Any discussion of multi-level selection thus requires that we first clarify how units of selection are defined and how these reproduce. Identifying the units of selection Because we can sometimes model evolution as though selection were acting on any one of a number of levels, we need some other criteria to decide what the mechanistic level of selection is in a particular case. We can see how this test works by applying it to the case of kin selection. Consider a population composed of a single family of full siblings. Now imagine that some of these offspring carry an allele for altruism and others do not.
Individuals are also related to one another and interact.
Thus, if kin selection is a case of selection acting on individuals or alleles, it should happen here. In fact, altruism will always decrease in this scenario, regardless of the costs and benefits. To see why this is so, consider the figure below. Thus, although the total number of altruists goes up, their frequency declines. This decline in altruism, despite the benefit being greater than twice the cost, results because selfish individuals always do better than their altruistic siblings.