Selection pressures are external agents which affect an organism’s ability to survive in a given environment

• Selection pressures can be negative (decreases the occurrence of a trait) or positive (increases the proportion of a trait)
  

• Selection pressures may not remain constant, leading to changes in what constitutes a beneficial adaptation for survival
  

Types of selection pressures include:

• Resource availability – Presence of sufficient food, habitat (shelter / territory) and mates
  

• Environmental conditions – Temperature, pH levels, weather and climate changes
  

• Biological factors – Exposure to predators and pathogens (diseases)

Selection pressures can be density-dependent (affected by population size) or density-independent (unaffected by population)

• Density-dependent factors include predators, availability of nutrients and shelter, pathogenic diseases and the accumulation of wastes
  

• Density-independent factors include abiotic conditions (such as temperature, pH, carbon dioxide levels), weather conditions and natural disasters
  

Selection Pressure Examples