   Darwinism also has challenges from the opposite direction.  In the 1970s a number of biologists working in the fields of paleontology and systematics challenged the Neo-Darwinian dogma that you could account for ‘macro-evolution’ by means of long term extrapolation from micro-evolution.  Gould, in particular, opens Part II of The Structure of Evolutionary Theory (Towards a Revised and Expanded Evolutionary Theory), with a chapter entitled ‘Species as Individuals in the Hierarchical Theory of Selection’.  That chapter title combines two conceptually distinct theses: first, the thesis defended by Michael Ghiselin (Ghiselin 1997) and championed and refined by David Hull (Hull 2001), that species are, in a robust sense of the term, ‘individuals’; and second, that there may well be selection among groups of organisms, qua groups. Gould’s title exemplifies one approach to group selection—the unit of selection is always the individual, but there are individuals other than individual organisms that are subject to selection.  A very different result emerges if one assumes that groups of organisms such as demes, kin-groups, or species, though not individuals, are nevertheless subject to selection. Adding to the conceptual complexity, some researchers propose that the term ‘group selection’ be restricted to the process whereby group-level traits provide advantages to one group over another, in which case there are strict conditions delimiting cases of group selection. Others define group selection primarily in terms of group level effects. Thus a debate analogous to that earlier discussed regarding the definitions of ‘fitness’ emerges here—by group selection do we mean a distinct type of causal process that needs to be conceptually distinguished from selection at the level of individual organism or gene, or do we merely mean a tendency within certain populations for some well defined groups to displace others over time?  (For further discussion, see Sterelny and Griffiths 1999, 151–179; Hull 2001, 49–90; and see the entry on:  levels and units of selection.)