808 THE STRUCTURE OF EVOLUTIONARY THEORY
components, or "atoms," of evolutionary trends in clades. The apparatus of
punctuated equilibrium then explains why trends, when necessarily described as
speciational, display a punctuated pattern at geological scales (as expressed in the
theory's basic components of stasis and geologically abrupt appearance). In a larger
sense, punctuational accounts of trends propose a similar allometric model for any
relevant scale—that is, any microscope placed over higher-level smoothness may
reveal an underlying "stair-step" pattern among constituent causal individuals
acting as Darwinian agents of the trend.
In sexually reproducing metazoa, species clearly play this role as causal
individuals (see Chapter 8). The theoretical validity of punctuated equilibrium
depends upon such a claim and model. But when we turn to such asexually
reproducing unicells as planktonic forams, designated "species" cannot be
construed as proper Darwinian individuals, and therefore cannot be primary causal
agents (or interactors) in evolutionary trends. To locate the proper agent, the
legitimate analog of the metazoan species, we must move "down" a level to the
clone—to what Janzen (1977), in a seminal paper, called the El, or "evolutionary
individual."
When we execute this conceptual downshift in levels to locate the focal
evolutionary individual in asexual and unicellular lineages, we recognize that the
foram "species" acts as an analog to the metazoan lineage or clade, not to the
metazoan species. The foram "species" represents a temporal collectivity whose
evolutionary pattern arises as a summed history of the Darwinian individuals—
clones in this case—acting as primary causal agents.
We can now shift the entire causal apparatus one level down to posit a
different locus of punctuational change in planktonic forams. Just as the
punctuational history of species generates smooth trends in the collectivity of a
lineage or clade in Metazoa, so too might the punctuational history of clones yield
gradualism in the collectivities so dubiously designated as "species" in asexual
unicells. In other words, foram "species" may exhibit gradualism because these
supposed entities are really results or collectivities, not proper Darwinian
individuals or causal agents. Eldredge and I first presented this argument in our
initial commentary on the debate about punctuated equilibrium (Gould and
Eldredge, 1977, p. 142, and Fig. 9-10):
We predict more gradualism in asexual forms on biological grounds. Their
history should be, in terms of their own unit, as punctuational as the history
of sexual Metazoa. But their unit is a clone, not a species. Their
evolutionary mode is probably intermediate between natural selection in
populations, and species selection in clades: variability arises via new
clones produced rapidly (in this case, truly suddenly) by mutation. The
phenotypic distribution of these new clones may be random with respect to
selection within an asexual lineage (usually termed a "species," but not
truly analogous with sexual species composed of interacting individuals).
Evolution proceeds by selecting subsets within the group of competing
clones. If we could enter the protists' world, we would view this process of
"clone selection" as punctuational. But we study