Species as Individuals in the Hierarchical Theory of Selection 607
since Buffon, as the chief criterion of "specieshood"). They function as a unit and
persist continuously. Above all, they do not change substantially in phenotype—
the crucial concept of stasis. Surely, the average species in stasis undergoes less
temporal change (with less directionality) than human bodies experience in our
passage from babyhood through adult vigor and into senescence. If humans retain
discrete personhood through all these slings and arrows of ontogeny, then species
(under punctuated equilibrium) function as equally good or even better individuals
by the same criteria of vernacular definition.
In describing exceptions and fuzzinesses in the application of these vernacular
criteria to organisms, and acknowledging that species face the same difficulties of
definition, Hull (1976, p. 177) wrote: "However, exactly the same questions arise
for both. If organisms can count as individuals in the face of such difficulties, then
so can species." But Hull assumed that these common problems plague species far
more intensely than they threaten organisms. I would suggest that the opposite
situation may prevail in nature: species may be even better individuated than
organisms when punctuated equilibrium applies (and we consider species at their
appropriate scales of geological time). This issue unites these two chapters in a
crucial link between the theory of punctuated equilibrium (Chapter 9) and the
classical debate about "units" or "levels" of selection (Chapter 8)—a conjunction
that underlies my views on the importance and validation of macroevolutionary
theory.
Interestingly, albeit through a glass darkly, Hull (1976) grasped the logical
link between the phenomenology of punctuated equilibrium and the definition of
species as individuals in his first important paper on this subject—even though he
had not, by this time, encountered our empirical and theoretical arguments for such
a pattern (Eldredge, 1971; Gould and Eldredge, 1971; Eldredge and Gould, 1972).
(In his more inclusive review of 1980, Hull then explicitly joined our particular
claims to the defense of species as individuals.) Hull begins by stating the problem
(1976, p. 185): "Earlier I described individuals as reasonably discrete,
spatiotemporally continuous and unitary entities individuated on the basis of
spatiotemporal location rather than similarity of some kind. But one might object
that species lack these characteristics. For example, in most cases new species arise
gradually."
Hull then recognized that some neontological models of speciation accelerate
the rate of branching relative to the supposedly standard rate of anagenesis within
species—and that such an acceleration will sharpen the definability of species by
the criterion of discrete birth: "But there are processes in nature which serve to
narrow the boundaries between ancestral and descendant species ... The end result
is that the number of organisms intermediate between the ancestral and descendant
species is reduced considerably" (1976, p. 185). Finally, Hull stresses the
important point that all individuation, at any appropriate scale, entails some
fuzziness at the boundaries—and that species therefore need not be construed as
"worse" individuals than bodies (1976, p. 185). "If processes similar to those just
described are common in nature, then the boundaries between ancestral and
descendant species can be narrowed