Overview

Natural selection is differential reproduction of genotypes in the absence of human intervention. The relative reproductive ability of a genotype is its fitness. Selection is a potentially strong force that can change both the population mean and its genetic variation; it is the principal agent of directional genetic change. The speed of evolution under natural selection is a function of fitness and allele frequencies. Rare deleterious alleles are eliminated very slowly from a population. One way genetic variation is maintained in a population is balancing selection, which favors heterozygotes.

Kinds of Selection

Selection occurs whenever one genotype reproduces at a higher or lower rate than other genotypes, on average, causing that genotype to contribute more or fewer copies of its genes to the population than other genotypes do. For example, a recessive lethal allele that acts at a prereproductive stage will be selected against because homozygotes will die before they can reproduce. Selection acts on all life forms, regardless of the mode of reproduction or the nature of the life cycle. It is the principal driving force of evolution. The speed of selection and its effect on equilibrium frequencies of alleles and genotypes can be quantified. Selection that acts on survival, fertility, and fecundity is called natural selection, while selection that acts on the mating success of sexual organisms is called sexual selection.