Interdemic (group) selection is 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; an example of such a trait would be altruistic social behavior in which an individual organism sacrifices its well-being for the good of other members of its species group. Such a trait would thus be selected for until every individual within the population has the trait (that is, the trait becomes fixed).

Some of the information in this writeup was taken from the science dictionary at; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.