The **theory of evolution** is a scientific explanation for how living organisms have changed over time through a process of gradual change and adaptation.

**Key points of the theory:**

1. **Common Ancestry:** All living things share a common ancestor. Over time, species have diverged from these common ancestors through a process called "descent with modification."

2. **Natural Selection:** Charles Darwin proposed that individuals in a species show variation in their traits (such as size, color, etc.). Some traits help individuals survive and reproduce better in their environment. Those individuals are more likely to pass on their traits to the next generation. Over many generations, this process causes species to adapt to their environments.

3. **Genetic Variation:** Differences among individuals are largely due to variations in their genes, which are inherited from their parents.

4. **Mutation and Recombination:** New genetic variations arise through random mutations and the recombination of genes during reproduction.

5. **Speciation:** As populations of a species become separated—by geography or other means—they may experience different environmental pressures and eventually evolve into distinct species.

**In summary:**  
The theory of evolution states that all life on Earth has changed and diversified over time through genetic variations and natural selection, leading to the wide variety of organisms that exist today.
