A unit of selection is a biological entity within the hierarchy of biological organisation (e.g. self-reproducing molecules, genes, cells, individuals, groups, species) that is subject to natural selection. For several decades there has been intense debate among evolutionary biologists about the extent to which evolution has been shaped by selective pressures acting at these different levels.
This debate has been as much about what it means to be a unit of selection as it has about the relative importance of the units themselves, e.g., is it group or individual selection that has driven the evolution of altruism? When it is noted that altruism reduces the fitness of individuals, gene- or individual-centered explanations for the evolution of altruism become more complex and rely on the use of game theory, for instance; see Kin selection and group selection.
Read more about Unit Of Selection: Fundamental Theory, Examples of Selection At Each Level
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