Evolution is the process by which living things change over many generations. Scientists have discovered that all species, from tiny bacteria to giant whales, gradually develop new traits over thousands or millions of years. These changes happen because individual organisms with helpful traits survive more often and pass those traits to their offspring.

Evolution explains why animals and plants fit their environments so well. Arctic foxes have thick white fur because, over countless generations, foxes with thicker, whiter coats survived the cold better and were harder for predators to spot. Giraffes have long necks because, over time, giraffes that could reach higher leaves got more food and had more offspring. These aren't conscious choices: nature simply favors traits that help organisms survive and reproduce.

Charles Darwin developed the theory of evolution in the 1800s after studying animals around the world. He noticed that species on isolated islands had unique features well suited to their surroundings. His book On the Origin of Species explained how this happens through a process he called natural selection: the environment “selects” which traits help organisms survive.

Evolution happens too slowly for us to see in most cases, but scientists can observe it in creatures with short life spans, like bacteria developing resistance to medicines. They also find evidence in fossils, which show how species looked millions of years ago and how they've transformed since then.

The word can also mean any gradual development or change. You might talk about the evolution of video games from simple pixels to realistic graphics, or the evolution of your own thinking as you learn new things.