Populations change over generations. That is evolution. The change is driven by several mechanisms, but natural selection is the most famous. Individuals with traits that help them survive and reproduce pass those traits on more often. Over time, the traits become more common. Charles Darwin and Alfred Russel Wallace described the mechanism in 1858, and Darwin published On the Origin of Species the following year. The evidence since then has been overwhelming. Fossils, DNA, and observed changes in living populations all point to the same conclusion.

Evolution is not progress. It is adaptation to local conditions. A trait that helps in one environment may hurt in another. Evolution also does not happen to individuals. It happens to populations, over generations. Mutations create variation. Selection filters it. Genetic drift, the random change in gene frequencies, can also shape populations, especially small ones. Gene flow, the movement of individuals between populations, mixes traits. Together these forces produce the diversity of life.

Mechanisms of evolution

• Natural selection: differential survival and reproduction.
• Mutation: source of new genetic variation.
• Genetic drift: random changes in small populations.
• Gene flow: movement of genes between populations.

Evolution is not a theory in doubt. It is the organizing principle of biology, supported by evidence from every field that studies life.