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Evolution | Definition, History, Types, & Examples | Britannica

evolution

Table of Contents

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 origen 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 origen 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

major evolutionary events

Why don't humans have tails?

How Charles Darwin developed the theory of evolution

evolution of the horse

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

homologies of vertebrate forelimbs

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?

For Students

Charles Darwin

evolution summary

evolution

scientific theory

Also known as: descent

Written by Francisco Jose Ayala

Fact-checked by The Editors of Encyclopaedia Britannica

Last Updated: Jul 31, 2025 • Article History

Key People:: Hugo de Vries; Robert Broom; Charles Darwin; Thomas Henry Huxley; Charles Lyell

Related Topics:: punctuated equilibrium model; convergence; catastrophe theory; divergence; molecular clock

On the Web:: Khan Academy - Evolution and classification link (July 31, 2025)

See all related content

News •

400-million-year-old fish exposes big mistake in how we understood evolution • July 29, 2025, 5:23 AM ET (ScienceDaily)

Tired of put-downs, Tennessee town corrects the record with play about the Scopes trial it hosted • July 18, 2025, 9:22 AM ET (AP)

evolution, theory in biology postulating that the various types of plants, animals, and other living things on Earth have their origen 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 to be discovered—from 10 million to 30 million, according to some estimates. What is impressive is not just the numbers but also the incredible heterogeneity ...(100 of 39215 words)

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