In biology, evolution is the process of change in the inherited traits of a population of organisms from one generation to the next. Genes that are passed on to an organism's offspring produce the inherited traits that are the basis of evolution. Mutations in genes can produce new or altered traits in individuals, resulting in the appearance of heritable differences between organisms. New traits may also arise from the transfer of genes between populations, as in migration, or between species, in horizontal gene transfer. In species that reproduce sexually, new combinations of genes are produced by genetic recombination, which can increase the variation in traits between organisms. Evolution occurs when these heritable differences become more common or rare in a population.

There are two major mechanisms that drive evolution. The first is natural selection, a process causing heritable traits that are helpful for survival and reproduction to become more common in a population, and harmful traits to become more rare. This occurs because individuals with advantageous traits are more likely to reproduce, so that more individuals in the next generation inherit these traits.{{cite journal |author=Lande R, Arnold SJ |year=1983 |title=The measurement of selection on correlated characters |journal=Evolution |volume=37 |pages=1210–26} |doi=10.2307/2408842}} Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of those variants best-suited for their environment. The second is genetic drift, an independent process that produces random changes in the frequency of traits in a population. Genetic drift results from the role probability plays in whether a given trait will be passed on as individuals survive and reproduce. Though the changes produced in any one generation by drift and selection are small, differences accumulate with each subsequent generation and can, over time, cause substantial changes in the organisms.

One definition of a species is a group of organisms that can reproduce with one another and produce fertile offspring. When a species is separated into populations that are prevented from interbreeding, mutations, genetic drift, and natural selection cause the accumulation of differences over generations and the emergence of new species. The similarities between organisms suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence.

Evolutionary biology documents the fact that evolution occurs, and also develops and tests theories that explain its causes. Studies of the fossil record and the diversity of living organisms had convinced most scientists by the mid-nineteenth century that species changed over time. However, the mechanism driving these changes remained unclear until the 1859 publication of Charles Darwin's On the Origin of Species, detailing the theory of evolution by natural selection. . Related earlier ideas were acknowledged in Darwin's work soon led to overwhelming acceptance of evolution within the scientific community. Joint statement issued by the national science academies of 67 countries, including the United Kingdom's Royal Society from the world's largest general scientific society In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis, in which the connection between the units of evolution (genes) and the mechanism of evolution (natural selection) was made. This powerful explanatory and predictive theory directs research by constantly raising new questions, and it has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth.