 · Answer

Its safe to say that its a call between B and D.
I think B is the answer. Genetics can conclusively confirm the lineage of any species. I don't know if you guys have watched the video in which several people are told about their places of origin through genetics. Several species are clubbed in a biological division based on their similarities in genetics.

D on the other hand says that fossils of the ancestors of the iguana has been found in Australia, which was once part of Gondwana, as was America. This doesn't weaken the argument at all. On the other hand, if the fossil in on another island, it may make the argument strong that iguanas can spread from one piece of land to another separated by a water body.

warrior1991 · Answer

Option B says "Genetic analysis indicates that the iguana species on the islands are different in several respects from those found in the Americas."

This option highlights the difference in characteristics between iguana species found on the islands and those found in the Americas.Look on the other side,there can be multiple similarities as well. This option didn't disregard the similarity point. So we cannot say that this option is correct. Actually we cannot conclude whether the species rafted across Pacific ocean. You see the outcome is undetermined.

Option D on the other hand says that fossils of the ancestors of the iguana has been found in Australia.

If fossils are found in Australia, the conclusion "MUST HAVE RAFTED" is weakened.

IMO D is correct.

Hope that helps!!

dcummins · Answer

Tough nut.

The   argument   is that the iguanas found on the islands near Australia must have rafted on floating debris from the Americas.
This  assumes  the iguanas originated in the Americas.
The argument is based on the fact that the iguanas are similar to those found in the Americas, but nowhere else. The argument is also dervied from the fact that the islands in question were formed after the the fragmentation of Gondwana (South America + Australia combined land).

We are asked to weaken the argument.
A is incorrect - the argument is concerned with iguanas, not other animals.
B is incorrect because the iguanas may still be descended from the Americas even if the present day Australian iguanas are different from the Americas Iguanas. 
C is incorrect- a lack of evidence to prove a claim is not sufficient to be taken as evidence against that claim - this is actually a logical fallacy.
D is  correct  because evidence of fossils found in Australia may suggest the iguanas descended from Australia, Not the Americas.
E is incorrect because (1) we are only concerned with the iguanas and this answer choice doesn't help us understand how the iguanas MAY NOT have originated from the Americas, and (2) the argument actually assumes that the iguanas lineages extend back before the fragmentation, but it assumes the lineage was in the Americas, not elsewhere.

ballest127 · Answer

Hi Experts,

In D, how do we know that the found fossils in Australia are not the one of these specie that had rafted on the floating debris and died on Australia ?

Please explain.

Thank you.

Bambi2021 · Answer

I dont like D.

Of course fossils are found in Australia if they have lived there for a very long period. We still dont know if they once rafted from America or not.

Posted from my mobile device

unraveled · Answer

Scientist: A small group of islands near Australia is inhabited by several species of iguana; closely related species also exist in the Americas, but nowhere else. The islands in question formed Jong after the fragmentation of Gondwana, the ancient supercontinent that included present-day South America and Australia.  Thus, these species' progenitors must have rafted on floating debris across the Pacific Ocean from the Americas.

Which one of the following, if true, most weakens the scientist's argument?

(A) A  number of animal species  that inhabit the islands are not found in the Americas.

(B)  Genetic analysis  indicates that the iguana species on the islands are different in several respects from those found in the Americas.

(C)  Documented cases  of iguanas rafting long distances between land masses are uncommon.

(D) Fossils of iguana species closely related to those that inhabit the islands have been found in Australia.

(E) The  lineages of numerous plant and animal species  found in Australia or in South America date back to a period prior to the fragmentation of Gondwana.

Yes, it is a valid question but is there a possibility that D weakens the conclusion(last sentence)? Yes, it does.
If Iguanas travelled from Australia which can be a possibility then they might not have rafted on floating debris across the Pacific Ocean from the Americas. 
We need not prove the weakening effect. We just need to identify that D can.

Everytime, if you try to find evidence of how the right option does what is required as per the question stem, then you might never find a right answer. We need to find a sufficient condition which D gives emphatically(relatively speaking).

HTH.

Mavisdu1017 · Answer

Hello expert,
I know D meant to say this species rafted from Australia but not America, but I have a doubt: is D contradictory with the premise? — —closely related species also exist in the Americas, but nowhere else. 
Need your help. Much thanks.

GMATNinja · Answer

(D) doesn't contradict the premise. In the passage, we learn that the closely related iguanas  exist  -- present tense -- on the islands, in the Americas, and nowhere else.

(D) tells us that  fossils  of closely related iguanas have been found in Australia. That means that the iguanas were in Australia in the  past , but not necessarily in the present.

As you've mentioned, (D) gives us an alternate origin story for the iguanas on the islands: maybe they came from Australia instead of the Americas. That weakens the argument that "these species' progenitors must have rafted on floating debris across the Pacific Ocean from the Americas."

I hope that helps!

Mavisdu1017 · Answer

Understand, and much thanks for your input expert.

Mislead · Answer

Hi  GMATNinja ,

Sorry about reopening this again but i have question on this one between B and D.
 
B: The genetics are different therefore, they are not from the same progenitor so they rafting across the ocean cannot be proved.
 D: T hey could have rafted across the ocean and could have inhabited the island 1st and then they could have moved to mainland australia therefore the similarity can be seen.

Cannot understand how D can weaken the argument more than B. Please help.

GMATNinja · Answer

Remember that we're not looking for something that DISPROVES the argument. We just need to weaken it.

If (D) is true, is it still POSSIBLE that the species' progenitors rafted on floating debris across the Pacific Ocean from the Americas? Sure. But the scientist's argument was that the species' progenitors MUST HAVE rafted on floating debris across the Pacific Ocean from the Americas. So by simply presenting an alternate (and highly plausible) possibility, (D) certainly weakens the argument.

(B) doesn't actually go against anything in the argument. The scientist says the species are CLOSELY RELATED, not IDENTICAL. So we expect there to be SOME genetic differences. That's consistent with choice (B).

I hope that helps!

rahul5657 · Answer

(A) A number of animal species that inhabit the islands are not found in the Americas.
This is irrelevant to the argument. The scientist is only concerned with the iguana species, not other animals. The fact that other species exist on the islands does not weaken the claim about how iguanas arrived.

(B) Genetic analysis indicates that the iguana species on the islands are different in several respects from those found in the Americas.
This does not necessarily weaken the argument. Evolutionary changes could have occurred after the iguanas arrived on the islands. The argument only states that their ancestors came from the Americas, not that they should be identical today.

(C) Documented cases of iguanas rafting long distances between land masses are uncommon.
This might suggest that rafting is unlikely, but it does not disprove that it happened in this case. Even if uncommon, it could still have occurred. The scientist’s argument does not depend on how frequently this happens, just that it did happen here.

(D) Fossils of iguana species closely related to those that inhabit the islands have been found in Australia.
This weakens the argument significantly. If closely related iguanas once lived in Australia, then the island species could have evolved from those ancestors instead of arriving from the Americas. This provides an alternative explanation, undermining the need for the rafting hypothesis.

(E) The lineages of numerous plant and animal species found in Australia or in South America date back to a period prior to the fragmentation of Gondwana.
This is a general statement about species but does not address the iguanas specifically. Even if other species originated from Gondwana, that does not mean the iguanas did.

Scientist: A small group of islands near Australia is inhabited

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Scientist: A small group of islands near Australia is inhabited

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