CAT 2025 Slot 2 VARC Question & Solution
Passage
The passage below is accompanied by four questions. Based on the passage, choose the best answer for each question.
Time and again, whenever a population [of Mexican tetra fish] was swept into a cave and survived long enough for natural selection to have its way, the eyes disappeared. “But it’s not that everything has been lost in cavefish . . . Many enhancements have also happened.” . . . Studies have found that cave-dwelling fish can detect lower levels of amino acids than surface fish can. They also have more tastebuds and a higher density of sensitive cells alongside their bodies that let them sense water pressure and flow. . . .
Killing the processes that support the formation of the eye is quite literally what happens. Just like non-cave-dwelling members of the species, all cavefish embryos start making eyes. But after
a few hours, cells in the developing eye start dying, until the entire structure has disappeared. [Developmental biologist Misty] Riddle thinks this apparent inefficiency may be unavoidable. “The
early development of the brain and the eye are completely intertwined—they happen together,” she says. That means the least disruptive way for eyelessness to evolve may be to start making an eye and then get rid of it. . . .
It’s easy to see why cavefish would be at a disadvantage if they were to maintain expensive tissues they aren’t using. Since relatively little lives or grows in their caves, the fish are likely
surviving on a meager diet of mostly bat feces and organic waste that washes in during the rainy season. Researchers keeping cavefish in labs have discovered that, genetically, the creatures are exquisitely adapted to absorbing and storing nutrients. . . .
Fats can be toxic for tissues, [evolutionary physiologist Nicolas] Rohner explains, so they are stored in fat cells. “But when these cells get too big, they can burst, which is why we often see
chronic inflammation in humans and other animals that have stored a lot of fat in their tissues.” Yet a 2020 study by Rohner, Krishnan and their colleagues revealed that even very well-fed cavefish had fewer signs of inflammation in their fat tissues than surface fish do. Even in their sparse cave conditions, wild cavefish can sometimes get very fat, says Riddle. This is presumably because, whenever food ends up in the cave, the fish eat as much of it as possible, since there may be nothing else for a long time to come. Intriguingly, Riddle says, their fat is usually bright yellow, because of high levels of carotenoids, the substance in the carrots that your grandmother used to tell you were good for your…eyes.
“The first thing that came to our mind, of course, was that they were accumulating these because they don’t have eyes,” says Riddle. In this species, such ideas can be tested: Scientists can cross surface fish (with eyes) and cavefish (without eyes) and look at what their offspring are like. When that’s done, Riddle says, researchers see no link between eye presence or size and the accumulation of carotenoids. Some eyeless cavefish had fat that was practically white, indicating lower carotenoid levels. Instead, Riddle thinks these carotenoids may be another adaptation to suppress inflammation, which might be important in the wild, as cavefish are likely overeating whenever food arrives.
Question 1
All of the following statements from the passage describe adaptation in Mexican tetra cavefish EXCEPT:
Solution:
Main Idea of the Passage
The passage explains how Mexican tetra cavefish have evolved specific adaptations that help them survive in cave environments. These adaptations include:
- Losing eyes to save energy
- Altering metabolism to cope with scarce and unpredictable food
- Regulating inflammation, even when storing large amounts of fat
The focus is on traits that directly help cavefish survive under conditions of limited resources.
Explanation of the Correct Answer
Why Option C Is Correct
Option C does not describe an adaptation in cavefish.
- This option explains how, in humans and other animals, fat cells can grow too large and burst, leading to inflammation
- It provides background information about why fat storage is usually harmful
- However, the passage explicitly contrasts this with cavefish, which do not experience this inflammation
Since this statement explains a problem in other organisms rather than an adaptive trait in cavefish, it does not describe a cavefish adaptation.
Why the Other Options Are Incorrect
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Option A:
- Describes the loss of energy-expensive tissues like eyes
- This is clearly presented as an adaptive advantage in caves
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Option B:
- Describes the meagre and unpredictable diet in caves
- Although it focuses on environmental pressure, it explains why cavefish adaptations evolved
- Therefore, it supports the adaptation narrative
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Option D:
- Describes cavefish becoming very fat when food is available
- This reflects their ability to overeat and store nutrients, a key survival adaptation
Final Answer
Statement That Does NOT Describe an Adaptation: Option C
Question 2
Which one of the following best explains why the “apparent inefficiency” is “unavoidable”?
Solution:
Main Idea of the Passage
The phrase “apparent inefficiency is unavoidable” refers to the fact that cavefish embryos initially begin developing eyes, even though these eye cells later die off. This sequence may seem wasteful, but the passage explains that it is biologically unavoidable due to constraints in early development.
The key idea is that evolution cannot simply skip the early stages of eye formation without disrupting other essential processes.
Explanation of the Correct Answer
Why Option D Is Correct
Option D correctly explains why this inefficient process cannot be avoided:
- Mexican tetra cavefish embryos initially develop in the same way as their surface-dwelling relatives
- In early development, eye and brain formation are completely intertwined
- Because of this overlap:
- Evolution cannot remove eye formation at the start
- Doing so would also interfere with brain development
As a result, cavefish must begin eye formation and then dismantle it later, making the inefficiency unavoidable.
Why the Other Options Are Incorrect
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Option A:
- Explains why eyes are costly in adult cavefish
- Does not address why embryos must start forming eyes
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Option B:
- Incorrectly attributes eye loss to darkness
- The passage focuses on developmental constraints, not environmental darkness
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Option C:
- Describes changes that occur after eye loss
- Fails to explain why the initial eye-development stage cannot be skipped
Final Answer
Correct Answer: Option D
Question 3
Which one of the following results for the cross between surface fish (with eyes) and cavefish (without eyes) would invalidate Riddle’s inference from the experiment?
Solution:
Main Idea of the Experiment
The experiment tests Riddle’s claim that carotenoid accumulation (yellow fat) in cavefish is not linked to eye loss, but instead serves another function, such as reducing inflammation.
To disprove this idea, the results would need to show a direct connection between the absence of eyes and carotenoid accumulation.
Explanation of the Correct Answer
Why Option C Is Correct
Option C would directly invalidate Riddle’s inference because:
- It shows that only eyeless offspring develop yellow fat
- This creates a strong causal link between:
- Loss of eyes
- Accumulation of carotenoids
Such a result would contradict Riddle’s statement that there is “no link between eye presence or size and the accumulation of carotenoids”.
Instead, it would support the earlier hypothesis that carotenoids build up because cavefish lack eyes, not because of an unrelated adaptive function.
Why the Other Options Do Not Disprove the Inference
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Option A:
- Shows white fat appearing regardless of eye presence
- Does not establish any link between eye loss and carotenoids
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Option B:
- Shows that fish with eyes can also have yellow fat
- This supports Riddle’s claim that carotenoid accumulation is independent of eye presence
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Option D:
- Again shows fat type appearing irrespective of eye condition
- Does not challenge the idea that carotenoids serve another purpose
Since these options do not establish a necessary connection between eye loss and carotenoid buildup, they fail to disprove Riddle’s inference.
Final Answer
Correct Answer: Option C
Question 4
On the basis of the information in the passage, what is the most likely function of carotenoids in Mexican tetra cavefish?
Solution:
Main Idea of the Passage
The passage discusses Riddle’s conclusion about the role of carotenoids in Mexican tetra cavefish. It explains that carotenoids are not linked to eye loss, but instead serve a different biological function.
The passage explicitly suggests that carotenoids act as an adaptation to suppress inflammation, especially when fat cells become large and could otherwise cause damage.
Explanation of the Correct Answer
Why Option D Is Correct
Option D correctly reflects Riddle’s conclusion because:
- The passage clearly states:
“Instead, Riddle thinks these carotenoids may be another adaptation to suppress inflammation.”
- This directly links carotenoids to controlling inflammation
- It fits the broader discussion about:
- Enlarged fat cells
- The potential for inflammatory damage
- How cavefish avoid this problem
Therefore, option D accurately captures the biological purpose of carotenoids as described in the passage.
Why the Other Options Are Incorrect
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Option A:
- Mentions a possible link between carotenoids and eye loss
- This is presented only as an initial hypothesis
- The passage explicitly rejects this idea after cross-breeding shows “no link between eye presence or size and the accumulation of carotenoids”
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Option B:
- Talks about nutrient storage
- While metabolic adaptation is discussed, carotenoids are not described as serving this function
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Option C:
- Focuses on the yellow colour of fat
- The passage treats this as a visible outcome, not the biological role of carotenoids