Logical Reasoning

PT148 · S4 · Q25 Scientist: A small group of islands

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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.

Conclusion (thus)

The ancestors of these iguana must have rafted on floating debris across the Pacific Ocean from the Americas.

Evidence

These iguanas inhabit several islands near Australia and some related species in the Americas, but nowhere else.

These islands were formed after the supercontinent connecting South America and Australia split apart.

Evaluate

The author is trying to solve a causal mystery here.

Curious Fact ... How did we end up with this group of closely related iguana species in the Americas and in the islands of near Australia?

Author's Explanation ... The ancestor of these iguanas lived on the American/Australian supercontinent. The supercontinent broke apart, and now there's a Pacific Ocean between the Americas and Australia. These islands later formed. Then, some iguanas from the American continent rafted across the Pacific Ocean on floating debris and landed at these islands near Australia, where the iguanas then proliferated into a local species.

Quite a story, right? Don't skimp on the stage where you're articulating for yourself the causal story that the author is selling. There are often unsaid pieces to that story that we want to get high on our mental radar.

Goal

As always, when we want to Weaken an Explain the Curious Fact argument, we go looking primarily for answers that - suggest an Alternate Explanation for the Curious Fact (how else could we have ended up with these related iguana species in the Americas / these islands and nowhere else)

- undermine the plausibility of the Author's Explanation (what would make it seem less plausible that present day iguanas in those near-Australian islands got there by rafting on debris over the Pacific)

25.

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

  1. A number of animal species

    No Impact

    This would be attempting to attack the plausibility of the author's story by saying, "Not all the animals on these islands seem to have ancestors that lived in America!" But the author is only claiming these iguanas are like that, not that any other species on the islands is like that.

    3% picked this

  2. Genetic analysis indicates that the

    Nothing New

    This isn't telling us anything we didn't already know, in a sense. The passage identified the island iguanas as being "closely related" to the American ones. If you're closely related, rather than same species, then of course you have genetic differences.

    24% picked this

  3. Documented cases of iguanas rafting

    Weaker Impact

    This somewhat calls into question the plausibility of the Author's Explanation. If it's uncommon for iguanas to raft long distances, then the author's hypothesis that these iguanas rafted from the Americas across the Pacific Ocean seems like a pretty exotic possibility. However, this answer isn't saying anything strong like, "An iguana couldn't possibly survive a rafting voyage from South America to Australian islands." It's just saying "documented cases are uncommon". If there are any documented cases, even if rare, that would strongly bolster the author's story by showing it is indeed possible for an iguana to raft that far. And given that we're talking about something that probably happened hundred of thousands of years ago, the lack of documentation seems less of a knock against this hypothesis. Because documented cases and uncommon leave lots of wiggle room (it's possible there are some rare documented cases / it's possible there are commonly undocumented cases of iguanas doing this), this answer doesn't have much punch to it. It definitely goes the right direction of lowering plausibility, but ever so faintly.

    9% picked this

  4. Correct

    Fossils of iguana species closely

    Why this is right

    This suggests an Alternate Explanation (the #1 pattern for Weaken on Explain Curious Fact arguments). It suggests that back when the supercontinent was together, there were iguanas throughout the regions that would later be Australia and America. Then the supercontinent split apart, and so you had some iguanas left on the American continent and some left on the Australian continetn Then the islands form and somehow the iguanas get to the islands. The author is thinking they probably rafted over the entire Pacific Ocean on a three month voyage on a piece of floating debris, from America. This answer suggests that the iguanas probably just floated over from Australia. Since the islands are near Australia, that possibility it at least as plausible as the idea that they floated over from America. Since we've exposed an alternate possibility, we've weakened the author's certainty that, "these progenitors' must have rafted on floating debris from the Americas". We can counterargue, "They didn't have to raft on debris from the Americas. They could have rafted over from Australia."

    Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.

    46% picked this

  5. The lineages of numerous plant

    No Impact

    This is just saying that other flora and fauna have an evolutionary past that goes back to the period before the supercontinent split up. That's a very generic fact about Earth's biological history that doesn't shed any light on this specific situation.

    17% picked this

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