In the theory of natural selection as currently conceived, there is both a chance component and a non-random component.
Genetic variation is seen as developing randomly, by chance, such as through mutations or genetic recombination.
However, even today, this theory remains controversial, with detractors in both the scientific and religious communities.
Main articles: Speciation and Species The concepts of speciation and extinction are important to any understanding of evolutionary theory.
Thus, chance (stochastic processes, randomness) also plays a major role in the theory of natural selection.
According to the theory of natural selection, natural selection is the directing or creative force of evolution.Mayr (2002) states that the production of genetic variation "is almost exclusively a chance phenomena." In every generation, new mutations and recombinations arise spontaneously, producing a new spectrum of phenotypes for natural selection—a non-random selective force (Mayr 2002)—to act upon.However, Mayr (2002) also notes that chance plays an important role even in "the process of the elimination of less fit individuals," and particularly during periods of mass extinction.A popular definition along these lines is that offered by Douglas J.Futuyma (1986) in Evolutionary Biology: "Biological evolution…is change in the properties of populations of organisms that transcend the lifetime of a single individual….Speciation is the term that refers to creation of new and distinct biological species by branching off from the ancestral population.Various mechanisms have been presented whereby a single evolutionary lineage splits into two or more genetically independent lineages.This theory encompasses both minor changes in gene frequency in populations, brought about by the creative force of natural selection, and major evolutionary changes brought about through natural selection, such as the origin of new designs.For Darwin, however, the term natural selection generally was used synonymously with evolution by natural selection.The validity of making such extrapolations has recently been challenged by some prominent evolutionists.The theory of natural selection received a much more contentious response than did the theory of descent with modification.