Directional selection

Immunology
In immunology positive selection is when: developing T-cells are selected for recognition of self MHC (and self peptides). This ensures an immune response can develop. Followed by negative selection.

Genetics
In population genetics, directional selection occurs when natural selection favors a single allele and therefore allele frequency continuously shifts in one direction. Under directional selection, the advantageous allele will increase in frequency independently of its dominance relative to other alleles (i.e. even if the advantageous allele is recessive, it will eventually become fixed). Directional selection stands in contrast to balancing selection where selection may favor multiple alleles, or purifying selection which removes deleterious mutations from a population. Directional selection is a particular mode or mechanism of natural selection.

Examples

 * A classic example is the evolution of the peppered moth, where the favored trait of moth color (light or dark) noticeably shifted to a darker shade in relation to the effects of human industrialization.