 Philosophers and biologists have been concerned to state the conditions for evolution by natural selection, many because they take it that there is a single theory, evolutionary theory, that governs Darwin’s process. In many ways, the attempt to state the conditions for natural selection is a typical philosophical undertaking. We know, for instance, that confirmatory evidence may be used to raise our confidence in what it confirms and this recognition spawns a debate over what, exactly, should count as confirmation (see entry on  confirmation).  Similarly, Darwin’s theory shows how some natural phenomena may be explained (including at least adaptations and speciation), and thus it is a similar philosophical concern to state exactly when the deployment of the theory is licensed. Such a statement would then issue in a verdict on what, beyond the phenomena targeted by Darwin, is equally explicable using his theory. Such a verdict could be used to arbitrate whether the spread of cultural variations, “memes”, is genuinely explicable using the theory, as Dawkins (1982) suggested. The mammalian immune system may equally involve dynamics that are explicable as selection processes (see section 4.1 of the entry on  replication and reproduction).  Zurek (2009) has even defended using the theory to explain phenomena in quantum mechanics.