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.

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, ...