The theory of evolution explains how living things change over generations and how the diversity of life on Earth arose.

At its core, it says:

1. **Living populations change over time**  
   Individuals in a species vary in traits, such as size, color, resistance to disease, or behavior. Some of these differences are inherited.

2. **Traits can be passed to offspring**  
   Genes carry hereditary information. When organisms reproduce, their offspring inherit genetic material, with some variation.

3. **Variation comes from mutation and recombination**  
   New genetic differences arise through mutations and through the reshuffling of genes during reproduction.

4. **Natural selection can favor useful traits**  
   If a trait helps an organism survive or reproduce in a particular environment, individuals with that trait tend to leave more offspring. Over many generations, the trait can become more common.

5. **Populations, not individuals, evolve**  
   An individual organism does not evolve during its lifetime. Evolution refers to changes in the genetic makeup of a population across generations.

6. **Species can split and diversify**  
   Over long periods, populations can become so different that they form new species. This process helps explain the huge variety of life.

7. **All life shares common ancestry**  
   Evolutionary theory says that organisms alive today are related through common ancestors. For example, humans and chimpanzees share a relatively recent common ancestor, while all living things share much more ancient ancestors.

Evolution is supported by many lines of evidence, including fossils, genetics, anatomy, embryology, and direct observations of evolution in organisms such as bacteria and insects.

In science, a “theory” is not a guess; it is a well-supported explanatory framework. The theory of evolution is the central explanation in biology for how life changes and diversifies over time.
