In biology, evolution is the change in the inherited traits of a population from generation to generation.
These traits are the expression of genes that are copied and passed on to offspring during reproduction.
Mutations in these genes can produce new or altered traits, resulting in heritable differences (genetic variation) between organisms.
New traits can also come from transfer of genes between populations, as in migration, or between species, in horizontal gene transfer.
Evolution occurs when these heritable differences become more common or rare in a population, either non-randomly through natural selection or randomly through genetic drift.
Natural selection is a process that causes heritable traits that are helpful for survival and reproduction to become more common, and harmful traits to become more rare.
This occurs because organisms with advantageous traits pass on more copies of these heritable traits to the next generation.
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.
In contrast, genetic drift produces random changes in the frequency of traits in a population.
Genetic drift arises from the role chance plays in whether a given individual will survive and reproduce.
One definition of a species is a group of organisms that can reproduce with one another and produce fertile offspring.
However, when a species is separated into populations that are prevented from interbreeding, mutations, genetic drift, and the selection of novel traits 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.
The theory of evolution by natural selection was proposed roughly simultaneously by both Charles Darwin and Alfred Russel Wallace, and set out in detail in Darwin's 1859 book On the Origin of Species.
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 has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth.