Levels of selection among interactors
Natural selection must always act on physical entities (interactors) that vary in aptitude for reproduction, either because they differ in the machinery of reproduction or in that of survival and resource capture on which reproduction depends. It is also necessary that there be what Darwin ( 1859, Chapter 4) called 'the strong principle of inheritance', so that events in the material domain can influence the codical record. Offspring must tend to resemble their own parents more than those of other offspring. Whenever these conditions are found there will be natural selection. Wherever they are found to great degree: inheritance strong enough, differences in fitness great enough, competing alternatives numerous enough, selection may produce noteworthy cumulative effects.
To Darwin and most of his immediate and later followers, the physical entities of interest for the theory of natural selection were discrete individual organisms. This restricted range of attention has never been logically defensible, especially not since Lewontin's ( 1970) lucid examination of the problem and of much relevant literature more than 20 years ago. Interactors can conceivably be selected at levels from molecule to ecosystem, and there has been helpful recent progress on this levels-of-selection question ( Sober 1984, Chapters 7-9; Brandon 1988, 1990; Damuth and Heisler 1988; Lloyd 1988).
Natural selection at any given material level, such as the individual soma, is expected to produce adaptation at that level, unless heritability is zero or selection is overcome by random processes or selection at other levels. If no evidence of adaptation is found, it must be that selection is absent