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On the Origin of Species (full title On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life) is a work of scientific literature by the English naturalist Charles Darwin, published on 24 November 1859 by John Murray of London. It presents the evidence that species are not fixed but change over time, that all living things descend from common ancestors, and that the main mechanism of change is natural selection. The book is the foundation of evolutionary biology. Darwin described it as an "abstract" of a much larger work that he never finished.

The argument rests on observations that were accessible to any naturalist: individuals of a species vary, much of the variation is inherited, more offspring are born than can survive, and so those with heritable traits that improve survival and reproduction leave more descendants. Darwin supported the argument with evidence from the breeding of domestic animals and plants, biogeography, the fossil record, comparative anatomy, embryology, and vestigial structures. The first edition ends with a single sentence that applies the argument to the history of life as a whole, and it avoids the word "evolution", using "evolved" as its last word.

The voyage of the Beagle

Darwin served as a naturalist on the second survey voyage of HMS Beagle from 1831 to 1836, which circled the globe. He read the first volume of Charles Lyell's Principles of Geology during the voyage, and its argument that the Earth was shaped by slow processes still in operation gave him a view of very long timescales. He observed fossils of large extinct mammals in South America that resembled living species of the region, the replacement of related species across the continent, and the distinctive species of the Galápagos Islands. Back in England, ornithologists identified the Galápagos mockingbirds and finches that he had collected as distinct, closely related species, which led him to consider that species might change.

Development of the theory

From 1837, Darwin recorded his ideas about transmutation in a series of private notebooks. One page of Notebook B, from about 1837, has a sketch of a branching tree labeled "I think". In September 1838, he read Thomas Malthus's An Essay on the Principle of Population, which argued that human populations grow faster than their food supply. Darwin applied the idea to all organisms and realized that, in a struggle for existence, favorable variations would tend to be preserved and unfavorable ones destroyed. He wrote a sketch of the theory in 1842 and a longer essay in 1844, which he showed only to a few friends, including the botanist Joseph Dalton Hooker. He then spent eight years (1846 to 1854) on a monograph of barnacles, which gave him experience of variation among species and standing as a taxonomist. In the 1850s, he did experiments and collected information from breeders, especially of pigeons, and began a large book to be titled Natural Selection.

Wallace and the joint papers

In June 1858, Darwin received a manuscript from Alfred Russel Wallace, who was collecting specimens in the Malay Archipelago. The essay, "On the Tendency of Varieties to Depart Indefinitely from the Original Type", described a process of natural selection very similar to his own. Darwin's friends Lyell and Hooker arranged for extracts from his earlier writings and Wallace's essay to be read together at the Linnean Society of London on 1 July 1858, with neither author present. Darwin's infant son had died of scarlet fever a few days before. The papers were published in the society's journal in August 1858 and attracted little immediate attention. Darwin then wrote an abstract of his larger work over about 13 months. The result was On the Origin of Species.

Print Runs and Textual Revisions Across Editions (1859–1872)

The first edition of about 500 pages was published at a price of 15 shillings, and a print run of 1,250 copies was oversubscribed by booksellers before its release. A second edition of 3,000 copies appeared in January 1860. Darwin revised the book through six editions during his lifetime, and the changes show how he responded to criticism.

The sixth edition is the one most widely reprinted, although the first edition is often preferred by scholars as the text of the original argument. The book has been translated into dozens of languages.

Architecture of the Fourteen Chapters

The book has 14 chapters. The introduction summarizes the argument, and the last chapter is a recapitulation. The sequence builds from familiar facts to the central mechanism, then answers objections, and then applies the theory to broad bodies of evidence.

Variation and the struggle for existence

Darwin began with domestic breeds, because breeders had produced great changes through the selection of heritable differences. The many breeds of domestic pigeons, which differ in beak, skeleton, and behavior, were an important example, and Darwin argued that they descended from a single wild species, the rock dove. He then showed that wild species also vary, and that the distinction between a well-marked variety and a species is arbitrary, which he took as evidence that varieties are incipient species.

His next point was that populations tend to grow geometrically and are limited by food, predators, disease, and climate. If a population of size N₀ grows by a factor of R each generation, the size after t generations is:

or, in continuous time at a per capita rate r, _N_(t) = N₀ e^rt. Darwin illustrated this with elephants, which he took to be the slowest breeders among animals. He assumed that a pair begins to breed at 30 years of age, breeds until 90, and has six young in that time, and he calculated that after 740 to 750 years there would be nearly 19 million elephants descended from the first pair. (Modern demographers note that his figure is approximate.) Because the numbers of most species stay roughly constant, most of the offspring must die.

