The theory of evolution explains how species change over time and how life's diversity developed. Here are its core principles:

## Main Ideas

**Descent with modification**: All living organisms descended from common ancestors, with species gradually changing across generations.

**Natural selection**: The primary mechanism driving evolution:
- Organisms produce more offspring than can survive
- Individuals vary in their traits
- Those with advantageous traits are more likely to survive and reproduce
- These beneficial traits become more common in populations over time

## Key Points

- **Gradual change**: Most evolutionary changes occur slowly over many generations, though the pace can vary

- **Adaptation**: Organisms develop traits suited to their environments through natural selection

- **Common ancestry**: Life shares a family tree—similar species share more recent common ancestors

- **Evidence sources**: Fossils, DNA comparisons, anatomy, embryology, and observed changes in populations all support the theory

## What It Doesn't Say

Evolution doesn't claim organisms evolve individually during their lifetimes, or that it's purely random (natural selection is non-random), or that it explains how life originally began (that's abiogenesis).

The theory is the unifying framework of modern biology, helping explain everything from antibiotic resistance to why different species share similar DNA.
