Evolution

In biology, evolution is change in the genetic material of a population of organisms through successive generations. Although the changes produced in a single generation are normally small, the accumulation of these differences over time can cause substantial changes in a population, a process that can result in the emergence of new species.^[1] Similarities among species suggest that all known species descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence.^[2]

The basis of evolution is the passing of genes from one generation to the next. Genes are what produce an organism's inherited traits. These vary within populations, with organisms showing heritable differences (variation) in their traits. Evolution is the product of two opposing forces: processes that constantly introduce variation, and processes that make those variants become either more common or rare. New variation arises in two main ways: either from mutations in genes, or from the transfer of genes between populations and between species. New combinations of genes are also produced by genetic recombination, which can increase variation between organisms.

Two major mechanisms determine if variants will become more common or rare in a population. One is natural selection, a process whereby helpful traits (those that increase the chance of survival and reproduction) become more common in a population while harmful traits become increasingly rare. This occurs because individuals with advantageous traits are more likely to survive and reproduce, resulting in more individuals of the next generation inheriting those traits.^[2][3] Adaptations occur over many generations through successive, small, random changes in traits combined with natural selection of those variants best-suited for their environment.^[4] The other major mechanism driving evolution is genetic drift, an independent process that produces random changes in the frequency of traits in a population. Genetic drift results from the role that chance plays in whether a given trait will be passed on as individuals survive and reproduce.

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.^[5][6] were independently proposed by However, the mechanism driving these changes remained unclear until the theories of natural selectionCharles 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,^[7] leading to the overwhelming acceptance of evolution among scientists.^{[8][9][10][11]} In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis,^[12] 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 diversity of life on Earth