In general it's hard to make value judgements about which traits are positive and which are negative. Natural selection cares only for survival advantage of off

In general it's hard to make value judgements about which traits are positive and which are negative. Natural selection cares only for survival advantage of offspring in the immediate environment. A good example is sickle cell trait, which provides a higher resistance to malaria, but a risk of a variety of other complications. In areas where malaria is a severe threat, the survival advantage of the higher resistance outweighs the other drawbacks. Medical intervention such as administering a malaria vaccine might reduce the threat of malaria such that there would be selection pressure against the sickle cell trait instead of for it.