The theory of evolution (by natural selection) explains how populations of living things change over generations and how all life is related through common ancestry.

**Main ideas:**

1. **Common ancestry**
   - All species are connected in a “family tree” of life.
   - New species arise when populations split and diverge over long periods.

2. **Variation within populations**
   - Individuals in a population aren’t identical. They differ in traits (size, color, physiology, behavior, etc.).
   - Some of this variation is **heritable**, meaning it can be passed from parents to offspring through genes.

3. **Natural selection**
   - More offspring are produced than can survive, so there is competition for limited resources.
   - Individuals with heritable traits that help them survive and reproduce in a particular environment tend to leave more offspring.
   - Over generations, those helpful traits become more common in the population.

4. **Other mechanisms also cause evolution**
   - **Mutation:** new genetic changes arise.
   - **Genetic drift:** random changes in trait frequencies, especially in small populations.
   - **Gene flow:** movement of genes between populations (e.g., migration).
   - **Sexual selection:** traits spread because they increase mating success, even if they don’t help survival.

5. **Adaptation and speciation**
   - **Adaptations** are traits shaped by natural selection that improve survival or reproduction in a given environment.
   - **Speciation** happens when populations become reproductively isolated and diverge enough to form separate species.

**What evolution does *not* say:**
- It does not claim individuals evolve during their lifetime; **populations** evolve over generations.
- It does not say evolution has a goal or direction toward “higher” forms.
- It does not claim “humans came from modern monkeys”; rather, humans and other primates share **common ancestors**.

If you tell me your background (e.g., middle school, high school, college), I can explain it at that level or give a concrete example.
