The neutral theory of molecular evolution states that the vast majority of evolutionary changes at the molecular level are caused by random drift of selectively neutral mutants (not affecting fitness). The theory was introduced by Motoo Kimura in the late 1960s and early 1970s. Neutral theory is compatible with Darwin's theory of evolution by natural selection: adaptive changes are acknowledged as present and important, but hypothesized to be a small minority of all the changes seen fixed in DNA sequences. Since then, this hypothesis has been tested using the McDonald-Kreitman test, and has not been supported in all species. Even in those species in which adaptive changes are rare, background selection at linked sites may violate neutral theory's assumptions regarding genetic drift.
Read more about Neutral Theory Of Molecular Evolution: Overview, The "neutralist–selectionist" Debate
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“The more specific idea of evolution now reached is—a change from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity, accompanying the dissipation of motion and integration of matter.”
—Herbert Spencer (1820–1903)