ABSTRACT
The units of selection problem are the problem of determining what kinds of adaptations are found in nature. The units of selection problem has been with us ever since Darwin. Although he is sometimes said to have held that traits evolve because they are "good for the species," Darwin almost never thought about natural selection in this way. If the organism is the exclusive unit of selection, then natural selection works against the evolution of altruism. If the group is sometimes a unit of selection, then natural selection sometimes favors altruistic traits. The units of selection problem cannot be settled by stipulative convention, because different views about the units of selection make contrary predictions about which traits evolve under natural selection. Selection processes require items that transmit their structure relatively unchanged from generation to generation (replicators) and things whose interactions with the environment affect their survivorship and reproductive success (interactors).
