Sometimes traits are related, or negative traits are only manifest in a recessive manner. In such a way positive traits can be selected for, which have a conseq

Sometimes traits are related, or negative traits are only manifest in a recessive manner. In such a way positive traits can be selected for, which have a consequence of a negative trait being present in a population. The example frequently citied in this area is sickle cell anaemia, which can be inadvertently selected for in areas where Malaria is common. Having one 'bad' gene protects the person from Malaria, having two means that one has sickle cell anaemia. This means that people with a single gene are more likely to survive, and hence pass on their gene, but this sometimes manifests as someone with sickle cell anaemia. See https://en.wikipedia.org/wiki/Sickle-cell_disease#Epidemiology https://en.wikipedia.org/wiki/Sickle-cell_disease#Epidemiolo...