Natural selection

Darwin joined the two observations. Because individuals vary, and some of the variation is inherited, the individuals that have traits that help them to survive and to leave offspring will pass those traits on more often. He called this process natural selection, in analogy with the selection practiced by breeders. In a simple model of one heritable trait, if a variant has a relative fitness 1 + s compared with 1 for the alternative, and p is its frequency, the frequency in the next generation (for haploid asexual reproduction) is:

so that even a small advantage can increase in frequency over many generations. Darwin did not use such equations. The mathematical theory was developed in the early twentieth century by Ronald Fisher, J. B. S. Haldane, and Sewall Wright, and the response of a quantitative trait to selection is now described by the breeder's equation R = h² S, where h² is the heritability and S is the selection differential.

Chapter IV also presents sexual selection, in which traits evolve through competition for mates or through mate choice, and the principle of divergence. Darwin argued that groups of descendants that differ more from each other can use more of the resources of an environment, so divergence is favored and a single ancestor gives rise to a tree of related forms. He described this tree in the only illustration of the book, a fold-out diagram of lines of descent in which some lineages diverge and persist, and others end in extinction.

Difficulties and objections

Darwin devoted several chapters to objections that he thought were the most serious. The most important were:

• Transitional forms. If species change gradually, the fossil record should show intermediate forms. Darwin replied that the geological record is very incomplete, because few organisms are fossilized and only some deposits are preserved or exposed. The discovery of Archaeopteryx, with features of both reptiles and birds, in 1861 gave an example.
• Complex organs. Darwin addressed the eye, which seemed to him at first "absurd in the highest degree" as a product of selection. He argued that a series of intermediate stages, from simple light-sensitive patches to complex eyes, exists among living animals, and that each stage is useful.
• Instinct. Complex behavior, such as the comb of the honeybee, the slave-making of some ants, and the sterile worker castes of social insects, were explained by selection acting on inherited variation in behavior. The sterile castes posed a particular difficulty, because they do not reproduce. Darwin proposed that selection acts on the family, which anticipated later ideas of kin selection.
• Hybrid sterility. If species descend from common ancestors, why is the offspring of crosses between species often sterile? Darwin argued that sterility is not a special endowment and arises as an incidental result of other differences between species.
• The age of the Earth. Darwin's argument required very long times. He estimated that the erosion of the Weald in southern England had taken about 300 million years, which was a calculation that critics soon attacked. Lord Kelvin later argued from the cooling of the Earth and the Sun that the time was much shorter, and the problem was resolved only after the discovery of radioactivity.

Geographic distribution

Darwin argued that species are related to those of nearby regions, and not to those of distant regions with a similar climate, which is explained by descent with modification and dispersal and not by independent creation for each environment. He did experiments on the ability of seeds to survive immersion in salt water and to germinate, and he examined how animals could be dispersed by floating, by birds, and by other means. The species of oceanic islands, which are few and often endemic, and which usually resemble those of the nearest mainland, were strong evidence. Darwin also used the glacial period to explain how alpine plants were distributed on mountain tops separated by lowlands.

Classification, homology, and embryology

The final part of the argument applies the theory to the natural system of classification, in which groups are nested within groups. Darwin proposed that this hierarchy exists because it reflects descent from common ancestors. Homologous structures, such as the arm of a human, the wing of a bat, and the flipper of a porpoise, have the same bones in the same arrangement because they are inherited with modification from a common ancestor. Embryos of different vertebrates are more similar than adults, which Darwin interpreted as evidence that embryos retain traces of the ancestral form. He argued that rudimentary or vestigial organs, which have no clear function, are expected under descent with modification. Although he did not discuss human origins in detail, he stated that "light will be thrown on the origin of man and his history".

Pangenesis, Fleeming Jenkin's Critique, and the Mendelian Resolution

Darwin did not have a correct theory of how traits are inherited. Chapter V reflects the common view that characters acquired by use or disuse can be inherited, a form of Lamarckism, to which Darwin gave a role besides natural selection. In 1868, Darwin proposed a theory of pangenesis, in which particles called gemmules from every part of the body are carried to the reproductive organs. It was not supported by experiments, including those of his cousin Francis Galton.

A serious objection was made by the engineer Fleeming Jenkin in 1867. He argued that under blending inheritance a new favorable variant would be diluted by crossing with the common type, so that its effect would be halved at each generation. Mendelian genetics later resolved this problem, because particulate genes are not blended. The work of Gregor Mendel on pea plants was published in 1866 and was not recognized until 1900. The synthesis of Mendelian genetics and natural selection, by Fisher, Haldane, Wright, Theodosius Dobzhansky, Ernst Mayr, and others, is called the modern synthesis.

