Species as Individuals in the Hierarchical Theory of Selection 621
struggle for differential reproductive success. To be causal units under this
criterion, organisms need only persist for the single generation of their lifetimes—
as they do. This endurance may not strike us as a long time in some intuitively
appealing psychological sense, or relative to the persistence of faithful gene
replicates, or considered in comparison with geological scales—but these temporal
frameworks are irrelevant to the question and theory at hand. Organisms last long
enough to act as units of selection in a Darwinian process; they therefore possess
the "sufficient stability" required of evolutionary individuals.
Of course, evolutionary individuals must all be able to pass—differentially
and in a heritable manner—their favorable properties into future generations. But
no aspect of this requirement implies or requires that units of selection must pass
copies of themselves, bodily and in their entirety, into the next generation. The
criterion of heredity only demands that units of selection be able to bias the genetic
makeup of the next generation towards features that secured the differential
reproductive success of parental individuals. Units of selection only need to plurify
their own representation in the next generation; they need not copy themselves.
Sexual organisms happen to plurify by disaggregation and subsequent differential
passage of genes and chromosomes. Other kinds of individuals, including genes,
asexual organisms and species, plurify more coherently. This common confusion
of plurifaction with faithful replication has erected a serious stumbling block to
proper understanding of the hierarchical theory of selection.
We can best clarify this crucial issue of the relationship between selective
agency and criteria of faithful replication vs. plurifaction if we drop, for a moment,
the conventional framework of replication vs. interaction, and return instead to a
different metaphor commonly invoked during 19th century debates about the
nature of Darwinism and natural selection—namely sieves.
We may use the classical metaphor of sieving to illustrate the
inappropriateness of faithful replication as a criterion for defining units of
selection. The "goal" of a unit of selection is not unitary persistence (faithful
replication)— and I can't quite figure out why so many late 20th century
Darwinians ever tried to formulate the concept in this manner. The "goal" of a unit
of selection is concentration by plurifaction—that is, the differential passage of
"youness" into the next generation, an increase in relative representation of your
heritable attributes (whether you pass yourself on as a whole, or in disaggregated
form, into the future of your lineage).
In the favored metaphor of Darwin's day, selection works like a sieve laden
with all the individuals of one generation. Surrounding environments shake the
sieve, and particles of a certain size become concentrated, while others pass
through the webbing (lost by selection). Sieving represents the causal act of
selection—the interaction of the environment (shaking the sieve) with varying
individuals of a population (particles on the sieve). As a result of this interaction,
some individuals live (remain on the sieve), while others die (pass through the
sieve)—and survival depends causally upon variation in emergent properties of the
particles (in this simplest case, large particles remain, and small particles pass
through to oblivion).