600 THE STRUCTURE OF EVOLUTIONARY THEORY
of selection" problem. In his initial article, Ghiselin (1974b, p. 543) dimly
perceived the key implication for hierarchical selection if species be construed as
individuals rather than classes, and selection (by Darwin's definition) works on
individuals—namely, that selection must also operate among species-individuals
(and, by extension, potentially at several levels in a hierarchy of units, each
properly construed as an "individual"). But Ghiselin did not complete his argument
and grant full evolutionary individuality to species. "Species are units, and they
have evolutionary importance, but the same may be said of organisms. Doubtless
both organisms and species specialize. And probably organisms become adapted
but species do not, except in so far as they consist of adapted organisms" (Ghiselin,
1974b, p. 543).
David Hull (1976), in the first major philosophical extension of Ghiselin's
proposal, firmly linked the concept of species as individuals to the older issue of
units (or levels) of selection, thus properly tying the rationale for a causal theory of
hierarchical selection to the generalization of Darwin's key insight that selection
can only operate by the differential reproductive success of "individuals": "Entities
at various levels of organization can function as units of selection if they possess
the sort of organization most clearly exhibited by organisms: and such units of
selection are individuals, not classes" (Hull, 1976, p. 182). In his important later
article—the locus classicus of the pivotal distinction between "replicators" and
"interactors" (see next section of this chapter)—Hull then added (1980, p. 315):
"Individuality wanders from level to level, and as it does, so too does the level at
which selection can occur."
If the rationale for a hierarchical theory of selection resides in the expansion
of "individuality" to several levels of biological organization (see Gould and
Lloyd, 1999), then we must specify a set of criteria that any material configuration
must meet to merit designation as an "individual." We may, I think (see Gould,
1994), most usefully divide these criteria into two categories: (1) requirements in
ordinary language for granting individuality to any configuration (vernacular
criteria); and (2) requirements in Darwinian theory for regarding any entity as an
evolutionary individual, or potential agent of selection (evolutionary criteria). (I
trust that, despite a traditional ethos contrary to such an admission, all thoughtful
and self-introspective scientists no longer feel threatened or disloyal in
acknowledging that all definitions must be theory laden—see Kuhn, 1962, for the
classic statement.)
We must also resolve one other terminological confusion before listing the
criteria of individuality. What word shall we use as a general term for the discrete
"thing" that can serve as a unit of selection at various hierarchical levels? In an
important article, a manifesto for reviving interest in the power of group selection
and the validity of the hierarchical model in general, D. S. Wilson and E. Sober
(1994) suggest that we use the term "organism" for the generality (and therefore
speak of "species organisms," "gene organisms," and so forth), while restricting the
word "individual" to organic bodies (you, me, the oak tree, and the barnacle) at the
conventional level of Darwinian concern. They choose this definition because they
emphasize—I would say