In biology, evolution is the well-supported explanation for how living things change over generations and how all species are related by common ancestry. In short: populations’ heritable traits shift over time, small changes accumulate, and lineages branch, producing the diversity of life.

Core ideas
- Variation: Individuals in a population differ genetically. New variation arises mainly through mutations and genetic recombination.
- Heredity: Some of those differences are heritable, passed from parents to offspring via DNA.
- Differential success (natural selection): In a given environment, individuals with certain heritable traits tend to survive and reproduce more, so those traits become more common. This produces adaptations—features that improve fit to local conditions.
- Other evolutionary forces:
  - Genetic drift: Random changes in trait frequencies, especially in small populations.
  - Gene flow: Movement of genes between populations (migration), which mixes variation.
  - Sexual selection: Traits spread because they help individuals secure mates, even if they’re costly in other ways.
- Speciation: If populations become reproductively isolated (by geography, behavior, or genetics), they can diverge so much that they form new species.
- Common descent: All organisms share ancestors. Humans did not evolve from modern apes; rather, humans and other apes share a common ancestor that lived in the past.
- No foresight or goal: Evolution has no predetermined direction or purpose; it favors traits that work here and now in a particular environment.

What evolution does not claim
- It does not explain the ultimate origin of life’s first molecules (that’s abiogenesis); it explains how life diversifies once replication with heredity exists.
- It is not “progress” toward perfection; “fitness” means better reproductive success in a context, not being stronger or “better” in general.

Evidence
- Fossils and transitional forms showing change over time and branching lineages.
- Comparative anatomy and embryology (shared structures and developmental patterns).
- Biogeography (predictable distribution of species on islands and continents).
- Genetics and genomics (shared genes, nested patterns of similarity, observed mutations).
- Direct observation of evolution today (e.g., antibiotic resistance in bacteria, insecticide resistance, changes in beak size in finches, rapid adaptation in lizards and stickleback fish).

In scientific terms, “theory” means a comprehensive, testable framework that explains a wide range of observations. Evolution is both a fact (populations change and share ancestry) and a theory (the mechanisms that explain how and why).
