Diseases
Diseases can also be selective pressures. One well-known example is the sickle cell trait in humans, caused by having one copy of the mutated hemoglobin allele that causes sickle cell anemia. People that have two copies of this mutated allele are likely to die of anemia at a young age, so you'd think that the allele would be strongly selected against, right? However, people with only one copy of the sickle cell allele have a survival advantage in malarial regions because they are more resistant to malaria. Scientists don't yet know exactly why they are more resistant, but in these regions, malaria is a selective pressure that keeps the sickle cell allele circulating in the population.