The theory of evolution explains how life on Earth has changed over time. Here are its core principles:

**Natural Selection** (the main mechanism)
- Individuals within a population vary in their traits
- Some traits improve survival and reproduction in a given environment
- Individuals with advantageous traits are more likely to pass them to offspring
- Over generations, beneficial traits become more common in the population

**Key Components**

- **Common descent**: All living organisms share ancestors if you trace back far enough
- **Gradual change**: Species change incrementally over long time periods
- **Speciation**: Populations can diverge into separate species when isolated or adapting to different conditions

**Supporting Evidence**
- Fossil record showing transitional forms
- DNA similarities across species
- Comparative anatomy (homologous structures)
- Direct observation in fast-reproducing organisms (bacteria, insects)
- Biogeography (distribution of species across regions)

**What It Doesn't Say**
- It doesn't address the origin of life itself (that's abiogenesis)
- "Survival of the fittest" doesn't mean strongest—it means best suited to an environment
- Evolution isn't goal-directed or progressing toward "higher" forms

The theory originated primarily with Charles Darwin's 1859 work *On the Origin of Species* and has been refined since with genetics and molecular biology (the "modern synthesis").

Would you like me to expand on any particular aspect?
