Natural Selection

What’s in This Unit?

In 1979, friends dared a 29-year-old man in Oregon to swallow a living, rough-skinned newt. What the man did not know is that rough-skinned newts can be extremely poisonous. A lethal, fast-acting poison called tetrodotoxin (TTX) oozes from their skin. The man swallowed the newt whole and started feeling weak a few minutes later. He described a numb feeling all over his body. His friends tried to take him to a hospital, but he refused. Just 20 minutes later, the man was dead. In the role of student biologists, students investigate what caused this newt population to become more poisonous—which serves as the anchor phenomenon for the unit. Using the Natural Selection Simulation, students investigate how the population of newts changed over time. Over the course of the unit, they gather evidence from the Simulation, hands-on activities, and texts to construct their own explanations of how the newts came to be so poisonous.

Why?

This unit helps students connect ideas about how the environment determines which traits are adaptive and non-adaptive, and how this affects the likelihood of survival and reproduction, to form an understanding of natural selection. The rough-skinned newt phenomenon motivates students to figure out concepts, such as variation, differential rates of survival and reproduction, adaptive traits, and mutations. By relating these ideas to changes in populations, students are challenged to think more deeply about why the distribution of traits in a population can change over time. Students’ hands-on role as student biologists adds a sense of responsibility and curiosity to this unit and inspires active, student-led learning in the classroom.

How?

The Natural Selection unit begins with a focus on genetic variation in populations. A series of tests in the Natural Selection Simulation highlight the important roles trait variation and the environment play in determining whether traits will be adaptive. Students tend to have the alternate conception that a helpful trait will become more common in a population over time—even if that trait is not present in the population. Students’ Sim tests in Chapter 1 are designed to address this common alternate conception. In the Simulation, they observe that new traits do not appear based on what would be ideal for the environment. Instead, the variation present in the population at any given time is the raw material for change. Once students are given information about the genetic variation of a population and the environment, they make and test predictions about how populations will change over time. Through these investigations, students learn that adaptive traits are the key to survival in an environment. Students apply these ideas to support and refute claims about the cause of the change in the rough-skinned newt population.

In Chapter 2, students deepen their understanding of how populations change over time by focusing on survival and reproduction. They observe that individuals with adaptive traits are more likely to survive. Investigating this pattern further, students observe that because individuals with adaptive traits are more likely to survive longer they also have a better chance of reproducing and passing on their traits to their offspring. At this point, students are able to point to adaptive and non-adaptive traits, specific environmental conditions, survival, and reproduction to explain how populations change over time.

In Chapter 3, students add more complexity to their understanding of natural selection by learning about the role of mutations in introducing new traits into populations. By reading case studies of different populations as well as making observations in the Simulation, students learn that if a randomly-introduced mutation is adaptive in a particular environment, this new trait is more likely to become more common in a population over time.