Species as Individuals in the Hierarchical Theory of Selection 713
open, but I am not confident that the argument can prevail (though Williams, 1992,
despite his past as an ardent gene selectionist, has become a strong advocate of this
view). What do we mean, for example, when we say that dinosaurs died and
mammals survived, or that brachiopods dwindled to a remnant while clams
continually expanded? Do these descriptive statements imply clade selection? A
general argument would have to be framed in the following way: any distinct clade
maintains defining autapomorphic characters expressed by all subparts (species). If
a clade survives, while another living in roughly comparable habitats, dies—and if
survival can be tied to autapomorphic characters held by the persisting clade (and
absent in the extinct clade)—may we not speak of clade selection based on a range
of variability that includes the key characters in the surviving case, but precludes
their expression in the extinct clade?
For example, if mammals survived in part by virtue of small body sizes, and
dinosaurs died for a set of consequences related to invariably (and substantially)
larger body size, couldn't we say that mammals, as a clade, possessed genetic
determinants (shared by homology in all subparts, with homology as the "glue" of
cladal coherence) that all dinosaurs lacked as a result of their own evolved cladal
distinctions? If such a scenario can count as clade selection (rather than just clade
sorting, as an obviously valid description), then selection at this highest level
becomes common in nature—for many clades yield in geological time to
phylogenetically distant clades that share sufficient similarity in habitat and
function to rank as genuine "replacements."
I am not comfortable with this general argument, for no one has yet
articulated firm and operational criteria for distinguishing true clade selection
(based on irreducible fitness conferred by a clade-level property) from descriptive
clade sorting (or differential survival as an effect of lower level properties
belonging to species or organisms, but translating upwards to success or failure of
a clade as a geologically persistent entity). Some examples probably do represent
genuine clade selection—as in Jablonski's (1987) case of clade survival (through
mass extinction), correlated with geographic range of the entire clade, but not with
ranges of component species. Most other examples, however, may not invoke any
genuine clade-level character (either aggregate or emergent), but only represent the
death of each species, item by item (part by part in cladal terms, for this highest-
level individual also maintains the peculiar property of relative immunity,
especially in clades with large numbers of widely distributed subparts, to the fate
of individual subparts). We may frame our best descriptions for such cases in terms
of clade sorting, but do they also qualify as cases of clade selection?
At a minimum, however, such arguments illustrate a need for
macroevolutionary accounts at all levels, even when causality arises from lower
levels and merely affects the fate of higher-level individuals. Thus, the explicit
study of macroevolution would remain vital even if traditionalists had been correct
in ascribing all causality to organismic selection. But we needn't take refuge in this
"minimalist" defense. Causal processes—and not only selection, as I shall
demonstrate in the next section—do operate at substantial (often controlling)