There are many conditions that confer a narrow fitness advantage and have significant disadvantages too: sickle cell anemia (malaria resistance), diabetes (cold

There are many conditions that confer a narrow fitness advantage and have significant disadvantages too: sickle cell anemia (malaria resistance), diabetes (cold tolerance) are two I can think of off the top of my head. Here's an interesting book on the topic: https://en.wikipedia.org/wiki/Survival_of_the_Sickest_(book) https://en.wikipedia.org/wiki/Survival_of_the_Sickest_(book)