evolution

Introduction
General overview
- The evidence for evolution
  - The fossil record
  - Structural similarities
  - Embryonic development and vestiges
  - Biogeography
  - Molecular biology
- History of evolutionary theory
  - Early ideas
  - Charles Darwin
  - Modern conceptions
    - The Darwinian aftermath
    - The synthetic theory
    - Molecular biology and Earth sciences
- The cultural impact of evolutionary theory
  - Scientific acceptance and extension to other disciplines
  - Religious criticism and acceptance
  - Intelligent design and its critics
The science of evolution
- The process of evolution
  - Evolution as a genetic function
    - The concept of natural selection
    - Genetic variation in populations
      The gene pool
      Genetic variation and rate of evolution
      Measuring gene variability
    - The origin of genetic variation: mutations
      Gene mutations
      Chromosomal mutations
  - Dynamics of genetic change
    - Genetic equilibrium: the Hardy-Weinberg law
    - Processes of gene-frequency change
      Mutation
      Gene flow
      Genetic drift
  - The operation of natural selection in populations
    - Natural selection as a process of genetic change
      Selection against one of the homozygotes
      Overdominance
      Frequency-dependent selection
    - Types of selection
      Stabilizing selection
      Directional selection
      Diversifying selection
      Sexual selection
      Kin selection and reciprocal altruism
- Species and speciation
  - The concept of species
  - The origin of species
    - Reproductive isolation
      Ecological isolation
      Temporal isolation
      Ethological (behavioral) isolation
      Mechanical isolation
      Gametic isolation
      Hybrid inviability
      Hybrid sterility
      Hybrid breakdown
    - A model of speciation
    - Geographic speciation
    - Adaptive radiation
    - Quantum speciation
    - Polyploidy
  - Genetic differentiation during speciation
- Patterns and rates of species evolution
  - Evolution within a lineage and by lineage splitting
  - Convergent and parallel evolution
  - Gradual and punctuational evolution
  - Diversity and extinction
  - Evolution and development
- Reconstruction of evolutionary history
  - DNA and protein as informational macromolecules
  - Evolutionary trees
    - Distance methods
    - Maximum parsimony methods
    - Maximum likelihood methods
    - Evaluation of evolutionary trees
- Molecular evolution
  - Molecular phylogeny of genes
  - Multiplicity and rate heterogeneity
  - The molecular clock of evolution
  - The neutrality theory of molecular evolution

References & Edit History Quick Facts & Related Topics

Images & Videos

A vertical geologic time scale chart spanning from 650 million years ago to the present, with colored bands for each era and period. Illustrations and labels highlight key evolutionary events such as the appearance of skeletal elements, first fishes, land plants, amphibians, reptiles, dinosaurs, mammals, and humans.

Why don't humans have tails?

How Charles Darwin developed the theory of evolution

This diagram traces the evolution of the horse from the small, multi-toed Hyracotherium (dawn horse) of the Eocene epoch to the modern single-toed Equus, showing changes in size, limb structure, and forefoot bones, with illustrations of key ancestral species to scale.

Collage showing a dog plus a saber-toothed cat equaling a prehistoric, quadrupedal creature with thick limbs and dark, hairless skin.

Illustration showing side-by-side reconstructions of five hominins in the human lineage: Australopithecus afarensis, Homo habilis, Homo erectus, Homo neanderthalensis, and Homo sapiens, with each figure labeled and depicted in a walking pose to highlight evolutionary changes in posture and body structure. A scale bar at the top right indicates relative height.

Diagram comparing the forelimb bone structures of a human, horse, dolphin, box turtle, fruit bat, and chicken, with color-coded bones showing homologous parts to illustrate evolutionary relationships.

The video thumbnail image shows a brown-speckled bird perched on a branch, singing with an open beak.

Brian Greene asks Richard Dawkins: Does God exist?

At a Glance

Portrait of Charles Darwin wearing a dark cloak and hat, with a long white beard, standing beside a wall with climbing vines.

evolution summary

A vertical geologic time scale chart spanning from 650 million years ago to the present, with colored bands for each era and period. Illustrations and labels highlight key evolutionary events such as the appearance of skeletal elements, first fishes, land plants, amphibians, reptiles, dinosaurs, mammals, and humans. — major evolutionary events The geologic time scale from 650 million years ago to the present, showing major evolutionary events.

evolution

scientific theory

Also known as: descent

Written by

Francisco Jose Ayala

Fact-checked by

Britannica Editors

Last updated

Nov. 13, 2026 •History

evolution, theory in biology postulating that the various types of plants, animals, and other living things on Earth have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations. The theory of evolution is one of the fundamental keystones of modern biological theory. The diversity of the living world is staggering. More than 2 million existing species of organisms have been named and described; many more remain...

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