Evolution

Evolution is the change in the heritable characteristics of biological populations over successive generations, driven by processes such as natural selection and genetic drift acting on genetic variation. Charles Darwin and Alfred Russel Wallace independently proposed natural selection as its central mechanism in the mid nineteenth century, an account Darwin set out in detail in On the Origin of Species. The theory rests on four observable facts: more offspring are produced than can survive, individuals vary in their traits, different traits affect survival and reproduction, and traits are passed between generations. Early twentieth century genetics combined this account with Mendelian inheritance and population genetics into the modern synthesis, with DNA now understood as the molecular basis of heredity. All known life is thought to descend from a last universal common ancestor that lived roughly 3.5 to 3.8 billion years ago, with the diversity of living things arising through repeated speciation and extinction across that history.