In lieu of an abstract, here is a brief excerpt of the content:

3Biological ontology and Hierarchical organization: A Defense of Rank freedom Samir okasha This chapter deals with the ontology of biology systems, with particular reference to hierarchical organization. That biological systems exhibit hierarchical structure is a commonplace: larger biological units, such as multicelled organisms, are composed of smaller biological units (e.g., cells), which themselves contain still smaller units (e.g., chromosomes). Commonplace though this observation is, it is unclear exactly how the biological hierarchy should be conceptualized. Is the biological hierarchy strictly nested, or does it permit overlapping? What determines the hierarchical level that a given biological unit occupies? What biological relation(s) bind(s) the smaller biological units into larger units? Moreover, it is unclear whether there is a single hierarchy, subsuming all biological units. Eldredge (1985, 2003) argues that there are actually two hierarchies, genealogical and ecological; Sarkar (1998) contrasts the “abstract” genetic hierarchy with the “spatial” hierarchy; while Brandon (1988) discerns a dual hierarchy of replicators and interactors. So there is clearly plenty to be said about the nature of hierarchical organization in biology. Building on the ideas of Eldredge, I examine the notion of hierarchy as it is featured in two quite different areas of modern biology. The first is the study of the “major evolutionary transitions,” sensu Maynard Smith and Szathmáry (1995), and the related discussion of multilevel selection. The second is phylogenetic systematics. Hierarchical organization is central to both of these areas of biology, though quite different types of entities are involved . In the major transitions case, new hierarchical levels are created when free-living biological units, capable of surviving alone, become integrated into a larger collective. In the phylogenetics case, new hierarchical levels are created through repeated cladogenesis, or lineage splitting, which leads to a nested hierarchy of monophyletic groups. A central idea in modern phylogenetic systematics is that of a rank-free hierarchy, that is, a hierarchy in which the various levels have no absolute meaning, unlike the traditional Linnaean hierarchy. I argue that the idea of rank freedom, which has proved so fruitful in phylogenetic systematics, can be extended to the quite different biological hierarchy that arises from evolutionary transitions. Applied to the latter hierarchy, rank freedom involves rejecting the idea that some biological units are organisms while others are suborganismic 54 Samir okasha or superorganismic; rather, all entities in the hierarchy are on a par, for there are no ranks. This fits well with the way that the concepts of “individual” and “group” are understood by multilevel selection theorists, and permits a useful new perspective on the old question, “What is an organism?” Hierarchy in Two Areas of Biology Beginning with Buss (1987), the literature on major transitions in evolution, also known as evolutionary transitions in individuality, has burgeoned (cf. Frank 1995; Maynard Smith and Szathmáry 1995; Michod 1999, 2005; Michod and Nedelcu 2003; Queller 2000; Reeve and Keller 1999; Sober and Wilson 1998). This is because biologists have come to realize that the coalescing of smaller biological units into larger ones is something that has occurred repeatedly in the history of life, generating the hierarchical complexity that we see in modern biological systems. A partial list of such transitions includes: single RNA replicators → networks of replicators, individual genes → chromosomes, prokaryotic cells → eukaryotic cells, single-celled organisms → multicelled organisms, solitary animals → integrated colonies.The challenge is to understand such transitions in Darwinian terms (as well as to piece together the actual sequence of stages in each transition). Why was it advantageous for the smaller biological units to sacrifice their individuality and form themselves into a corporate body? And how could such an arrangement, once evolved, be stable against invasion by cheats? These are the questions that a theory of evolutionary transitions must answer. The study of evolutionary transitions has led to a reassessment of the traditional levels of selection question, familiar from the sociobiology debates of the 1960s and 1970s (Okasha 2005, 2006). Clearly, in any evolutionary transition, the potential exists for selection to act at more than one hierarchical level. For example, in the transition to multicellularity, selection could act on variant cell types within the emerging multicellular aggregate, and also on the aggregates themselves. Though the traditional levels of selection debate did not explicitly deal with evolutionary transitions, many themes and lessons from the former have proved useful for understanding the latter (Michod 1999; Queller 2000)—for example , that individual and group interests can pull in opposite directions, that high relatedness favors the evolution of cooperation...

Share