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How Natural Selection Works

8th grade nonfiction passage about evolution and life science: 263 words, eight questions, an answer key and vocabulary.

8th grade nonfiction evolution and life science 263 words

How Natural Selection Works

Living things of the same kind are not exactly alike. Look at any group of dogs, beetles, or tomato plants, and you will see differences in size, color, speed, and many other traits. Some of these differences are inherited, which means they are passed from parents to offspring.

In the 1800s, the naturalist Charles Darwin proposed that these differences help explain how species change over time. His idea is called natural selection. It rests on a few observations. Living things make more offspring than can survive. They compete for limited resources, such as food, water, and space. Individuals vary, and some of the variations are inherited.

Here is an imagined example. A population of beetles lives on a forest floor covered with dark soil. Some of the beetles are green, and others are brown. Birds hunt the beetles by sight. Against the dark soil, the brown beetles are harder to see, so more of them escape. The green beetles are eaten more often.

The brown beetles live longer on average, so they have more chances to reproduce, and their offspring inherit the brown color. Over many generations, brown becomes more common in the population. No beetle changed its color. The population changed because the environment favored some individuals over others.

Natural selection is not planning, and it does not make living things better in any absolute way. A trait is helpful only in a particular place and time. If the soil turned light, brown might become a disadvantage. Over very long periods, natural selection, along with other processes, can lead to new species.

Questions

  1. What does it mean for a trait to be inherited? in the text

  2. What observations does natural selection rest on? in the text

  3. In the beetle example, why do more brown beetles survive? in the text

  4. What happens to the brown color over many generations? in the text

  5. Did any beetle change its color, and how do you know? think about it

  6. Why does the author call the beetle story an "imagined" example? think about it

  7. What would probably happen if the soil turned light? think about it

  8. Why is it wrong to say natural selection makes living things better in an absolute way? think about it

Words to know

traits
features of a living thing, such as its size, color, or speed
inherited
passed down from parents to their young
offspring
the young that a living thing produces
population
all the living things of one kind in a place
generations
groups of living things born around the same time, one after another
natural selection
the process in which living things with helpful traits survive and reproduce more

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put traits, inherited, offspring, population, generations and natural selection on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 3 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 5 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "How Natural Selection Works"?

It was written for 8th grade. It has 263 words and works out to a Flesch-Kincaid grade level of about 7.3. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are eight. 4 ask for something the text says directly and 4 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 3 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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