Evolution is a change in the genetic makeup of a population within a species. Since the emergence of modern genetics in the 1940s, evolution has been defined more specifically as a change in the frequency of alleles from one generation to the next. The word "evolution" is often used as a shorthand for the modern theory of evolution of species based upon Charles Darwin's theory of natural selection, which states that all modern species are the products of an extensive process that began over three billion years ago with simple single-celled organisms, and Gregor Mendel's theory of genetics. As the theory of evolution by natural selection and genetics has become universally accepted in the scientific community, it has replaced other explanations including creationism and Lamarckism. Skeptics, often creationists, sometimes deride evolution as "just a theory" in an attempt to characterize it as an arbitrary choice and degrade its claims to truth. Such criticism overlooks the scientifically-accepted use of the word "theory" to mean a falsifiable and well-supported hypothesis.