Evolution is the change in the inherited traits of a population of organisms through successive generations. After a population splits into smaller groups, these groups evolve independently and may eventually diversify into new species. A nested hierarchy of anatomical and genetic similarities, geographical distribution of similar species and the fossil record indicate that all organisms are descended from a common ancestor through a long series of these divergent events, stretching back in a tree of life that has grown over the 3,500 million years of life on Earth.
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Evolution is the product of two opposing forces: processes that constantly introduce variation in traits, and processes that make particular variants become more common or rare. A trait is a particular characteristic, such as eye color, height, or a behavior, that is expressed when an organism's genes interact with its environment, translating its genotypic predispositions into phenotypic phenomena. Genes vary within populations, so organisms show heritable differences (variation) in their traits.
The main cause of variation is mutation, which changes the sequence of a gene. Altered genes, or alleles, are then inherited by offspring. There can sometimes also be transfer of genes between species. Two main processes cause variants to become more common or rare in a population. One is natural selection, which causes traits that aid survival and reproduction to become more common, and traits that hinder survival and reproduction to become more rare. Natural selection occurs because only a few individuals in each generation will survive, since resources are limited and organisms produce many more offspring than their environment can support. Over many generations mutations produce successive, small, random changes in traits, which are then filtered by natural selection and the beneficial changes retained. This adjusts traits so they become suited to an organism's environment: these adjustments are called adaptations. Not every trait, however, is an adaptation. Another cause of evolution is genetic drift, an independent process that produces entirely random changes in how common traits are in a population. Genetic drift comes from the role that chance plays in whether a trait will be passed on to the next generation.
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 theories of natural selection were independently proposed by Charles Darwin and Alfred Wallace. In 1859, Darwin's seminal work On the Origin of Species brought the new theories 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 units of evolution (genes) 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 therefore applied and studied in fields as diverse as ecology, anthropology, conservation biology, paleontology, agriculture, medicine, psychology, philosophy and others.