Thirdly, we need to understand that the mechanisms of heritable variation and differential reproduction may apply at many levels, systems and sub-systems, simultaneously. The extent to which it is useful to explain the adaptation of a system under these mechanisms is dependent on both the extent to which the characteristics of the system are heritable, and the extent to which the system can be differentiated from other systems. That is, selection acts on an entity to the extent that that entity is coherent. Thus, the level of individual is a useful one is explaining adaptation. Another useful level is the gene.
Genes are highly heritable and easily differentiated.
However, note that neither of these levels alone provides a full explanation. The complete set of characteristics that describes an individual is not perfectly heritable under sexual reproduction because subsets of genes are crossed-over. And, conversely, one gene is not completely differentiable from another - they are often tightly coupled within an individual. Put simply, genes are more likely to reproduce together if they come from the same individual than if they come from different individuals. Niether the gene level nor the individual level tells the whole story.