A type of natural selection which acts upon populations within a species. Interdemic selection occurs when populations die out or give rise to new populations at different rates, depending on whether they have traits that are beneficial or harmful to the population as a whole. It is also possible for a trait to be harmful to the individuals within the population which have it, but be beneficial to the population as a whole; such a trait would thus be selected for until every individual within the population has the trait (that is, the trait becomes fixed).