My justification for (A) over (D) is different from other members on here, and I argue that others' justification for (A) is incorrect. Here's why:
A. A characteristic developed for a particular ability does not remain after that ability is lost.
vs.
D. Today’s flightless birds are direct descendants of feathered reptiles.
In the end, both
work, but I'd argue that "flight" is too much to assume and is not necessary for the statement to hold true. There are four strong cases for this:
1. Red herring: Sentence (A) and the passage never refer to flight anywhere, hence the find the assumption question. However, it never refers to birds in general (both flying and flightless birds) for the assumption of flying birds to be relevant. E.g., if we compare hairy ancient reptiles to today's hairy apes, and say that these two very different species share hair because it has to do with body temp regulation, then does that having anything to do with today's humans who have only a little bit of hair for a different reason, perhaps to keep water, dirt, etc. from entering our eyes and other orifices? Bringing in the outside info of humans benefiting from hair in a different way is unnecessary. Similarly, today's flying, feathered birds are unnecessary. Assuming the combination of flight and feathered birds is a red herring, no pun intended.
2. Begs the question: if body temp is the main reason for feathers, then why are feathers necessary for flying, which is a completely different, mutually exclusive trait; then why didn't flightless birds lose their feathers if they lost the ability to fly? Because feathers came from ancient flightless reptiles to control body temp? Then why do flying birds still have feathers if its purpose of body temp regulation may have been lost, and feathers somehow remained over millions of years in a state of uselessness until the benefit of flight happened to come along? If flying birds also use feathers for body temp in a dual purpose sense, then the body temp benefit is never lost, but then why do flightless birds still have wings if that's for flight?
3. Contradiction: You don't know that flightless birds are directly descended from today's feathered flying birds. The strong language of "direct" is precisely why we didn't choose (D). So, why should be apply the same language between the relationship of today's flying and flightless birds? It's equally unjustified, which is why the genesis theory of flightless birds' feathers is made in the first place.
4. If-then transposition rule: Note that if A then B = if not B, then not A. If we would have chosen (D) save for its strong language, and if (A) says the exact same thing as (D) by way of if-then logic (again, minus the use of "direct"), then (A) must be chosen for the same reason as (D).
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For (A), I can see the argument that flightless birds will lose their feathers if they lose their ability to fly, since there's no use for them. But, who says that the biologist is necessarily assuming that? What if the biologist believes that the flightless birds were a different branch of evolution from the ancestors of birds? So, say scaled flying birds developed into two distinct groups: flying feathered birds and flightless feathered birds. Both develop feathers independently, which is a possibility.
(D) can be read as "If today's flightless birds have feathers, then ancient reptiles are
directly related because they have feathers, too." I see option D as making too strong of a claim, that today's featherless birds are
direct descendants of feathered reptiles, which cannot be inferred (for instance, ancient reptiles and today's flightless birds may share a common ancestor that had feathers, but the two may only be related indirectly by way of the ancestor (= evolutionary fork in the road). And, the language of the passage seems to reinforce that
today's flightless birds are very different from ancient feathered reptiles.
Option A is sort of saying D by negating it. So, negative of (D) is, "If not for the feathers on ancient reptiles, then no feathers on today's flightless birds." Notice I dropped the strong word,
direct.
There is a transposition rule of logic that comes in handy here: If X, then Y = If not Y, then not X.
So, A reads as: "If not feathers on ancient reptiles (-Y), then trait is lost and not feathers on today's flightless birds (-X). Feathers do exist on today's flightless birds and trait is not lost (+x), then it must have come from ancient reptiles."
A is exactly the same as D but differs in that it doesn't use as strong a language.So, our justifications for (A) differ by placing a higher priority on the following assumptions:
- Mine: Animals are more likely to carry the same traits as very different species through direct or indirect genetic inheritance, and not as likely to develop the same trait for the same purpose independently.
vs.
- Others: Birds can have a dual purpose for feathers (flight and body temp regulation), whether both are primary, one is secondary, or body temp regulation became a sort of unintended benefit for flightless birds so they found a use after losing their ability to fly.