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

evolution: Media

scientific theory

Videos

Why don't humans have tails?

Why don't humans have tails?

Scientists have narrowed the answer down to a single genetic mutation.

Video: Encyclopædia Britannica, Inc.

How Charles Darwin developed the theory of evolution

How Charles Darwin developed the theory of evolution

Overview of Charles Darwin's life, with a focus on his work involving evolution.

Video: Contunico © ZDF Studios GmbH, Mainz; Thumbnail © Nicku/Dreamstime.com; © Ke77kz/Dreamstime.com; Courtesy, Biodiversity Heritage Library

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

What came before the dinosaurs?

Humans might be related to these weird, hairless dogs.

Video: Encyclopædia Britannica, Inc.

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

Why don't birds have teeth?

Birds descended from dinosaurs, but they don't have their ancestors' bite.

Video: Encyclopædia Britannica, Inc.

Brian Greene asks Richard Dawkins: Does God exist?

Brian Greene asks Richard Dawkins: Does God exist?

Brian Greene and Richard Dawkins discussing their notions of God in the context of...

Video: © World Science Festival (A Britannica Publishing Partner)

Watch Earth's Continents Move

Watch Earth's Continents Move

Watch a time-lapse of Earth changing through geologic time, from the Cambrian Era...

Video: Adapted from C.R. Scotese, The University of Texas at Arlington

A human hand and a gorilla's hand both reach for a gavel, against a blue-tinted background image of a courtroom.

What happened at the Scopes Monkey Trial?

This court case went bananas.

Video: Encyclopædia Britannica, Inc.

Evolution and diversity in Galapagos finches

Evolution and diversity in Galapagos finches

The 14 species of Galapagos finches differ from each other mainly in beak structure...

Video: Encyclopædia Britannica, Inc.

Where did human evolution begin?

Where did human evolution begin?

Learn about the Australopithecus genus.

Video: Contunico © ZDF Studios GmbH, Mainz

Know how the Galapagos finches from Charles Darwin's specimen collection helped him in the formulation of his theory of evolution, particularly the woodpecker finch

Know how the Galapagos finches from Charles Darwin's specimen collection helped him in the formulation of his theory of evolution, particularly the woodpecker finch

Learn about the Galapagos finches, particularly the woodpecker finch (Camarhynchus...

Video: © Open University (A Britannica Publishing Partner)

See how game theory applies to the peacock's tail evolution

See how game theory applies to the peacock's tail evolution

Learn how game theory applies to the evolution of the peacock's tail.

Video: © Open University (A Britannica Publishing Partner)

How evolution reveals the differences between humans and apes

How evolution reveals the differences between humans and apes

Learn about human evolution.

Video: Contunico © ZDF Studios GmbH, Mainz; Thumbnail © anne sarfina agustina dimyadi/Dreamstime.com

Hear psycholinguist Steven Pinker outlining the evolution of several unique traits in humans as compared with other species

Hear psycholinguist Steven Pinker outlining the evolution of several unique traits in humans as compared with other species

Psycholinguist Steven Pinker outlining uniquely human traits.

Video: © World Science Festival (A Britannica Publishing Partner)

Images

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...

Encyclopædia Britannica, Inc.

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.

evolution of the horse

Evolution of the horse over the past 55 million years. The present-day Przewalski's...

Encyclopædia Britannica, Inc.

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.

human lineage

Five hominins—members of the human lineage after it separated at least seven million...

Encyclopædia Britannica, Inc.

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.

homologies of vertebrate forelimbs

Homologies of the forelimb among vertebrates, giving evidence for evolution. The...

Encyclopædia Britannica, Inc.

Charles Darwin

undated portrait of English naturalist Charles Darwin

Universa Images Group/Getty Images

Darwin: On the Origin of Species

Darwin: On the Origin of Species

Title page of the 1859 edition of Charles Darwin's On the Origin of Species by...

Library of Congress, Washington, D.C. (neg. no. LC-USZ62-95224)

British naturalist Alfred Russel Wallace

British naturalist Alfred Russel Wallace

Alfred Russel Wallace, detail of a painting over a photograph; in the National Portrait...

Courtesy of the National Portrait Gallery, London

German biologist August Weismann

German biologist August Weismann

August Weismann, German biologist and one of the founders of the science of genetics.

