Logical Reasoning

PT103 · S2 · Q14 The common ancestors of Australian

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The common ancestors of Australian land-and tree-dwelling kangaroos had prehensile (grasping) tails and long opposable thumbs, attributes that are well-adapted to tree-dwelling but offer kangaroos few advantages on land.

Given that ...

Ancient kangaroos had grasping tails and long opposable thumbs, which are good for tree-dwelling

How can it be that ...

Modern tree-dwelling kangaroos now lack these traits?

Goal

Essentially we need a story where the ancestor of the modern day tree-dwelling kangaroos could lose some traits that seem like they would be adaptive (provide some benefit to the modern tree-kangaroos' environment).

Maybe there was an in-between stage when the kangaroos weren't in trees at all, and thus these tree-friendly traits eventually evaporated from the gene pool.

14.

Which one of the following, if true, most helps explain the puzzling fact cited above?

  1. Modern tree-dwelling kangaroos must back

    Opposite: exacerbates paradox

    This answer just underscores how USEFUL it would have been to the modern tree-kangaroo to keep those traits that the ancient kangaroos had. So we're even more confused why evolution edited out those traits.

    13% picked this

  2. Modern tree-dwelling kangaroos are smaller

    Irrelevant Comparison

    This gives us a size gradient for the three styles: Ancient K's < Modern Tree K's < Modern Land K's But that gradient doesn't give us any causal explanation for how the ancient K's lost the tree-friendly traits. There's no common sense link from, "modern tree K's are somewhat bigger" to "that's why they no longer have these tree-friendly traits".

    1% picked this

  3. Modern tree-dwelling kangaroos’ tails cannot

    Cheats Paradox, if anything

    This doesn't explain how the ancient K's lost their tree-friendly traits en route to evolving into the modern tree K's. If anything, it tries to make it seem like modern tree K's do still have tree-friendly adaptations.

    4% picked this

  4. Correct

    Modern tree-dwelling kangaroos are descended

    Why this is right

    This shows that the modern Land and Tree kangaroos didn't both evolve from the ancient ones. Rather, ancient K's evolved into Land K's, and then Land K's evolved into Tree K's. Since there was a long stage in between (for many generations), there was time for Land Kangaroos to lose those tree-friendly traits that the ancient kangaroos had. After all, those traits wouldn't have been particularly useful to land kangaroos, and so it's perfectly plausible that those traits would be edited out by natural selection (useful traits persist .. harmful or useless traits tend to disappear).

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

    81% picked this

  5. Modern tree-dwelling kangaroos have smaller

    Irrelevant Comparison / Unrelated to Goal

    This doesn't seem related to our goal of explaining how ancient kangaroos lost their tree-friendly traits along the evolutionary path to modern tree kangaroos. This just compares modern land K's to modern tree K's, but the distinction doesn't tell us anything about how modern tree K's "lost" the tree-friendly traits that the ancient K's had.

    2% picked this

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