Why do cave fish lose their eyes?

What ability have many cave fishes lost?

• the ability to swim

• the ability to smell

• the ability to see

• the ability to hear

To organize this text, the author divides it into sections with subheadings. What is described in the section with the subheading "Two Answers"?

• answers scientists have come up with about why some species of cave fishes are blind

• answers scientists have come up with about why some caves receive no light from the sun

• answers scientists have come up with about why some crustaceans have evolved to live without light

• answers scientists have come up with about why regressive evolution occurs in salamanders

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People's genes determine which traits they inherit. What information in the article supports this statement?

• "How did that happen? How can evolution cause a species to lose a trait? It’s a mystery that evolutionary scientists have been struggling to unravel. The search for an answer gives us a fascinating look at how evolution works."

• "We usually think of evolution as a process in which species acquire new traits. But in cave fishes we have an example of regressive evolution, a process in which species lose a trait—in this case, the ability to see."

• "...you have five fingers on each hand because of the genes you got from your parents. However, if you have an accident and lose a finger, your children will still be born with five fingers on each hand. If you lift weights and become a body builder, it doesn’t mean your children will be born with bulging biceps. In each case, your genes haven’t changed—even though your body has."

• "The scientists knew that there are many genes responsible for the development of each part of an eye (for example, the retina, iris, cornea and lens). Each part develops independently. The results of the experiment showed that the genes for eye development in the Mexican tetra were all ready to work properly, given the correct signal."

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Read these sentences from the text.

“The researchers did indeed find one of those genes. It is nicknamed Hedgehog or the Hh gene. They discovered that the Hedgehog gene does more than cause blindness in cave tetras—when the fish develops without eyes, the skull bones move into the empty eye socket, which at the same time enlarges its nose. Unlike other vertebrates, fishes use their nose only for smelling. It could be that the same control gene (Hh) that stops eye development in the fish also enhances its sense of smell. An enhanced sense of smell would be a definite advantage for a fish that lives in darkness.”

Based on this information, what can you conclude about the effect that the size of a cave tetra's nose has on the cave tetra's sense of smell?

• The smaller a cave tetra's nose is, the better the cave tetra's sense of smell will be

• The bigger a cave tetra's nose is, the better the cave tetra's sense of smell will be.

• The size of a cave tetra's nose has no effect on the cave tetra's sense of smell.

• Any change in the size of a cave tetra's nose will make it more difficult for the cave tetra to recognize smells.

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What is the main idea of this text?

• More than one hundred species of cave fishes live in constant darkness

• If an animal is born with a trait that gives it an advantage over other individuals, it will be more successful at having offspring, and its offspring will inherit the advantageous trait.

• The neutral mutation hypothesis is based on the fact that natural selection does not just reward success but also weeds out failures.

• Many cave fishes are blind, and an experiment carried out by scientists suggests that blindness in these fishes is the result of a mutation that also improves their sense of smell

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The title of this text is "Why Do Cave Fish Lose Their Eyes?" Why might the author have written the title as a question?

• to prepare readers for a discussion of possible answers to this question in the article

• to encourage readers to answer the question on their own before they read the article

• to criticize scientists for not having reached a definite answer about why cave fishes lose their eyes

• to praise scientists for the effort they have put into understanding the cause of blindness in cave fishes

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Read these sentences from the text.

“The fact that cave fishes don’t use their eyes has absolutely no effect on the DNA in their chromosomes. They are blind because something happened to the genes that control the development of their eyes. This change is passed on from parent tooffspring. That explains why a blind fish would have blind offspring. But it doesn’t explain how a whole species of blind fish came to exist.”

How could you rewrite the last sentence without changing its meaning?

• In particular, it doesn’t explain how a whole species of blind fish came to exist.

• Therefore, it doesn’t explain how a whole species of blind fish came to exist.

• For example, it doesn’t explain how a whole species of blind fish came to exist.

• However, it doesn’t explain how a whole species of blind fish came to exist.

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