 Consider the following naive argument against the conceptual possibility of group-level  selection.[12]  Suppose that we define the fitness of a group as the average fitness of its individuals. In this context, altruistic individuals are, by definition, those with traits that reduce their individual fitness while improving the fitness of other group members. For instance, crows that issue warning cries when a predator approaches benefit the group while increasing the chances of being harmed themselves. Natural selection explains the evolution of traits on the basis that individuals with the trait are fitter than those without it (all else being equal). Since selfish individuals are by definition fitter than altruistic ones, it follows that groups with more altruistic individuals cannot be fitter. Or so one might argue.