Selective pressure is any phenomena which alters the behavior and fitness of living organisms within a given environment. It is the driving force of evolution and natural selection, and it can be divided into two types of pressure: biotic or abiotic.

Biotic pressure affecting an organism are living organisms within the same ecosystem that interact with the affected organism. Abiotic pressure is created by non-living factors within the organism's environment, such as light, wind, and soil. All of these factors interact with the organism to provide opposition to its continued survival. Some characteristics are better suited to survive with a particular set of conditions, and these conditions are the selective pressures that drive the course of evolution for the organism. Some selective pressure is neutral and an example of stabilizing selection. Other pressures that influence the development of an organism are examples of directional selection and disruptive selection.