In biology, "evolution" is the change in the heritable traits of a population over successive generations, as determined by shifts in the allele frequencies of genes. Through the course of time, this process results in the origin of new species from existing ones (speciation). All contemporary organisms are related to each other through common descent, the products of cumulative evolutionary changes over billions of years. Evolution is the source of the vast diversity of extant and extinct life in the world.
The basic mechanisms that produce evolutionary change are natural selection (which includes ecological, sexual, and kin selection) and genetic drift; these two mechanisms act on the genetic variation created by mutation, genetic recombination and gene flow. Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce. If those traits are heritable, they pass them to their offspring, with the result that beneficial heritable traits become more common in the next generation.