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.

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? Moderate

1. Some offspring with eyes had yellow fat.

2. Only eyeless offspring had yellow fat.

3. Some offspring with eyes had white fat.

4. Some eyeless offspring had white fat.

Answer

Correct Option: 2

Rationale: Riddle’s inference from the experiment is that there is no link between the presence of eyes and the accumulation of yellow fat (carotenoids). She concludes that the fat color is likely an adaptation for inflammation control, not merely a buildup of unused material. If the experiment had shown that only eyeless offspring had yellow fat, it would establish a direct, exclusive link between eyelessness and carotenoid accumulation. This result would support the initial hypothesis she rejected (that they accumulate it simply because they lack eyes to use it) and would directly contradict her finding that eye presence and fat color are unrelated.

Why other options wrong: Option 1 supports Riddle’s conclusion. It proves that having eyes doesn’t prevent yellow fat accumulation, confirming there is no exclusive link. Option 3 is a neutral or supportive finding, consistent with the idea that the traits are independent. Option 4 is the actual result mentioned in the passage (“Some eyeless cavefish had fat that was practically white”). This finding was the key evidence leading to her conclusion that being eyeless doesn’t guarantee yellow fat.

Difficulty: Moderate

Which one of the following best explains why the “apparent inefficiency” is “unavoidable”? Moderate

1. The inefficiency resulting from eyelessness is compensated by enhancements like more tastebuds in Mexican tetra cavefish.

2. The lack of light in the caves kills the eye cells in the developing Mexican tetra cavefish embryo.

3. Mexican tetra cavefish are similar to non-cave-dwelling variants in their early stages of development.

4. The caves have poor and inconsistent availability of food and nutrition for Mexican tetra cavefish.

Answer

Correct Option: 3

Rationale: The “apparent inefficiency” refers to the process where cavefish embryos start developing eyes only to destroy them later. The passage explains this is unavoidable because the early development of the brain and the eye are completely intertwined. This means the cavefish must follow the same early developmental path as their sighted ancestors to ensure their brains develop correctly. Option 3 captures this by stating the cavefish are similar to the non-cave variants in these early stages; this shared developmental constraint is the fundamental reason the inefficiency cannot be avoided.

Why other options wrong: Option 1 discusses enhancements (tastebuds) that compensate for the loss of sight, but does not explain why the developmental process of making and killing the eye is necessary. Option 2 attributes the loss to lack of light, but the passage attributes the cell death to genetic programming (“natural selection to have its way,” “cells… start dying”), not direct environmental exposure. Option 4 discusses food availability, which explains why they need to be efficient (save energy), but not why the specific inefficient mechanism of “build-then-destroy” is unavoidable.

Difficulty: Moderate

All of the following statements from the passage describe adaptation in Mexican tetra cavefish EXCEPT: EASY

1. “It’s easy to see why cavefish would be at a disadvantage if they were to maintain expensive tissues they aren’t using.”

2. “‘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.’”

3. “Even in their sparse cave conditions, wild cavefish can sometimes get very fat, says Riddle.”

4. “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.”

Answer

Correct Option: 2

Rationale: Option 2 is the only statement that does not describe a trait or behavior of the Mexican tetra cavefish. Instead, it is a general explanation by physiologist Nicolas Rohner regarding the biological mechanism of inflammation in humans and other animals when fat cells burst. It describes the problem (toxicity of fat) that the cavefish have successfully overcome, rather than the adaptation itself. The passage later contrasts this by stating that cavefish, unlike the subjects in this statement, show fewer signs of inflammation.

Why other options wrong: Option 1 describes the evolutionary pressure (avoiding “expensive tissues”) that drives the adaptation of losing eyes. Option 3 describes a behavioral and physiological adaptation: the ability to “get very fat” to store energy. Option 4 describes the fish “surviving” on a unique diet, implying the adaptation effectively allows them to live in a niche where “relatively little lives.”

Difficulty: Easy

On the basis of the information in the passage, what is the most likely function of carotenoids in Mexican tetra cavefish? EASY

1. To act as a substitute for eyes.

2. To render bright yellow colour to the cavefish.

3. To control inflammation from the bursting of fat cells.

4. To help the fat cells store nutrients.

Answer

Correct Option: 3

Rationale: The passage explicitly states Riddle’s conclusion regarding the function of the yellow fat: “Riddle thinks these carotenoids may be another adaptation to suppress inflammation, which might be important in the wild, as cavefish are likely overeating.”

Why other options wrong: Option 1 was the initial hypothesis (“accumulating these because they don’t have eyes”) which was tested and rejected by Riddle. Option 2 states the yellow color is a visual side effect of the carotenoids, not their biological function. Option 4 suggests carotenoids help the fat cells store nutrients, but the passage implies they protect the tissue from inflammation caused by the storage.

Difficulty: Easy