Glossary · Sinh học
Evolution
The change in the genetic composition of a population of organisms over time — specifically over generations — arising from changes in the genome. Over successive generations, in some cases spanning millions of years, a species may take on divergent functions or physical traits, or may even evolve into a different species.
Individuals do not evolve: the changes in an individual's lifetime are development. Evolution shows up in populations, as a shift in the average value of a trait over generations. The genetic variation evolution works on has two main sources: mutation — the ultimate source of new alleles — and sexual reproduction, which assembles the parents' alleles into new genotypes in each offspring.
The mechanism of natural selection was described independently by Charles Darwin and Alfred Russel Wallace, who presented their papers together at the Linnean Society in London in 1858. Individuals with favourable traits reproduce more, and because traits are inherited, those traits are better represented in the next generation; over generations the population changes — a process Darwin called descent with modification. Natural selection is the only known mechanism for adaptive evolution, but not the only evolutionary force: in addition to it, genetic drift (chance changes in a population's gene pool), gene flow, mutation, nonrandom mating and environmental variation could be in play.