From The American Museum Journal, Volume XV by The American Museum of Natural History, 1915.

Michelangelo: Creation of Adam

Michelangelo: Creation of Adam

Creation of Adam, detail of the ceiling fresco by Michelangelo, 1508–12;...

© Art Images/Getty Images

Scopes Trial

Scopes Trial

William Jennings Bryan (lower left, with fan) and Clarence Darrow (center right,...

Library of Congress, Washington, D.C.

Diagram showing how point mutations, or single nucleotide changes in a codon, result in different amino acids in a protein. The central isoleucine codon (AUA) is surrounded by other codons, each differing by one nucleotide, with corresponding amino acid molecular structures and color-coded atoms.

point mutation

The effect of base substitutions, or point mutations, on the messenger-RNA codon...

Encyclopædia Britannica, Inc.

Diagram illustrating the Hardy-Weinberg law for two alleles, showing a mother and father each contributing alleles A and a to offspring, with resulting genotype frequencies p squared (AA), 2pq (Aa), and q squared (aa) represented in babies.

Hardy-Weinberg law

The Hardy-Weinberg law applied to two alleles.

Encyclopædia Britannica, Inc.

Diagram illustrating three types of natural selection-stabilizing, directional, and diversifying-showing how each affects the distribution of beetle phenotypes in a population over multiple generations. Shaded areas indicate phenotypes selected against, and arrows show shifts in population traits.

Types of natural selection

Three types of natural selection, showing the effects of each on the distribution...

Encyclopædia Britannica, Inc.

light gray peppered moth (Biston betularia)

light gray peppered moth (Biston betularia)

A light gray peppered moth (Biston betularia) and a darkly pigmented variant...

From the experiments of Dr. H.B.D. Kettlewell, University of Oxford; photographs by John S. Haywood

dark coloured peppered moth (Biston betularia)

dark coloured peppered moth (Biston betularia)

Against the background of a lichen-covered oak tree, a darkly pigmented peppered...

From the experiments of Dr. H.B.D. Kettlewell, University of Oxford; photographs by John S. Haywood

red deer stags (Cervus elaphus)

red deer stags (Cervus elaphus)

A pair of red deer stags (Cervus elaphus) competing for possession of a...

Stefan Meyers GDT/Ardea London

Two Japanese macaques at Jigokudani Monkey Park in Nagano, Japan, with one grooming the other's fur as they sit close together.

Japanese macaque (Macaca fuscata)

Members of a group of Japanese macaques groom each other. Grooming is a type of altruistic...

© bigy9950/stock.adobe.com

A liger, a large feline hybrid of a lion and a tiger, stands on grass and dirt near fallen logs with a rocky wall in the background.

liger

A liger, the result of a mating between a male lion and a female tiger in a captive...

© mcherevan/Shutterstock.com

Adaptive radiation in Galapagos finches

Adaptive radiation in Galapagos finches

Fourteen species of Galapagos finches that evolved from a common ancestor. The different...

Encyclopædia Britannica, Inc.

Bar chart comparing genetic identity and genetic distance across five levels of Drosophila willistoni populations, from local populations to morphologically different species, with illustrations of fruit flies.

genetic differentiation

Genetic differentiation between populations of the fruit fly Drosophila willistoni.

Encyclopædia Britannica, Inc.

parallel evolution of marsupial and placental mammals

parallel evolution of marsupial and placental mammals

Parallel evolution of marsupial mammals in Australia and placental mammals on other...

Encyclopædia Britannica, Inc.

Diagram showing rib strength evolution in the brachiopod Eocelia, with horizontal bars representing the range of rib height-to-width ratios for fossil samples over time. Four species are illustrated on the left, with time in millions of years ago marked vertically. Yellow bars indicate confidence intervals, and sample means are shown as dots. Explanatory text describes how rib strength data distinguishes species and evolutionary transitions.

morphological evolution

Morphological evolution in a lineage of brachiopods, presented as an illustration...

Encyclopædia Britannica, Inc.

Line graph showing the number of marine animal families from the Precambrian era to the present, with illustrations of key groups such as trilobites, ammonoids, marine reptiles, and diatoms. The graph highlights major extinction events and a rise to 1,900 living families today.

marine family diversity

The diversity of marine animal families since late Precambrian time. The data for...

Encyclopædia Britannica, Inc.

