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

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

• Darwin's understanding of evolution and his explanation of blindness in cave fishes

• Darwin's daily life in the 19th century and the places where he studied blindness in cave fishes

• the animals Darwin studied as a young man that led him to develop his theory of evolution

• the objections that have been raised to the theory of evolution developed by Darwin

An organism's genes determine which traits it inherits.

What evidence in the article supports this statement?

• "In some of these underground caves, there are fishes, crustaceans, salamanders and other organisms that have evolved to live without light. For example, more than one hundred species of cave fishes live their lives in perpetual darkness. They depend on senses other than sight to hunt, eat and reproduce."

• "We usually think of evolution in a positive sense, that is, 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."

• "...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."

• "Jean-Baptiste Lamarck... suggested that giraffes stretched their necks to reach higher leaves, and as a result their offspring were born with longer necks. The idea that cave fishes lost their eyesight because generations of fish didn’t use their eyes is a Lamarckian mistake."

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Based on the information about evolution in this text, what effect does the trait of blindness have on a fish living in a dark cave?

• The trait of blindness has a positive effect; it gives the fish an evolutionary advantage.

• The trait of blindness has a negative effect; it puts the fish at an evolutionary disadvantage.

• The trait of blindness has a neutral effect; it is neither an advantage nor a disadvantage.

• The trait of blindness has a mixed effect; it is an advantage for the fish at certain times and a disadvantage at other times.

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What is the main idea of this text?

• There are caves deep underground where the sun never shines, and in some of these underground caves, there are fishes, crustaceans, salamanders, and other organisms that have evolved over many years to live without light.

• Charles Darwin was a scientist living in the 19th century who was convinced that cave fishes lost their eyesight because they did not use their eyes.

• The neutral mutation hypothesis about the blindness of cave fishes is based on the idea that the mutations that cause blindness have no effect (or a neutral effect) on the survival of a fish living in a dark cave.

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

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

• o 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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Choose the answer that best completes the sentence.

Many species of cave fishes are blind or nearly blind. _______, they all evolved from fishes that could see.

• Consequently

• Primarily

• However

• For instance

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What is the first hypothesis that scientists have about blindness in the Mexican tetra.

• First hypothesis is fish are blind because the cave is dark.

• The first hypothesis is that blindness gives the tetra some sort of evolutionary advantage. That advantage might be the result of pleiotropy, which is when multiple effects are caused by the same mutation in one gene. Scientists think that the mutation that causes blindness in the tetra might also cause a "seemingly unrelated change in the fish that is beneficial."

• The first hypothesis is that fish are blind because they were not born with eyes and adjusted to darkness in the cave.

• Fish are blind because the cave don't have any light.

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One effect of the Hedgehog gene is to make cave tetras go blind. What is another effect it might have?

• Another effect it might have is to improve cave tetras' sense of taste

• Another effect it might have is to improve cave tetras' sense of smell.

• Another effect it might have is to improve cave tetras' sense of hearing

• Another effect it might have is to improve cave tetras' sense of feel.

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As a result of the experiment scientists did with Mexican tetras, it seems likely that their first hypothesis about blindness in the tetras is right. Explain how the result of the experiment supports their first hypothesis.

• Scientists' first hypothesis was that blindness gives the tetras some sort of evolutionary advantage. They thought the advantage might be the result of pleiotropy, which is when multiple effects are caused by the same mutation in one gene. The experiment led scientists to discover a gene that seems to have multiple effects. It causes blindness in cave tetras and may also improve their sense of smell. An improved sense of smell would be an evolutionary advantage for a fish that lives the dark. In this way, blindness seems to be linked to an evolutionary advantage in cave tetras. The link supports the first hypothesis scientists have about blindness in the tetras.

• The experiment led scientists to discover a gene that seems to have multiple effects.

• In this way, blindness seems to be linked to an evolutionary advantage in cave tetras.

• The link supports the first hypothesis scientists have about blindness in the tetras.

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