Scientist: A small group of islands near Australia is inhabited by several species of iguana; closely related species...

Mehran on October 17, 2018

@UTSmtihie let's break it down.

The conclusion here is, "Thus, these species' progenitors must have rafted on floating debris across the Pacific Ocean from the Americas."

The support?

"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."

This is a cause and effect argument.

The observed effect the author is trying to explain? The presence of iguanas in the Americas and on a small group of islands near Australia but nowhere else.

His proposed cause? The species' progenitors floated on debris across the Pacific Ocean from the Americas.

As you pointed out, this is a Weaken question so we are looking for the answer choice that weakens this argument.

And what is one way to weaken a cause and effect argument? By showing an alternative cause.

The most obvious alternative cause is the one referred to in the stimulus, which is that South America and Australia used to be the supercontinent of Gondwana before it fragmented.

So let's take a look at (D):

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

Does this weaken? Absolutely! How? Because it points out the possibility that the iguanas on the islands near Australia could have come from Australia as opposed to the Americas.

As such, (D) would be the correct answer.

Hope that helps! Please let us know if you have any other questions.

UTSmtihie on October 19, 2018

Thanks! I'd also like to talk about the answer that I chose. Is (C) the wrong answer choice because is merely says that the documented cases are uncommon? I'm guessing uncommon still leaves the possibility that the theory could have happened so it doesn't weaken the argument. Please explain what is wrong about answer choice (C).

Mehran on October 20, 2018

Hi @UTSmtihie, thanks for your post. As noted above, this is a cause and effect argument. The observed effect the author is trying to explain is the presence of iguanas in the Americas and on a small group of islands near Australia, but nowhere else. The proposed cause is that the species' progenitors floated on debris across the Pacific Ocean from the Americas.

The question stem asks us to Weaken this argument. One way to weaken a cause and effect argument is to show an alternative cause, as answer choice (D) does. Answer choice (C) does not posit an alternative cause, nor does it undermine the possibility in *this* case that *this* species rafted on floating debris. As you correctly note, the fact that something is "uncommon" does not mean it is "impossible," so (C) does not weaken this argument.

Hope this helps! Please let us know if you have any additional questions.

UTSmtihie on October 20, 2018

got it. thanks!