A vertical diagram shows five stages in the evolution of eye complexity in mollusks, from simple pigment spot and pigment cup eyes in Patella and Pleurotomaria, to a pinhole-lens eye in Nautilus, an eye with a primitive lens in Murex, and a complex camera-type eye in octopus and squid, with labeled structures such as epithelium, photoreceptor layer, optic nerve, retina, lens, cornea, and iris.

steps in the evolution of the eye in living mollusk species

Steps in the evolution of the eye as reflected in the range of eye complexity in...

Encyclopædia Britannica, Inc.

Diagram comparing two phylogenetic trees based on molecular changes: one tree shows evolutionary relationships among four species using myoglobin changes, and the other compares humans, rhesus monkeys, and horses using cytochrome c changes, with arrows and numbers indicating the amount of molecular change between branches.

two phylogenies based on the amount of change in macromolecules

(Left) Amount of change in the evolutionary history of three hypothetical living...

Encyclopædia Britannica, Inc.

A branching phylogenetic tree diagram shows evolutionary relationships among various organisms, from Neurospora mold to humans, based on differences in cytochrome c gene sequences. Each branch is labeled with nucleotide substitution values and illustrated with animal or organism icons along the tree.

phylogeny

Phylogeny based on differences in the protein sequence of cytochrome c.

Encyclopædia Britannica, Inc.

Diagram tracing the evolutionary history of globin genes from a single ancestral globin gene, with branching lines and dots marking gene duplication events over millions of years, leading to myoglobin and various hemoglobin genes on chromosomes 16 and 11.

evolutionary history of the globin genes

Evolutionary history of the globin genes.

Encyclopædia Britannica, Inc.

A line graph compares the evolutionary rates of three proteins-fibrinopeptides (very fast), hemoglobin (intermediate), and cytochrome c (slow)-by showing amino acid changes per 100 residues over millions of years since divergence from a common ancestor, with geological time periods and major organism groups labeled along the axes.

evolutionary rates in proteins

Three proteins with very different evolutionary rates: fibrinopeptides (very fast),...

Encyclopædia Britannica, Inc.

Haeckel, Ernst: evolutionary scheme

Haeckel, Ernst: evolutionary scheme

Ernst Haeckel's evolutionary scheme presented in the form of a tree. From The...

© Photos.com/Thinkstock

A colorful diagram of the geologic time scale shows major eons, eras, and periods, with illustrations of key prehistoric life forms such as bacteria, jellyfish, trilobites, fishes, dinosaurs, mammals, and humans, spanning from 4,000 million years ago to the present. Measurement bars at the top and bottom indicate the passage of time in millions of years, and each section is color-coded to highlight evolutionary milestones.

timeline of life on Earth

Over hundreds of millions of years, life spread through the seas and over Earth's...

Encyclopædia Britannica, Inc.

Figure 6: Evolution of the chordates.

Figure 6: Evolution of the chordates.

Encyclopædia Britannica, Inc.

A diagram showing the tentative evolutionary relationships among hominid species over the past four million years, with skull images and labeled species names connected by solid and dotted lines to indicate proposed evolutionary pathways.

human evolutionary pathways

Possible pathways in the evolution of the human lineage.

Encyclopædia Britannica, Inc.

A timeline chart shows major events in plant evolution alongside the geologic time scale, with illustrations of algae, mosses, ferns, ginkgoes, conifers, cycads, and flowering plants marking their first appearances from the Ordovician to the Cenozoic era. Key milestones such as the emergence of early vascular plants and seed plants are labeled with approximate millions of years ago.

Figure 5: Significant events in plant evolution.

Encyclopædia Britannica, Inc.

Diagram illustrating the evolutionary stages of turtle shell development, showing skeletal outlines of Eunotosaurus, Pappochelys, Odontochelys, and Proganochelys with red highlighting the expanding shell area over time. Below, side-by-side illustrations compare the body structure and stride length of early reptiles with flexible spines and fast crawling to Eunotosaurus and later turtles with rigid spines and slower movement.

turtle shell evolution

Broad-ribbed bony structures likely evolved to provide early turtles with more stability...

Encyclopædia Britannica, Inc./Eliana Tobin

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

Charles Darwin

Charles Darwin leaning against a column on the veranda of Down House in 1881.

Hulton Archive/Getty Images

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