Use evidence to explain how certain traits increase survival or reproduction

Have you ever wondered why some animals blend into their surroundings? Or why certain plants grow faster than others? Scientists have been asking these questions for hundreds of years. The answers they found changed how we understand all living things.

Our anchoring phenomenon for this lesson is the peppered moth in England. Before the 1800s, most peppered moths were light-colored. During the Industrial Revolution, dark-colored moths became much more common. By the 1950s, over 90% of peppered moths near cities were dark. Why did this dramatic change happen?

These discoveries raised a big question: How can we use evidence to explain why certain traits help organisms survive and reproduce? In this lesson, you will learn to think like a scientist. You will look at real data and build explanations from evidence.

Before we look at evidence, let's define some key ideas. A trait is a feature of an organism. Traits can be physical (like fur color) or behavioral (like hunting at night). Some traits help organisms survive and have offspring. Others do not help, or may even be harmful.

Let's return to our anchoring phenomenon — the peppered moth. The diagram below shows what happened over about 150 years. Look at how the environment changed the moths' chances of survival.

Notice the Cause and Effect pattern in this example. The cause was pollution darkening the tree bark. The effect was that dark-colored moths had a selective advantage — they were harder for predators to spot. More dark moths survived, and they passed on their dark color to offspring.

Natural selection follows a pattern. It is not random luck. Scientists have identified the steps that must happen for a trait to become more common in a population. Let's walk through each step.

Here is the key idea: natural selection only works when there is variation. If every organism were identical, there would be nothing for the environment to "select." Variation comes from differences in genetic information (the DNA instructions inside cells). These genetic differences produce different traits.

Scientists don't just guess about which traits help survival. They gather evidence (observations and data that support or challenge a claim). Let's look at the main types of evidence used to study traits and survival.

The finch data above shows the Pattern crosscutting concept in action. You can see a clear pattern: the population shifted toward larger beaks. This pattern is your evidence. You use it to explain that larger beaks gave finches a survival advantage during the drought.

Now let's practice the key science skill: constructing an explanation from evidence. We will use the Galápagos finch data to write a complete scientific explanation.

Traits that help organisms survive and reproduce come in many forms. Some help with avoiding predators. Others help with finding food, attracting mates, or resisting disease. Let's compare the main categories.

What you have learned about traits and survival is a foundation for bigger ideas. In high school and beyond, you will explore these concepts in more depth. Here is a preview of how this lesson connects to advanced topics.

The Stability and Change crosscutting concept ties everything together. Populations can be stable for a long time. But when the environment changes, traits that were once unimportant can suddenly become critical for survival. Change in the environment leads to change in the population.

Test your understanding with these five questions. Each one asks you to think like a scientist — using evidence, identifying traits, and explaining cause and effect.