Evolution is the change in the inherited characteristics of biological populations over successive generations. It is the central organizing principle of biology — nothing in the field makes sense without it, as Theodosius Dobzhansky put it in 1973.
The basic mechanism is straightforward. Organisms vary. Some of that variation is heritable — encoded in DNA and passed from parent to offspring through reproduction. When heritable variation affects survival or reproduction, natural selection occurs: organisms with traits better suited to their environment leave more offspring, and those traits become more common in subsequent generations. Over long periods, this process produces adaptation — the fit between organisms and their environments — and, when populations diverge, new species.
Charles Darwin and Alfred Russel Wallace independently formulated the theory of evolution by natural selection in the 1850s. Darwin’s On the Origin of Species (1859) provided the detailed argument and extensive evidence. The theory did not initially include a mechanism of heredity — Darwin did not know about genes or DNA. The modern synthesis of the 1930s-40s united Darwinian selection with Mendelian genetics, establishing that evolution is the change in allele frequencies within populations over time.
Natural selection is not the only mechanism of evolution. Genetic drift — random changes in allele frequency due to chance — is significant in small populations. Gene flow — the movement of genes between populations through migration — can introduce new variation or homogenize populations. Mutation — errors in DNA replication — is the ultimate source of all new genetic variation, though most mutations are neutral or harmful. Sexual selection — differential reproduction based on mate choice or competition for mates — can drive the evolution of traits (peacock tails, elk antlers) that have no survival advantage and may even be costly.
Evolution operates at multiple scales. Microevolution is change within populations over generations — the shift in allele frequencies that population genetics tracks. Macroevolution is the larger pattern: the origin of new body plans, mass extinctions, adaptive radiations, the branching tree of life. Whether macroevolution is fully explained by the accumulation of microevolutionary changes or requires additional principles (developmental constraints, punctuated equilibrium, niche construction) is an active area of research.
The evidence for evolution is extensive and converges from independent sources: the fossil record, comparative anatomy, embryology, biogeography, molecular biology, and direct observation of evolution in laboratory and field populations. Bacterial populations evolve antibiotic resistance in days. Darwin’s finches’ beak sizes shift measurably across drought years. Speciation has been observed in both laboratory and natural settings.
Related concepts
- Natural Selection — the primary mechanism of adaptive evolution
- Morphogenesis — the developmental processes that evolution shapes and is constrained by
- Niche Construction — how organisms modify the selective environments that drive evolution
- Symbiosis — evolutionary relationships between species, including endosymbiosis
Related terms
- Gene — the unit of heredity on which evolution acts
- DNA — the molecular substrate of heritable variation
- Species — the taxonomic unit produced by evolutionary divergence
- Reproduction — the process through which hereditary information is transmitted