PrepTest 103, Section 2, Question 14
The common ancestors of Australian land- and tree-dwelling(50 more words not shown)
Which one of the following,(10 more words not shown)
- AModern tree-dwelling kangaroos(27 more words not shown)
- BModern tree-dwelling kangaroos(13 more words not shown)
- CModern tree-dwelling kangaroos'(18 more words not shown)
- DModern tree-dwelling kangaroos(21 more words not shown)
- EModern tree-dwelling kangaroos(22 more words not shown)
Reading the argument
Why lose the tools that help you live in trees?
A grasping tail works like an extra way to hold on, and opposable thumbs help with gripping. The passage says these traits are useful in trees but offer few advantages on land. That makes their disappearance in land-dwelling kangaroos understandable. Their absence in the modern tree-dwellers is the puzzle.
Maybe there was a land-dwelling stage in between.
Imagine descendants of the common ancestor spending many generations on land and losing those features during that period. A later branch could move into the trees after the features had already disappeared. Living in trees now would not mean every generation of its ancestors had continuously lived there.
What we’re looking for
Another possibility would be some different evolutionary pressure that outweighed the usefulness of the traits. We have not been told what that pressure might be. My more concrete prediction for the explanation question is the intermediate land-dwelling ancestor: the loss happened before this branch became tree-dwelling.
Working the answers
- A
This shows a disadvantage associated with lacking the thumbs: the modern animals descend slowly, while the ancestors used their thumbs to go down quickly. That tells us something about the consequences of the difference. It does not explain how or why the useful trait was lost. If anything, it reinforces why losing it seems puzzling.
- B
The sizes are different, but no connection is given between those sizes and losing grasping tails or opposable thumbs. A size change could matter if we were told how it altered the usefulness of those traits. Simply listing who is larger than whom does not supply that explanation.
- C
We learn more about the modern tails: they cannot grasp, but they are longer and more flexible than the land-dwellers' tails. That does not explain the loss of grasping ability or opposable thumbs. Describing what the animals are like now is not the same as explaining how they came to lack these ancestral features.
- DCorrect
This gives us the intermediate stage we predicted. The modern tree-dwellers descended from kangaroos that had spent many generations on land. The passage already makes the loss of the traits during land-dwelling understandable because the traits offer few advantages there.
The later tree-dwellers therefore came from a lineage that had passed through conditions under which those traits could have disappeared. We do not have to imagine the animals losing useful gripping tools while remaining in the trees the whole time. Their current habitat does not tell us their entire evolutionary history.
- E
We are given another present-day difference between the groups: the tree-dwellers have weaker hind legs and move more slowly on land. That does not explain the missing thumbs or grasping tails. Those traits, and their loss along the tree-dwellers' ancestry, are the specific puzzle.
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