Species as Individuals in the Hierarchical Theory of Selection 623
pluralism in arguing that identification of causal agency must include statements
about both the faithfulness of replicators and the potency of inter-actors.
Individuals need not replicate themselves faithfully to be units of selection. Rather,
they must contribute to the next generation by hereditary passage, and they must
plurify their contributions relative to those of other individuals. But the
contributions themselves can be wholes or parts; faithful replicates or
disaggregated bits of functional heredity. Selection demands plurifaction, not
faithful replication.
The simple observation of plurifaction—the relative increase of an
individual's representation in the heredity of subsequent generations—does not
suffice to identify the operation of natural selection, for plurifaction can occur by
nonselective means, and phenotypes can increase in frequency but then be unable
to plurify. Consider the primary example of each phenomenon. First, individuals
may plurify by accidents of genetic drift. Suppose that individuals fall through the
sieve of selection at random, but survivors show increased frequency of certain
heritable traits by accident. These surviving individuals will plurify, but they have
not operated as active units of selection. Second, individuals may increase in
frequency for phenotypic reasons unrelated to heredity. Suppose that large
individuals remain differentially on the sieve, but that individuals grow larger than
average for purely ecophenotypic reasons uncorrelated with any aspect of heredity
that can pass to subsequent generations. Large phenotypes have increased in
frequency for causal reasons—but they will not be able to plurify because they
cannot bias the heredity of subsequent generations.
So selection demands plurifaction because evolutionary individuals must
maintain lineages by hereditary passage, and selection occurs by increase in
relative representation. But plurifaction can only represent a necessary condition,
not a cause. We define selection as occurring when plurifaction results from a
causal interaction between traits of an evolutionary individual (a unit of selection)
and the environment in a manner that enhances the differential reproductive
success of the individual. Thus, and finally, units of selection must, above all, be
interactors. Selection is a causal process, not a calculus of results—and the
causality of selection resides in interaction between evolutionary individuals and
surrounding environments. The study and documentation of group and higher-level
selection has been stymied and thrown into disfavor by our confusion over these
issues—and especially by the blind alley of a logically false argument that
identified replicators rather than interactors as units of selection, and then
constructed a fallacious, reductionistic theory, precisely opposite in structure to the
hierarchical model, by specifying genes (because they replicate faithfully) as
ultimate or exclusive units of selection. In this context, I note with delight that
group selection has risen from the ashes to receive a vigorous rehearing (Sober and
Wilson, 1998, for a full treatment; Lewin, 1996, for a popular account under the
title "Evolution's new heretics"; and Gould and Lloyd, 1999, for resolution of a
final logical problem). This potent revival rests upon two proposals that, as
centerpieces of this book, could not gain my stronger assent: the identification of
evolutionary