Scientific response

The book was widely reviewed, and its first printing sold quickly. The geologist Charles Lyell and the botanist Hooker supported Darwin, although Lyell was slow to accept the extension to humans, and the American botanist Asa Gray defended Darwin in the United States. Thomas Henry Huxley was Darwin's most active public supporter and was called "Darwin's bulldog". Opponents included the anatomist Richard Owen and the geologist Adam Sedgwick, and the Swiss-American zoologist Louis Agassiz. At a meeting of the British Association in Oxford in June 1860, the bishop Samuel Wilberforce and Huxley debated the book. The events of the debate are known from conflicting accounts, and the popular version that Huxley decisively won is partly a later reconstruction.

By the 1870s, most scientists accepted that species had evolved through common descent, but many doubted that natural selection was the main mechanism. From the 1880s to the 1930s, alternatives such as neo-Lamarckism, orthogenesis, and saltationism had a wide following, a period that Julian Huxley called the "eclipse of Darwinism". The development of population genetics in the 1920s and 1930s and the modern synthesis restored natural selection as the main explanation of adaptation.

Religious and social response

The book was seen as challenging the argument from design by William Paley and the idea that each species was specially created. Some religious leaders opposed it, whereas others, including some clergy, accepted evolution as compatible with faith, and Darwin himself was careful in the first edition not to discuss human origins. The phrase "survival of the fittest" and the ideas of social Darwinism, which applied misinterpretations of evolution to society, were developed by Spencer and others, and they are not part of Darwin's science. Darwin discussed human evolution in The Descent of Man, and Selection in Relation to Sex (1871) and The Expression of the Emotions in Man and Animals (1872).

Molecular Phylogenetics, Radiometric Dating, and Experimental Evolution

Darwin's theory remains the central explanatory framework in biology. The book's arguments have been supported and extended by evidence that Darwin could not have known, including:

• The discovery of many transitional fossils, such as Tiktaalik and the fossils of early whales and of feathered dinosaurs.
• Genetics and molecular biology, which showed the physical basis of heredity and variation, and gave molecular evidence of common descent in the universal genetic code and in shared genes.
• Radiometric dating, which showed that the Earth is about 4.5 billion years old.
• Plate tectonics, which explains many patterns of biogeography.
• Direct observation of evolution in natural and laboratory populations, as in the beaks of Darwin's finches, and in the E. coli long-term evolution experiment.

The theory has also been modified. Neutral processes such as genetic drift and neutral molecular evolution, horizontal gene transfer among microbes, and the role of development and plasticity are now recognized as part of the study of evolution. Darwin was buried in Westminster Abbey in 1882 near Isaac Newton. The 150th anniversary of the publication, in 2009, was marked by events worldwide.

Why did Fleeming Jenkin's blending inheritance objection threaten Darwin's theory of natural selection?

In 1867, Scottish engineer Fleeming Jenkin demonstrated mathematically that if parental traits blended irreversibly like mixed paints (blending inheritance), any newly arising advantageous variant mating with the wild-type population would have its phenotypic deviation halved in each successive generation, swamping the trait before natural selection could fix it. Gregor Mendel's particulate inheritance resolved this paradox by proving that discrete alleles segregate intact during meiosis without dilution.

How did the discovery of radioactivity resolve Lord Kelvin's objection to Darwinian timescales?

Physicist Lord Kelvin calculated that if the Earth and Sun cooled purely by thermal conduction and gravitational contraction without an internal heat source, the Earth could be only 20 to 40 million years old—far too brief for gradual natural selection. The discovery of radioactive decay by Henri Becquerel, Marie Curie, and Ernest Rutherford revealed both that radiogenic heat in the mantle slows planetary cooling and that radiometric dating places the Earth's age at 4.54 billion years.

What is the "Principle of Divergence" illustrated in the book's sole tree diagram?

Darwin's Principle of Divergence states that within any ecological habitat, descendant lineages that evolve the greatest morphological and physiological differences from one another can partition distinct ecological niches and minimize intraspecific competition for limiting resources. Over geological timescales, this ecological character displacement drives branching speciation, transforming intraspecific varieties into distinct species, genera, and families along a bifurcating phylogenetic tree.

• Charles Darwin
• Alfred Russel Wallace
• Natural selection
• Evolution
• History of evolutionary thought
• Modern synthesis (20th century)
• Darwin–Wedgwood family