Two useful introductions to the fundamental theory underlying the unit of selection issue and debate, which also present examples of multi-level selection from the entire range of the biological hierarchy (typically with entities at level "N"-1 competing for increased representation, i.e., higher frequency, at the immediately higher level "N", e.g., organisms in populations or cell lineages in organisms), are Richard Lewontin's classic piece "The Units of Selection" and John Maynard-Smith and Eörs Szathmáry's co-authored book, "The Major Transitions in Evolution". As a theoretical introduction to units of selection, Lewontin writes: