Natural selection is described as the only known mechanism for:

1. Several processes change allele frequencies in a population. 2. Only one of them systematically improves the fit between organism and environment. 3. Ultimately, natural selection leads to greater adaptation of the population to its local environment; it is the only mechanism known for adaptive evolution. 4. Drift and mutation change populations but not in a consistently adaptive direction. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 18 'Evolution and the Origin of Species', sections 18.1-18.3_