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Evolution by natural selection
1858–1859· Theory or idea· Animals
Well establishedStrong fossil, genetic or geological evidence.
The theory that species change over generations because individuals with helpful inherited traits leave more offspring.
See it on the Life map →What happened
Darwin and Wallace reasoned from four facts: individuals vary, much of that variation is inherited, more young are born than can survive, and survival is not random. Individuals whose traits suit their surroundings leave more offspring, so useful traits spread. Over many generations populations change and split into new species. Their joint papers were read in London in July 1858; Darwin's On the Origin of Species followed in November 1859. The process itself needs no designer or goal.
Why it matters
It explains the fit between living things and their surroundings without a designer, and became the organizing idea of biology, medicine and crop and animal breeding.
Think of it like
Like a leasing market: nobody plans it, but over the years tenants who suit the location stay and expand, and the rest move out.
The evidence (fossils, genes, rocks)
Seen directly in Galápagos finches (Peter and Rosemary Grant, since 1973), peppered moths, drug-resistant microbes and Richard Lenski's E. coli experiment running since 1988; backed by fossils, anatomy and DNA.
Still debated
How much evolution is driven by selection versus random genetic drift, and whether selection also acts on genes or whole groups, not just individuals.
Inside this (7)
Connections (10)
▸Archaeopteryx gave early support to Evolution by natural selectionWell established
The connection
Two years after Darwin's book, quarry workers in Bavaria found a skeleton with feathers and wings but also teeth, clawed fingers and a long bony tail: an apparent link between reptiles and birds. Thomas Henry Huxley used it, along with small dinosaurs like Compsognathus, to argue that birds evolved from reptiles, a striking case of the transitional forms Darwin predicted.
When and how
Solnhofen limestone, Bavaria, Germany, 1861; bought by the British Museum in London in 1862 and described by Richard Owen in 1863; Huxley's arguments followed from 1868.
The evidence
Darwin added Archaeopteryx to later editions of the Origin, and about a dozen skeletons are now known.
Debates and open questions
Whether Archaeopteryx is on the direct line to modern birds or a side branch among feathered dinosaurs, and how well it could fly.
▸Agriculture begins inspired the 'artificial selection' analogy of Evolution by natural selectionWell established
The connection
Darwin opened On the Origin of Species with a chapter on domestic breeds. Farmers and fanciers, choosing which animals and plants to breed, had turned wild ancestors into very different pigeons, dogs, cattle and crops. Darwin reasoned that nature, through differences in survival and reproduction, could do the same, more slowly but over far longer times.
When and how
England, 1850s: Darwin bred pigeons at Down House and joined London pigeon clubs; 'Variation under Domestication' is the Origin's first chapter (1859).
The evidence
Darwin's text and letters spell out the analogy, and his two-volume The Variation of Animals and Plants under Domestication (1868) expanded it.
Debates and open questions
Early domestication was largely unplanned, which Darwin called 'unconscious selection'; how far goal-directed breeding parallels goal-less natural selection is still discussed.
▸Antibiotic resistance demonstrates in real time Evolution by natural selectionWell established
The connection
An antibiotic kills susceptible bacteria, but any carrying a resistance mutation or gene survive and multiply, so the next generation is mostly resistant. With bacteria dividing as often as every 20 minutes, this can happen within days. In a 2016 Harvard experiment, E. coli evolved to survive 1,000 times the dose that killed the original strain in about 11 days.
When and how
Hospitals worldwide since the 1940s; the 2016 'MEGA-plate' experiment by Michael Baym and Roy Kishony at Harvard Medical School.
The evidence
Sequencing tracks specific resistance mutations as they arise and spread, matching what natural selection predicts step by step.
Debates and open questions
Why some resistance persists even after a drug is withdrawn, and whether drug combinations or rotation can reliably slow it.
▸Charles Darwin proposed Evolution by natural selectionWell established
The connection
On the Beagle voyage Darwin saw fossils resembling living South American animals and island species that differed from mainland ones. In 1838, reading Thomas Malthus on populations outgrowing food, he realized the struggle to survive would favor useful variations. He wrote sketches in 1842 and 1844 but held off publishing for twenty years, gathering evidence.
When and how
HMS Beagle, 1831–1836; London, September 1838; Down House in Kent, where he wrote On the Origin of Species, published 24 November 1859.
The evidence
His dated private notebooks, the 1844 essay and his 1857 letter to Asa Gray, read in London in 1858, document the idea's development.
Debates and open questions
Why he delayed so long (fear of controversy, perfectionism or illness), and how much 'Darwin's finches' really shaped his thinking; mockingbirds mattered more at first.
