Unit of selection

A unit of selection is a biological entity within the hierarchy of biological organization (for example, an entity such as: a self-reproducing 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.

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 what is at stake vis-a-vis units of selection, Lewontin writes:

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