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Evolution (also known as biological, genetic or organic evolution) is the change in the inherited traits of a population of organisms through successive generations. This change results from interactions between processes which introduce variation into a population, and other processes which remove it. As a result, variants with particular traits become more, or less, common. A trait is a particular characteristic, anatomical, biochemical or behavioural, that is the result of gene–environment interaction.

The main source of variation is mutation, which introduces genetic changes. These changes can be passed on through reproduction, and give rise to alternative varieties, or alleles, of traits in organisms. Another source of variation is genetic recombination which shuffles the genes into new combinations that can result in organisms exhibiting different traits. Under certain circumstances, variation can also be increased by the transfer of genes between species, and by the extremely rare, but significant, wholesale incorporation of genomes through endosymbiosis.

Two main processes cause variants to become more common or rare in a population. One is natural selection, through which traits that aid survival and reproduction become more common, while traits that hinder survival and reproduction become more rare. Natural selection occurs because only a small proportion of individuals in each generation will survive and reproduce, since resources are limited and organisms produce many more offspring than their environment can support. Over many generations, heritable variation in traits is filtered by natural selection and the beneficial changes are successively retained through differential survival and reproduction. This iterative process adjusts traits so they become better suited to an organism's environment: these adjustments are called adaptations.

However, not all change is adaptive. Another cause of evolution is genetic drift, which leads to random changes in how common traits are in a population. Genetic drift is most important when traits do not strongly influence survival, particularly so in small populations where chance plays a disproportionate role in the frequency of traits passed on to the next generation. Genetic drift is important in the neutral theory of molecular evolution, and plays a role in the molecular clocks that are used in phylogenetic studies.

A key process in evolution is speciation, in which a single ancestral species splits and diversifies into multiple new species, and there are several modes through which this occurs. Ultimately, all living (and extinct) species are descended from a common ancestor via a long series of speciation events. These events stretch back in a diverse "tree of life" that has grown over the 3.5 billion years in which life has existed on Earth. This is visible in anatomical, genetic and other similarities between groups of organisms, geographical distribution of related species, the fossil record and the recorded genetic changes in living organisms over many generations.

Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes. The study of evolutionary biology began in the mid-nineteenth century, when research into the fossil record and the diversity of living organisms convinced most scientists that species changed over time. The mechanism driving these changes remained unclear until the theory of natural selection was independently proposed by Charles Darwin and Alfred Wallace. In 1859, Darwin's seminal work On the Origin of Species brought the new theory of evolution by natural selection to a wide audience, leading to the overwhelming acceptance of evolution among scientists. In the 1930s, Darwinian natural selection became understood in combination with Mendelian inheritance, forming the modern evolutionary synthesis, which connected the substrate of evolution (inherited genetics) and the mechanism of evolution (natural selection). This powerful explanatory and predictive theory has become the central organizing principle of modern biology, directing research and providing a unifying explanation for the history and diversity of life on Earth. Evolution is applied and studied in fields as diverse as agriculture, anthropology, conservation biology, ecology, medicine, paleontology, philosophy, and psychology along with other specific topics in the previous listed fields.

The roots of scientific thinking can be dated to at least the 6th century BCE, with the Greek philosopher Anaximander. However, the growth of modern biology out of natural history is fairly recent. The word evolution (from the Latin evolutio, meaning "to unroll like a scroll") appeared in English in the 17th century. As biological knowledge grew in the 18th century, proto-evolutionary ideas were set out by a few natural philosophers such as Pierre Maupertuis in 1745 and Erasmus Darwin in 1796. The ideas of the biologist Jean-Baptiste Lamarck about transmutation of species influenced radicals, but were rejected by mainstream scientists. Charles Darwin formulated his idea of natural selection in 1838 and was still developing his theory in 1858 when Alfred Russel Wallace sent him a similar theory, and both were presented to the Linnean Society of London in separate papers. At the end of 1859, Darwin's publication of On the Origin of Species explained natural selection in detail and presented evidence leading to increasingly wide acceptance of the occurrence of evolution.

Debate about the mechanisms of evolution continued, and Darwin could not explain the source of the heritable variations which would be acted on by natural selection. Like Lamarck, he thought that parents passed on adaptations acquired during their lifetimes, a theory which was subsequently dubbed Lamarckism. In the 1880s, August Weismann's experiments indicated that changes from use and disuse were not heritable, and Lamarckism gradually fell from favour. More significantly, Darwin could not account for how traits were passed down from generation to generation. In 1865 Gregor Mendel found that traits were inherited in a predictable manner. When Mendel's work was rediscovered in 1900s, disagreements over the rate of evolution predicted by early geneticists and biometricians led to a rift between the Mendelian and Darwinian models of evolution.