A Unit of Selection, is, in evolutionary theory theory, the hypothetical basis upon which organisms diverge and mutate.

One of the primary debates in evolutionary theory has been the drive to explain phenomeon such as altruism and kin selection, as these types of behavior do not seem to agree with the concept of individual survival of the fittest, and the idea of perfect selfishness. The unit of selection concept tries to explain these behaviors by generalizing fitness to a specific attribute of a species.

One of the first units of selection to be proposed was the species or group, as in the study of group selection. More recently, the concept of the selfish gene, as proposed primarily by Richard Dawkins has gained strength as a probable alternative.