In Chapter 1, we noted that the levels-of-selection question has undergone a subtle transformation in recent years. In early discussions, the existence of hierarchical organization was taken for granted, as if it were simply a brute fact about the biological world; the aim was to understand selection and adaptation at pre-existing hierarchical levels. But recent theorists, beginning with Buss (1987), have sought to understand how the biological hierarchy evolved in the first place, thus transforming the levels-of-selection question. This chapter examines the implications of this change in focus.

Maynard Smith and Szathmary (1995) discuss the evolution of hierarchical organization under the heading 'the major transitions' in evolution; in a similar vein, Michod (2005) talks about 'evolutionary transitions in individuality'. These transitions include: (i) solitary replicators s networks of replicators enclosed in compartments; (ii) unlinked genes s chromosomes; (iii) prokaryotic cells s eukaryotic cells with intra-cellular organelles; (iv) single-celled s multicelled organisms; and (v) solitary organisms s colonies. In each case, a number of smaller units, originally capable of surviving and reproducing on their own, formed themselves into a larger unit, creating a new level of organization. As Maynard Smith and Szathmary say, 'entities that were capable of independent replication before the transition can replicate only as part of a larger whole after it' (1995 p. 6). The challenge is to understand these transitions in Darwinian terms. Why was it advantageous for the lower-level units to sacrifice their individuality and form themselves into a corporate body? And how could such an arrangement, once first evolved, be evolutionarily stable?

This immediately raises the levels-of-selection issue. For during an evolutionary transition, lower-level selection may frustrate the evolution of the higher-level unit. In the transition to multicellularity, for example, selection between competing cell lineages may disrupt the integrity of the emerging multicelled creature. However, if selection also acts on the higher-level units, this may promote the evolution of adaptations for suppressing internal conflict. Thus in the multicellularity case, Buss (1987) argues that germ-line sequestration is such an adaptation, for it reduces the probability that mutant cells, arising during ontogeny, will pass to the next generation. This particular argument has been contested, but the general idea that evolutionary transitions involve an interaction between levels of selection is widely accepted. In large part, this explains the recent resurgence of interest in multi-level selection theory among biologists (cf. Reeve and Keller 1999).

Section 8.1 explores how the traditional levels-of-selection question has been transformed by the recent developments. Section 8.2 looks at competing methodologies for studying evolutionary transitions; the 'genic' approach of Maynard Smith and Szathmary (1995) is contrasted with the 'hierarchical' approach of Buss (1987). Section 8.3 asks what becomes of the contrast between MLS1 and MLS2 in the context of the major transitions; I argue that both types of multi-level selection are relevant, but at different stages of a transition. This point is illustrated in Section 8.4, with reference to Michod and co-workers' models of the transition to multicellularity (Michod 1997, 1999, 2005; Michod and Roze 1999; Roze and Michod 2001; Michod and Nedelcu 2003). Section 8.5 draws some tentative conclusions.

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