I remember reading about a concept—in evolutionary biology or natural selection, I think—whereby a particular trait is advantageous to the population or species but only so long as that trait is only exhibited by a minority of the population. That is, the population is more likely to survive if the trait's existence and expression is maintained, but only if that expression is limited to a small percentage of the population. If the trait becomes too common, the selection advantage of having it will decrease, and the fact that most of the population has it may even negatively impact the survivability of that population.
I can't recall where I encountered it. It may have been in the context of reading about ADD, Aspberger's/Autism, neurotypicality, introversion, or mania/OCD. I want to revisit the topic and learn some more about it, but Google is failing me.
Can anyone tell me the name of or term for the concept I'm thinking of?