Selective pressure is an evolutionary force that causes a particular phenotype to be more favorable in certain environmental conditions. It is a quantitative description of the amount of change occurring in processes investigated by evolutionary biology. Any cause that reduces or increases reproductive success in a portion of a population potentially exerts selective pressure, driving natural selection. Selection pressures are external agents that affect an organisms ability to survive in a given environment. They can be negative (decreases the occurrence of a trait) or positive (increases the proportion of a trait) . Types of selection pressures include:

• Resource availability: Presence of sufficient food, habitat (shelter/territory), and mates.
• Environmental conditions: Temperature, weather conditions, or geographical access.
• Biological factors: Predators and pathogens (diseases).

Selective pressures can be density-dependent (affected by population size) or density-independent (unaffected by population) . Examples of selective pressures include antibiotic use against pathogenic microbes, resource availability, and predation.