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A unit of selection is a biological entity within the hierarchy of biological organisation (e.g. 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, i.e., is it group or individual selection that has driven the evolution of altruism? When it is noted that altruism reduces the fitness of individuals, it is difficult to see how altruism has evolved within the context of Darwinian selection acting on individuals; see Kin selection.