Have you ever wondered why there are so many different kinds of living things on earth? In North America alone there are over 300 different kinds of amphibians, 900 different kinds of birds, and over 3,000 different kinds of spiders. Where does this biodiversity come from? When scientists look at fossils that lived millions of years ago, they see that many living things looked much different than they do today.

How does this change happen?

Photo by Whylogwhy (flickr).

Natural Selection

Natural selection is the process through which populations of organisms change over time in response to environmental pressures. This process can help living things survive and thrive in their natural habitat. Environmental pressures can include things like predators, disease, and changes in the abundance of food.

Photo by Anthony Masi (flickr).

Natural selection isn't about changes that happen to one individual in the course of a single lifetime. It's about changes that happen over many generations and that get passed down from parents to children.

Here are a few key terms that we'll think about more detail later:

  • variation in a population of living things
  • inheritance of traits from one generation to the next
  • selection pressures that act on a population and make it more likely for some individuals to survive and reproduce than others.

Programming Frogs

To start exploring natural selection we're going to use a simulated ecosystem. This is a virtual frog pond filled with some dragonflies and a few small blue frogs. The frogs aren't based on any real species—in fact they're more like mindless robot frogs that will do whatever you tell them. But they can still help us think about natural selection.

To get started, use the blocks to create a program, and then press the play button. You can also press the plus (+) and minus (-) keys to zoom in or out. Here are a few things to notice as you try different programs:

  1. Frogs come in different sizes. Some are big, some are little, and some are medium-sized. This is an example of variation in a population.
  2. Big frogs hop farther than little frogs. This means that they're more likely to fall into the water.
  3. Big frogs also have longer tongues than little frogs. This means that they're better at hunting flies.
  4. Little frogs use less energy than big frogs. This isn't immediately obvious, but it's important because it means that little frogs don't need to eat as much to survive.


OK. Ready for some evolution challenges?