Evolution

In biology, evolution is descent with inherited modification. At a small scale, it can be measured as a change in the frequency of inherited genetic variants in a population. Across longer spans, populations diverge and lineages accumulate differences, contributing to the diversity of species. Evolution concerns changes passed between generations; an individual organism's growth or a temporary change during its lifetime is not population evolution. The evidence supports deep common ancestry among living things.

Evolution has more than one mechanism. Mutations introduce changes in DNA; their effects may be harmful, neutral or beneficial, and they do not arise because an organism needs a particular trait. Natural selection occurs when heritable variants differ in reproductive success in a given environment, making some variants more common over generations. Genetic drift changes variant frequencies by chance rather than because a variant improves adaptation; chance effects are especially important in small populations. These processes can operate together, and their outcomes depend on a population's history and environment.

Several independent lines of evidence fit an evolutionary history. Fossils document organisms from different periods and show sequences of change and extinction. Similarities in anatomy and development, the geographic distribution of organisms, and patterns in DNA provide additional evidence about relationships among lineages. Scientists compare these records: agreement among them helps reconstruct common ancestry, while new discoveries can refine particular branches and dates. Evolutionary change is also observed in populations, providing evidence about how its mechanisms work in real time.

In science, a theory is a broad explanation supported by extensive evidence, not a guess. Evolutionary theory connects observations about inherited change with mechanisms such as selection and genetic drift, and it provides a framework for testing questions about life's history. Like other scientific explanations, it can be refined when new evidence warrants it; continued research develops its details without making evolution a merely speculative idea.

Evolution provides a framework for explaining biological diversity and adaptation. It also has practical value: tracking genetic change in disease-causing organisms helps researchers understand how pathogens change and informs work on diagnosis, treatment and prevention. Evolutionary biology is therefore used in research across fields including medicine, agriculture and conservation.