Natural Science Passages · Lesson 3

Hypotheses and evidence in science writing

Many science passages tell the story of a question: several possible explanations, called hypotheses, and the observations that favored one or ruled another out. Questions then ask what each hypothesis claimed, which finding supported or weakened it, and what the passage says is still unsettled.

Keep a small ledger as you read. For each hypothesis, note its claim in a few words and mark the evidence beside it: camouflage, against: lions can’t see stripes from far off. Cooling, against: no consistent effect. Flies, for: fewer landings on striped coats. A finding counts as support only if it fits that hypothesis better than the others. A true fact about the topic that doesn’t favor any hypothesis is just background.

Watch the passage’s level of confidence. Words such as suggest, may, one proposed explanation, has not been confirmed and the best-supported answer rather than the only one mean the author is not claiming certainty. A correct answer keeps the same level; a choice that turns best supported into proved is wrong.

Why Does a Zebra Have Stripes?

This passage is from an article about the explanations scientists have proposed for zebra stripes.

1Few animal patterns are as easy to recognize as a zebra’s stripes, and few have produced as many competing explanations. Over the years, scientists have proposed that the stripes hide zebras from predators, help them keep cool, or protect them from biting flies. Each idea is a hypothesis, a possible explanation that can be tested against observations.

2The oldest idea is camouflage. Perhaps, the reasoning went, the stripes break up a zebra’s outline in tall grass, or make it hard for a lion to single out one animal in a running herd. The trouble is that lions hunt mostly at dusk and at night, and studies of how well lions see suggest that at the distances from which they usually spot prey, the stripes would blur into an even gray. If the stripes are hard to see from far away, they are unlikely to be fooling the predator they were supposed to fool.

3A second idea holds that the stripes help a zebra stay cool. Black hair absorbs more sunlight than white hair, so the black stripes heat up more, and some researchers suggested that the difference might stir small breezes over the skin. Measurements on striped and unstriped animals standing in the sun, however, have not shown a consistent cooling effect, and the hypothesis remains unsupported.

4The third idea has gathered the most support. Biting flies, such as horseflies, spread diseases among grazing animals and are common where zebras live. In one kind of experiment, researchers dressed horses in coats printed with black and white stripes and filmed the flies around them. Flies approached the striped coats about as often as plain ones, but far fewer of them landed. On video, many seemed to fail to slow down properly and bumped into the coat or flew past it. Zebras also tend to live in the regions where biting flies are most active, a pattern the other hypotheses do not explain as well.

5Even so, the question is not closed. The experiments show that flies avoid landing on stripes, but they do not yet show why. One suggestion is that the narrow bands confuse the way a fly judges its speed as it closes in, but that explanation has not been confirmed. And stripes could serve more than one purpose. Most researchers now treat fly protection as the best-supported answer rather than the only one.

Worked example Easy

Show the whole passage

Why Does a Zebra Have Stripes?

This passage is from an article about the explanations scientists have proposed for zebra stripes.

1Few animal patterns are as easy to recognize as a zebra’s stripes, and few have produced as many competing explanations. Over the years, scientists have proposed that the stripes hide zebras from predators, help them keep cool, or protect them from biting flies. Each idea is a hypothesis, a possible explanation that can be tested against observations.

2The oldest idea is camouflage. Perhaps, the reasoning went, the stripes break up a zebra’s outline in tall grass, or make it hard for a lion to single out one animal in a running herd. The trouble is that lions hunt mostly at dusk and at night, and studies of how well lions see suggest that at the distances from which they usually spot prey, the stripes would blur into an even gray. If the stripes are hard to see from far away, they are unlikely to be fooling the predator they were supposed to fool.

3A second idea holds that the stripes help a zebra stay cool. Black hair absorbs more sunlight than white hair, so the black stripes heat up more, and some researchers suggested that the difference might stir small breezes over the skin. Measurements on striped and unstriped animals standing in the sun, however, have not shown a consistent cooling effect, and the hypothesis remains unsupported.

4The third idea has gathered the most support. Biting flies, such as horseflies, spread diseases among grazing animals and are common where zebras live. In one kind of experiment, researchers dressed horses in coats printed with black and white stripes and filmed the flies around them. Flies approached the striped coats about as often as plain ones, but far fewer of them landed. On video, many seemed to fail to slow down properly and bumped into the coat or flew past it. Zebras also tend to live in the regions where biting flies are most active, a pattern the other hypotheses do not explain as well.

5Even so, the question is not closed. The experiments show that flies avoid landing on stripes, but they do not yet show why. One suggestion is that the narrow bands confuse the way a fly judges its speed as it closes in, but that explanation has not been confirmed. And stripes could serve more than one purpose. Most researchers now treat fly protection as the best-supported answer rather than the only one.

