Fig. 1. Complex traits, such as behavior, are composed of numerous lower-level (subordinate) traits, themselves interrelated in a strongly hierarchical fashion. In general, natural and sexual selection will tend to act more strongly at higher levels of biological organization, as indicated by the relative thickness of the black arrows. As typically viewed by organismal and evolutionary biologists, selection acts on phenotypic variation (which reflects variation in gene expression), but does not generally act directly on genetic variation (e.g. at the level of DNA sequences). Exceptions to this point can occur via such phenomena as genomic conflict (e.g. Stearns and Hoekstra, 2005).