The hierarchical expansion of sorting and selection: sorting and selection cannot be equated

In a nonhierarchical world, where selection on organisms regulated all nonrandom evolutionary change, the traditional equation of selection (a cause of sorting) with sorting itself (differential birth and death among varying organisms within a population) would rarely lead to error, even though the phenomena are logically distinct (for sorting is a simple description of differential "success," and selection a causal process). But in a hierarchical world, with entities acting as evolutionary individuals (genes, organisms, and species among them) at several levels of ascending inclusion, sorting among entities at one level has a great range of potential causes. Direct selection upon entities themselves is but one possibility among many. This paper discusses why hierarchy demands that sorting and selection be disentangled. It then presents and illustrates an expanded taxonomy of sorting for a hierarchical world. For each of three levels (genes, organisms, and species), we show how sorting can arise from selection at the focal level itself, and as a consequence either of downward causation from processes acting on individuals at higher levels or upward causation from lower levels. We then discuss how hierarchy might illuminate a range of evolutionary questions based on both the logical structure of hierarchy and the historical pathways of its construction—for hierarchy is a property of nature, not only a conceptual scheme for organization.