According to the passage, which observation counts against the hypothesis that stripes hide zebras from lions?

  1. From the distances at which lions spot prey, stripes would likely blur into gray.Answer
  2. Black hair absorbs more sunlight than white hair does, so the black stripes heat up more.
  3. Flies approached striped and plain coats about equally often in the experiments.
  4. Zebras tend to live in the regions where biting flies are most active.

How to solve it

  1. Find the camouflage hypothesis: paragraph 2, “The oldest idea is camouflage.”
  2. Look for the turn: “The trouble is that...” introduces the evidence against it.
  3. Match the choice to that evidence and reject choices that belong to the other hypotheses’ paragraphs.

Why each choice is right or wrong

  • A. Correct. Paragraph 2 says studies “suggest that at the distances from which they usually spot prey, the stripes would blur into an even gray,” so the stripes are “unlikely to be fooling the predator they were supposed to fool.”
  • B. Incorrect. Sunlight and hair color belong to the cooling hypothesis in paragraph 3, not to camouflage.
  • C. Incorrect. The fly experiments in paragraph 4 test the fly hypothesis; they say nothing about lions.
  • D. Incorrect. Where zebras live is evidence for the fly hypothesis, “a pattern the other hypotheses do not explain as well,” not evidence about lions’ eyesight.

Worked example Medium

Show the whole passage

Why Does a Zebra Have Stripes?

This passage is from an article about the explanations scientists have proposed for zebra stripes.

1Few animal patterns are as easy to recognize as a zebra’s stripes, and few have produced as many competing explanations. Over the years, scientists have proposed that the stripes hide zebras from predators, help them keep cool, or protect them from biting flies. Each idea is a hypothesis, a possible explanation that can be tested against observations.

2The oldest idea is camouflage. Perhaps, the reasoning went, the stripes break up a zebra’s outline in tall grass, or make it hard for a lion to single out one animal in a running herd. The trouble is that lions hunt mostly at dusk and at night, and studies of how well lions see suggest that at the distances from which they usually spot prey, the stripes would blur into an even gray. If the stripes are hard to see from far away, they are unlikely to be fooling the predator they were supposed to fool.

3A second idea holds that the stripes help a zebra stay cool. Black hair absorbs more sunlight than white hair, so the black stripes heat up more, and some researchers suggested that the difference might stir small breezes over the skin. Measurements on striped and unstriped animals standing in the sun, however, have not shown a consistent cooling effect, and the hypothesis remains unsupported.

4The third idea has gathered the most support. Biting flies, such as horseflies, spread diseases among grazing animals and are common where zebras live. In one kind of experiment, researchers dressed horses in coats printed with black and white stripes and filmed the flies around them. Flies approached the striped coats about as often as plain ones, but far fewer of them landed. On video, many seemed to fail to slow down properly and bumped into the coat or flew past it. Zebras also tend to live in the regions where biting flies are most active, a pattern the other hypotheses do not explain as well.

5Even so, the question is not closed. The experiments show that flies avoid landing on stripes, but they do not yet show why. One suggestion is that the narrow bands confuse the way a fly judges its speed as it closes in, but that explanation has not been confirmed. And stripes could serve more than one purpose. Most researchers now treat fly protection as the best-supported answer rather than the only one.

Which result of the experiments described in paragraph 4 most directly supports the hypothesis that stripes protect zebras from biting flies?

  1. Flies approached the striped coats about as often as they approached plain coats.
  2. Horseflies spread diseases among the grazing animals that live near zebras.
  3. Far fewer flies landed on the striped coats than on the plain coats.Answer
  4. Many flies seemed to fly toward the coats without any trouble.

How to solve it

  1. State the hypothesis: stripes protect zebras from biting flies.
  2. Ask what result would favor it: fewer flies reaching the skin, that is, fewer landings.
  3. Separate support from background. Disease and equal approaches are in the paragraph but don’t show protection.

Why each choice is right or wrong

  • A. Incorrect. Equal approaches show that stripes did not keep flies away from a distance, so this result alone gives no support.
  • B. Incorrect. Disease explains why fly protection would matter, but it is background, not a result of the experiments.
  • C. Correct. The experiments found that “far fewer of them landed,” which is exactly the protection the hypothesis predicts.
  • D. Incorrect. This misreads the video: “many seemed to fail to slow down properly and bumped into the coat or flew past it.”

Trap Treating a maybe as a fact

When a passage says an explanation is proposed, suggested or not yet confirmed, the answer must keep that hedge. A choice that says the explanation is proved, or that one hypothesis has been ruled out completely, goes further than the passage. The reverse also traps students: best supported is stronger than one idea among many, so don’t water it down either.