A unit of selection is a biological entity within the hierarchy of biological organization (for example, an entity such as: a self-replicating molecule, a gene, a cell, an organism, a group, or a species) that is subject to natural selection. There is debate among evolutionary biologists about the extent to which evolution has been shaped by selective pressures acting at these different levels. There is debate over the relative importance of the units themselves. For instance, is it group or individual selection that has driven the evolution of altruism? Where altruism reduces the fitness of individuals, individual-centered explanations for the evolution of altruism become complex and rely on the use of game theory, for instance; see kin selection and group selection. There also is debate over the definition of the units themselves, and the roles for selection and replication, and whether these roles may change in the course of evolution. Fundamental theory 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: The generality of the principles of natural selection means that any entities in nature that have variation, reproduction, and heritability may evolve. ...the principles can be applied equally to genes, organisms, populations, species, and at opposite ends of the scale, prebiotic molecules and ecosystems." (1970, pp. 1-2) Elisabeth Lloyd's book The Structure and Confirmation of Evolutionary Theory provides a basic philosophical introduction to the debate. Two more recent introductions include Samir Okasha's book Evolution and the Levels of Selection and Pierrick Bourrat's book Facts, Conventions, and the Levels of Selection. Selection at each level Below, cases of selection at the genic, cellular, individual and group level from within the multi-level selection perspective are presented and discussed. Nucleic acid George C. Williams in his influential book Adaptation and Natural Selection was one of the first to present a gene-centered view of evolution with the gene as the unit of selection, arguing that a unit of selection should exhibit a