• Adaptations evolve by means of natural selection. When natural selection acts, it alters the frequencies of entities at many levels in the hierarchy of biological levels of organization. It also produces adaptations that benefit entities at many levels.

• The discussion of units of selection aims to find out which level natural selection directly acts on, and which ones it affects only incidentally.

• Evolutionary biologists are interested in what the unit of selection is both in order to understand why adaptations evolve and also in order that, when they study adaptations, they can concentrate on theoretically sensible hypotheses.

• We can find out which level of organization shows adaptations by considering a series of adaptations at genic, cellular, organismic, and group levels and asking which evolves most often.

• Segregation distortion is an adaptation of a gene against its allelic alternatives. Examples of this kind are rare.

• In organisms like humans, with separate germ and somatic cell lines, selection between cell lines is a weak force. But many species do not have separate germ lines and in these we expect cell lines to evolve adaptations enabling them to proliferate at the expense of other cell lines.

• Adaptations are common at the level of organisms. When genetic relatives interact, adaptations may evolve for the benefit of kin groups (kin selection).

• Group selection, in which selection produces adaptations for the benefit of groups of unrelated individuals, is thought to be a weak force.

• Adaptations are possessed by the levels in the hierarchy of life that show heritability, in the sense that genetic changes are inherited by the progeny at that level. Group selection is weak because of the low genetic correlation (heritability) between succeeding generations of groups.

• Natural selection only adjusts the frequencies of entities that are sufficiently permanent over evolutionary time. It therefore fundamentally adjusts the frequency of small genetic units. This small genetic unit is called the replicator. The gene can be so defined to be the unit of selection; but it is then not necessarily always a cistron in length.

• Adaptations evolve because they increase the replication of genes. The replication of genes, in the real world, is enhanced by adaptations that benefit entities that show heritability.

• The question of whether natural selection adjusts the frequencies of genes or of organisms is distinct from the question of the relative power of individual, kin, and group selection.