Evolution (or more specifically biological or organic evolution) is the change over time in one or more inherited traits found in populations of individuals. Inherited traits are distinguishing characteristics, for example anatomical, biochemical or behavioural, that are passed on from one generation to the next. Evolution occurs when there is variation of inherited traits within a population over time. The major sources of such inherited variants are mutation, genetic recombination and gene flow. Evolution has led to the diversification of all living organisms from a common ancestor, which are described by Charles Darwin as "endless forms most beautiful and most wonderful".
There are four common mechanisms of evolution. The first mechanism is natural selection, a process in which there is differential survival and reproduction of entities that differ in one or more inherited traits. Selection can act at multiple levels of organization, for example differential survival and/or reproduction of organisms, populations, or gene variants. A second mechanism is genetic drift, a process in which there are random changes to the proportions of two or more inherited traits within a population. A third mechanism is biased mutation, which can affect phenotypes expressed across multiple levels of organisation. Finally, the fourth mechanism is gene flow, which is the incorporation of genes from one population into another.
Evolution may in the long term lead to speciation, whereby a single ancestral species splits into two or more different species. Speciation 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. Common descent stretches back over 3.5 billion years during which life has existed on earth. Both evolution within populations and speciation between them are thought to occur in multiple ways such as slowly, steadily and gradually over time or rapidly from one long static state to another.
The scientific study of evolution began in the mid-nineteenth century, when research into the fossil record and the diversity of living organisms convinced most scientists that species evolve. The mechanisms driving these changes remained unclear until the theory of natural selection was independently proposed by Charles Darwin and Alfred Wallace in 1858. In the early 20th century, Darwinian theories of evolution were combined with genetics, palaeontology and systematics, which culminated into a union of ideas known as the modern evolutionary synthesis.The synthesis became a major principle of biology as it provided a coherent and unifying explanation for the history and diversity of life on Earth.
Evolution is currently applied and studied in various areas within biology such as conservation biology, developmental biology, ecology, physiology, paleontology and medicine. Moreover, it has also made an impact on other disciplines such as agriculture, anthropology, philosophy and psychology. Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes.
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