Environment and Traits

After eight weeks, Maria's tomato plants are tall, green, and covered in flowers. James's plants are short, pale yellow-green, and have no flowers at all. Both students used the same exact seeds — so why do the plants look so different?

• What do you think caused Maria's and James's plants to look so different?
• If the seeds were exactly the same, what else could have made the difference?
• What would you want to test to find out for sure?

Living things — like plants, animals, and people — have traits. A trait is any feature or characteristic you can observe. Plant height, leaf color, and flower size are all traits. The color of your hair, your height, and the shape of your ears are traits, too.

Many traits are inherited — that means they are passed down from parents to offspring. That's why a sunflower seed grows into a sunflower, not a daisy. But here's the important idea: the environment can also affect traits. Even though two organisms have the same inherited information, they might end up looking different because of the conditions around them.

What scientists do: Constructing Explanations from Evidence

The question: Does the amount of sunlight affect how tall a bean plant grows?

The investigation: A class of third graders planted 12 bean seeds from the same seed packet in identical pots with the same soil. They placed 4 pots in a sunny window, 4 pots under a desk lamp (medium light), and 4 pots in a dark closet (no light). Every group got the same amount of water every day. After 3 weeks, they measured the height of each plant.

Materials:

• 12 bean seeds (same packet)
• 12 identical pots with the same soil
• Water (same amount for each pot)
• Three locations: sunny window, desk lamp, dark closet
• Ruler for measuring height

What made this a fair test: Everything was kept the same except the amount of light. This way, scientists can be confident that any differences in plant growth were caused by the light, not something else.

The data from the investigation tells a clear story. All 12 bean plants came from the same seed packet, so they started with the same inherited information. They all got the same amount of water and the same type of soil. The only thing that was different was the amount of light.

The plants in the sunny window grew the tallest, had the darkest green leaves, and the strongest stems. The plants in the dark closet were tiny, yellow, and weak. This is strong evidence that the environment — in this case, sunlight — affected the plants' traits, even though the plants had the same inherited information.

When scientists see a pattern like this, they use the evidence to construct an explanation. Here's how a scientist would explain what happened:

Notice that the explanation uses evidence (the data from the investigation) and connects it to a cause (sunlight provides energy for growth). This is exactly how scientists build explanations — they don't just guess, they support their ideas with evidence.

So when we look back at our phenomenon — Maria's and James's tomato plants — we can now explain the difference using evidence. The plants had the same inherited traits, but Maria's plants got more sunlight, better soil, and more water. Those environmental differences caused her plants to grow taller, greener, and with more flowers.

In this lesson, we are exploring an important pattern that scientists see all over the natural world: cause and effect. When one thing (a cause) happens, it leads to a change (an effect). In our investigation, the cause was the difference in sunlight, and the effect was the difference in plant height and color.

This pattern of cause and effect shows up in many areas of science — not just with plants! Let's look at some examples across different parts of the natural world.

Do you see the pattern? In every example, a change in the environment (the cause) leads to a change in an organism's observable traits (the effect). Scientists look for cause-and-effect patterns like this to help them predict what will happen and explain why things are the way they are.

Understanding how the environment affects traits is not just interesting — it's actually very useful! People use this science knowledge every day to solve real-world problems.

• How would you provide enough light?
• How would you make sure the plants get the right amount of water?
• What kind of soil would you use?
• How would you keep the temperature just right?

Engineers use the same kind of evidence that scientists collect to design solutions like this. They test their designs, collect data, and improve them — just like we did in our investigation!

📖 KEY VOCABULARY

• Trait — A feature or characteristic of a living thing that you can observe, such as height, color, or shape.
• Inherited trait — A trait that is passed down from parents to their offspring. For example, a sunflower seed will always grow into a sunflower because that information is inherited.
• Environment — Everything around a living thing that can affect how it grows and develops, including sunlight, water, temperature, soil, and food.
• Evidence — Information collected through observations or investigations that supports an explanation. Data from experiments is one kind of evidence.
• Fair test — An investigation where only one thing is changed (the variable being tested) and everything else is kept the same, so you can be sure what caused the results.
• Cause and effect — A pattern where one event (the cause) makes another event (the effect) happen. For example, less sunlight (cause) leads to shorter plants (effect).
• Organism — Any living thing, including plants, animals, and people.