Positive selection

Positive selection (also called directional selection or adaptive selection) is the evolutionary process that increases the frequency of beneficial alleles in a population. It is the creative face of natural selection, responsible for adaptation, the origin of new traits, and the divergence of species. Unlike purifying selection, which removes deleterious mutations, positive selection actively promotes genetic change — but it is far rarer across the genome.

Detecting positive selection requires distinguishing it from background processes: genetic drift, demographic bottlenecks, and relaxed purifying selection all increase allele frequencies without requiring beneficial effects. The signature of positive selection is typically a localized region of reduced diversity (a selective sweep), an excess of nonsynonymous substitution relative to synonymous substitution (dN/dS > 1), or an unusual frequency spectrum of alleles.

Positive selection is the headline of evolutionary biology, but purifying selection is the fine print. The ratio of attention between them is the inverse of the ratio of their occurrence.