▸Alfred Russel Wallace co-discovered Evolution by natural selectionWell established
The connection
While collecting animals in what is now Indonesia, Wallace hit on the same idea during a fever in February 1858: in each generation the less fit die and the fittest survive, so species change. Like Darwin, he credited Malthus. He mailed his essay to Darwin, and their papers were presented together.
When and how
Moluccas (Ternate or Halmahera), in today's Indonesia, February 1858; joint presentation at the Linnean Society of London, 1 July 1858.
The evidence
Wallace's essay, 'On the Tendency of Varieties to Depart Indefinitely from the Original Type', was printed with Darwin's extracts in the Society's journal in August 1858.
Debates and open questions
Wallace later argued natural selection could not explain the human mind, parting from Darwin; historians still weigh whether he gets due credit.
▸Jean-Baptiste Lamarck proposed an earlier rival to Evolution by natural selectionWell established
The connection
Lamarck's 1809 Philosophie zoologique offered the first full theory that species change over time. His mechanism differed: organisms change through use and disuse of body parts and pass those acquired traits on, driven by a tendency toward complexity. Darwin proposed selection instead, though he too accepted some inheritance of acquired traits.
When and how
Paris, at the National Museum of Natural History, 1800–1809; August Weismann's germ-line theory of the 1880s argued that acquired traits are not inherited.
The evidence
Weismann cut the tails off mice for generations without producing tailless offspring, and genetics later found no general route from body changes to eggs or sperm.
Debates and open questions
Epigenetics, chemical tags on DNA, can sometimes pass environmental effects to offspring for a few generations; how much this matters in evolution is debated.
▸Evolution by natural selection implies Common descent & the tree of lifeWell established
The connection
If species change and split over generations, then every living species descends from earlier ones, and tracing them back joins all life into one family tree. Natural selection supplied the mechanism; common descent is the pattern it predicts. Shared features, from bone layouts to the genetic code, then become family resemblances.
When and how
On the Origin of Species, 1859; Ernst Haeckel drew the first detailed trees of life in 1866.
The evidence
All known life uses the same genetic code and core genes, and trees built independently from DNA and from fossils largely agree.
Debates and open questions
Whether all life traces to one population, the last universal common ancestor (LUCA), and how gene swapping between microbes blurs the tree's base.
▸Evolution by natural selection was merged with genetics in The modern synthesisWell established
The connection
By 1900 many biologists doubted natural selection, a period later called the 'eclipse of Darwinism', and rediscovered Mendelian genetics seemed to favor sudden jumps. From 1918, Fisher, Haldane and Wright showed mathematically that selection acting on many small Mendelian differences produces Darwin's gradual change, putting selection back at the center of evolution.
When and how
Britain and the U.S., from Fisher's 1918 paper on inheritance and continuous variation to the 1940s; the name came from Julian Huxley's 1942 book.
The evidence
The mathematics of Fisher's 1930 book and its match to field and lab studies, such as Dobzhansky's work on wild fruit fly populations.
Debates and open questions
How much evolution is driven by neutral drift rather than selection, a question sharpened by Motoo Kimura's neutral theory in 1968.
▸Evolution by natural selection was challenged on its pace by Punctuated equilibriumDebated
The connection
Darwin pictured evolution as mostly slow and steady. Eldredge and Gould argued that fossils show species staying the same for millions of years, then being replaced abruptly, and that this pattern is real. Selection still drives change, they said, but mostly in short bursts when new species form, not continuously.
When and how
A 1972 essay in the volume Models in Paleobiology, by Niles Eldredge (American Museum of Natural History) and Stephen Jay Gould (Harvard).
How sure are we?
Decades of fossil studies show both stasis-and-burst and gradual patterns; the debate fills a large literature, including Gould's final book in 2002.
Debates and open questions
Whether it really challenges Darwin or just fine-tunes him; Darwin himself noted that rates of change vary and many species stay unchanged for long periods.
▸Deep time & the age of the Earth gave the time needed for Evolution by natural selectionWell established
The connection
Darwin read Charles Lyell's Principles of Geology on the Beagle; Lyell argued that slow processes over vast time shaped Earth. Darwin applied this logic to life: small changes add up over enough time. When physicist Lord Kelvin argued in the 1860s that Earth was probably under 100 million years old, Darwin called it one of his 'sorest troubles'.
When and how
Lyell's book, 1830–1833; Darwin's Origin, 1859; Kelvin's estimates from 1862; radioactivity, discovered in 1896, later removed the conflict.
The evidence
Darwin's letters and the Origin show his reliance on Lyell and his worry over Kelvin; radiometric dating later gave billions of years.
Debates and open questions
How fast evolution's earliest steps were remains open, since life seems to have appeared fairly soon after Earth formed.