Evolutionary pressure or selection pressure can be formalized as an external pressure applied to a process, thereby pushing that process in a distinct direction.

A process driven by evolutionary pressure is, for example, the natural selection for erythrocytes carrying the sickle cell hemoglobin gene mutation (Hb S)—causing sickle cell anaemia—in areas where malaria is a major health concern, which grants some resistance to this infectious disease. Therefore, the concept can be described as the application of Charles Darwin's principle of "survival of the fittest" (which actually should be understood as "extinction of the un-fittest") via some selection mechanism. Darwins illustrations of beak variation in the finches of the Galápagos Islands, which hold 13 closely related species that differ most markedly in the shape of their beaks. ... Human red blood cells Red blood cells are the most common type of blood cell and are the vertebrate bodys principal means of delivering oxygen to body tissues via the blood. ... Sickle-shaped red blood cells Sickle cell anemia (American English), sickle cell anaemia (British English) or sickle cell disease is a genetic disease in which red blood cells may change shape under certain circumstances. ... 3-dimensional structure of hemoglobin. ... For other meanings of this term, see gene (disambiguation). ... It has been suggested that mutant be merged into this article or section. ... HB is an abbreviation of: Hampton and Branchville Railroad (AAR reporting mark HB) hard black – a medium grade of pencil lead Helvetischer Bund, used as ICAO aircraft registration prefix (since 1935, before: CH) for Switzerland Hemoglobin is often abbreviated as Hb HB ice cream in Ireland, originally an abbreviation for... Sickle-shaped red blood cells Sickle cell anemia (American English), sickle cell anaemia (British English) or sickle cell disease is a genetic disease in which red blood cells may change shape under certain circumstances. ... Malaria is a vector-borne infectious disease that is widespread in tropical and subtropical regions. ... This false-colored electron micrograph shows a malaria sporozoite migrating through the midgut epithelia. ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ...

It is a quantitative description of the amount of change occurring in processes investigated by evolutionary biology, but the formal concept is often extended to other areas of research. Evolutionary biology is a subfield of biology concerned with the origin and descent of species, as well as their change, multiplication, and diversity over time. ...

In population genetics, selection pressure is usually expressed as a selection coefficient. In population genetics, selection coefficient is the Fitness (biology) deviation, measuring the intensity of natural selection acting on the genotypes in the population. ...